176,99 €
Mehr erfahren.
- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Serie: Institute of Food Technologists Series
- Sprache: Englisch
The increasing demand for healthy foods has resulted in the food industry developing functional foods with health-promoting and/or disease preventing properties. However, many of these products bring new challenges. While drugs are taken for their efficacy, functional foods need to have tastes that are acceptable to consumers. Bitterness associated with the functional foods is one of the major challenges encountered by food industry today and will remain so in years to come. This important book offers a thorough understanding of bitterness, the food ingredients that cause it and its accurate measurement. The authors provide a thorough review of bitterness that includes an understanding of the genetics of bitterness perception and the molecular basis for individual differences in bitterness perception. This is followed by a detailed review of the chemical structure of bitter compounds in foods where bitterness may be considered to be a positive or negative attribute. To better understand bitterness in foods, separation and analytical techniques used to identify and characterize bitter compounds are also covered. Food processing can itself generate compounds that are bitter, such as the Maillard reaction and lipid oxidation related products. Since bitterness is considered a negative attribute in many foods, the methods being used to remove and/mask it are also thoroughly discussed.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 514
Veröffentlichungsjahr: 2017
Ähnliche
Table of Contents
Cover
Title Page
Copyright
List of Contributors
Preface
Section I: The Biology of Bitterness Perception
Chapter 1: Biochemistry of Human Bitter Taste Receptors
1.1 Introduction
1.2 Bitter Taste Receptors: T2Rs
1.3 T2R Signal Transduction
1.4 Bitter Taste Perception and T2R Polymorphisms
1.5 Ligand Binding and Activation Mechanisms of T2Rs
1.6 Nutrigenomics of Taste
1.7 Bitter Taste Blockers
1.8 Expression of T2Rs in Extraoral tissues
1.9 Conclusion
Acknowledgement
References
Chapter 2: Physiological Aspects of Bitterness
2.1 Introduction
2.2 Anatomy
2.3 Taste Signal Transduction
2.4 Gustatory Bitter Taste Receptor Gene Expression
2.5 Extragustatory Bitter Taste Receptors
2.6 Outlook
Acknowledgements
References
Chapter 3: Bitterness Perception in Humans: An Evolutionary Perspective
3.1 Bitter Taste Receptors - A Group of G Protein-Coupled Receptor (GPCR) Members
3.2
Tas2R
Gene Family - A Highly Diverse Family in Vertebrates
3.3 The Evolution of
Tas2R
Gene Family in Vertebrates
3.4 Diverse Selective Forces Drove the Evolution of
Tas2R
Genes in Primates
3.5 Genetical Basis of Tasteblindness – Human PTC Perception as an Example
3.6 PTC Tasteblindness in Humans and Chimpanzees - Shared Phenotype Resulted From Unshared Genotypes
3.7 Closing Remarks
Acknowledgement
References
Section II: The Chemistry of Bitterness
Chapter 4: Fruits and Vegetables
4.1 Introduction
4.2 Fruits
4.3 Vegetables
4.4 Future Progress
References
Chapter 5: Bitterness in Beverages
5.1 Introduction
5.2 Bitterness in Tea
5.3 Bitterness in Coffee
5.4 Bitterness in Cocoa/Hot Chocolate
5.5 Bitterness in Beer
5.6 Bitterness in Wine
5.7 Bitterness in Cider
References
Chapter 6: Structural Characteristics of Food Protein-Derived Bitter Peptides
6.1 Introduction
6.2 Bitter Peptides Preparation and Taste Evaluation
6.3 Role of Amino Acid Composition and Position Arrangement in Determining Peptide Bitterness Intensity
6.4 Peptide Debittering Methods
6.5 Conclusions
Acknowledgement
References
Section III: Analytical Techniques for Separating and Characterizing Bitter Compounds
Chapter 7: Sensory Evaluation Techniques for Detecting and Quantifying Bitterness in Food and Beverages
7.1 Screening Methods
7.2 Test Methods
7.3 Techniques to Maximize Bitterness Perception
7.4 Use of Standards
7.5 Conclusion
References
Chapter 8: Analysis of Bitterness Compounds by Mass Spectrometry
8.1 Introduction
8.2 Overview of LC-MS
8.3 Data Acquisition in LC-MS
8.4 LC-MS Application of Bitterness Compounds
8.5 Challenges and Future Perspectives
8.6 Optimisation of Mass Spectra Parameters
8.7 Recording of MS Profile
8.8 Challenges in the Collection of HRMS Data
8.9 Conclusions
References
Chapter 9: Evaluation of Bitterness by the Electronic Tongue: Correlation between Sensory Tests and Instrumental Methods
9.1 Introduction
9.2 The Electronic Tongue
9.3 The Electronic Tongue and Food Production
9.4 Electronic Tongue and Bitterness
9.5 Evaluating Bitterness in Food Products Using Electronic Tongues
9.6 Conclusion
References
Section IV: Methods for Removing Bitterness in Functional Foods and Nutraceuticals
Chapter 10: Methods for Removing Bitterness in Functional Foods and Nutraceuticals
10.1 Introduction
10.2 Reducing and Removing Bitter Components
10.3 Conclusion
References
Index
End User License Agreement
Pages
xiii
xiv
xv
xvi
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
37
38
39
40
41
42
43
44
45
46
47
48
49
50
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
133
134
135
136
137
138
139
140
141
142
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
211
212
213
214
215
216
217
219
220
221
222
224
225
226
227
228
229
230
231
232
233
234
235
236
237
239
240
241
242
243
Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Chapter 1: Biochemistry of Human Bitter Taste Receptors
Figure 1.1 Predicted secondary structure model of the bitter taste receptor T2R4. The coding region is 299 amino acids long, has a short extracellular N-terminus, three extracellular loops, seven transmembrane (TM1-TM7) helices, three intracellular loops and a short C-terminus.
Figure 1.2 Bitter taste
signaling pathway (IP
3
pathway) Abbreviations: PLCβ2, phospholipase C β2; PIP
2
, phospatidyl-inositol-biphosphate; DAG, diacylglycerol; IP
3
, inositol triphosphate; ER, endoplasmic reticulum; Ca
2+
, calcium; Na
+
, sodium.
Chapter 2: Physiological Aspects of Bitterness
Figure 2.1
Signal transduction in bitter taste receptor cells
. The bitter taste receptor (here shown as a seven transmembrane domain glycoprotein) transmits its activation via heterotrimeric G proteins (consisting of α-, β-, and γ-subunits). After the dissociation of the G protein complex, the βγ-subunits induce the turnover of phosphatidylinositol-4,5-bisphosphate leading to the generation of diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP
3
) by the activity of phospholipase Cβ2. The IP
3
triggers the release of calcium ions (Ca
2+
) from intracellular stores via activation of the type III IP
3
-receptor (IP
3
R). Increasing intracellular calcium ion levels in turn result in the opening of the cation channel TRPM5 leading to depolarization and subsequent release of the neurotransmitter ATP (not shown).
Figure 2.2
Bitter taste receptor expression in human circumvallate papillae
. Cross-section through a human circumvallate papillae stained with antibodies specific for the human bitter taste receptor TAS2R38. Note that within each taste bud several cells express TAS2R38 (white). The taste buds are circled for easier visibility.
Figure 2.3
Bitter taste receptor gene expression throughout the human body
. Tissues reported to express bitter taste receptor genes in human or other mammals are labeled in red. Note that large parts of the respiratory tract and the alimentary canal express bitter taste receptor genes. Additional sites of expression are brain, heart, urinary bladder and testes.
Chapter 3: Bitterness Perception in Humans: An Evolutionary Perspective
Figure 3.1
The
Tas2R
gene repertoires in vertebrates.
Dietary information and the number of
Tas2R
genes are shown after each species name. The scale beneath the phylogeny indicates the divergence time and the one below the bars indicates the total number of
Tas2R
genes (intact, partial and pseudogenes) in each species. (Modified from Li & Zhang, 2013).
Chapter 4: Fruits and Vegetables
Figure 4.1 Delayed bitterness in citrus, formation of Limonin derived from Nomilin via Limoneic acid A-ring lactone
Figure 4.2 Flavanone neohesperidoside Naringin inducing bitterness in grapefruit and other citrus
Figure 4.3 Cyanogenic glycosides amygdalin and prunasin and release of glucose, benzaldehyd and HCN
Figure 4.4 Glucosinolates in Brassica
Figure 4.5 The phenolic bitter compound 3-Methyl-6-methoxy-8-hydoxy-3,4-dihydroisocoumarin named 6-Methoxymellein (6-MM) in carrots
Figure 4.6 Polyacetylenes (Falcarinols) in carrot and other Apiaceae food plants (Czepa and Hofmann, 2003, Christenen and Brandt, 2006).
Figure 4.7 Glycoalkaloids in potato: α-solanine, α-chaconine and β
2
-chaconine (Fenwick
et al.
, 1990).
Figure 4.8 Glycoalkaloid Tomatin in tomato (Fenwick
et al.
, 1990).
Chapter 5: Bitterness in Beverages
Figure 5.1 Major catechins of green tea (Narukawa
et al
., 2011)
Scheme 5.1 Bitter lactones identified in coffee. 5-
0
-caffeoyl-muco-γ-quinide (2), 3-
0
-caffeoyl-γ-quinide (3), 4-0-caffeoyl-muco-γ-quinide (4), 5-
0
-caffeoyl-epi-δ-quinide (5), and 4-
0
-caffeoyl-γ-quinide (6), as well as the novel 3-0-caffeoyl-epi-γ-quinide (7) formed upon thermal treatment (30 min., 230°C) of 5-
0
-caffeoylquinic acid (1)
Scheme 5.2 Reaction mechanism leading to the formation of (furan-2-yl) methylated benzene diols and triols
1,3,5,6
from furfuryl alcohol and 5-0-chlorogenic acid upon coffee roasting
Figure 5.2 Scheme showing conversion of α-acids to iso-α-acids and tetra-iso-α-acids (Humulones: R = isobutyl, cohumulones: R = isopropyl, adhumulones: R =
sec
butyl)
Figure 5.3 Phenolics in cider.
Chapter 6: Structural Characteristics of Food Protein-Derived Bitter Peptides
Figure 6.1 Correlation between bitterness intensity of cheese and β-casein f193-209 concentration in the aqueous extract of cheese at 180 days () and 270 days ().
Figure 6.2 Relative activation rates of T2R1 in response to different peptides calculated based on their EC
50
values. All values were normalized to the EC
50
value of FFF.
List of Tables
Chapter 4: Fruits and Vegetables
Table 4.1 Bitter phytonutrients in fruits and vegetables
Chapter 6: Structural Characteristics of Food Protein-Derived Bitter Peptides
Table 6.1 Bitterness intensity of peptides (X-X) and equivalent free amino acid mixtures (X+X)
Table 6.2 Structure-taste properties of typical bitter peptides
Chapter 7: Sensory Evaluation Techniques for Detecting and Quantifying Bitterness in Food and Beverages
Table 7.1 Review of sensory panels with bitter products
Chapter 8: Analysis of Bitterness Compounds by Mass Spectrometry
Table 8.1 The list of compounds and the analytical methods employed for their determination in various foods
Chapter 9: Evaluation of Bitterness by the Electronic Tongue: Correlation between Sensory Tests and Instrumental Methods
Table 9.1 Summary of selected studies with application of electronic tongue systems in bitterness analysis in food products
Chapter 10: Methods for Removing Bitterness in Functional Foods and Nutraceuticals
Table 10.1 Reducing, Removing and Masking Bitter Compounds
Bitterness
Perception, Chemistry and Food Processing
First Edition
Edited by
Michel Aliani & Michael N. A. Eskin
Manitoba, Canada
The IFT Press series reflects the mission of the Institute of Food Technologists \endash\ to advance the science of food contributing to healthier people everywhere. Developed in partnership with Wiley Blackwell, IFT Press books serve as leading-edge handbooks for industrial application and reference and as essential texts for academic programs. Crafted through rigorous peer review and meticulous research, IFT Press publications represent the latest, most significant resources available to food scientists and related agriculture professionals worldwide.
Founded in 1939, the Institute of Food Technologists is a nonprofit scientific society with 22,000 individual members working in food science, food technology, and related professions in industry, academia, and government. IFT serves as a conduit for multidisciplinary science thought leadership, championing the use of sound science across the food value chain through knowledge sharing, education, and advocacy.
This edition first published 2017 © 2017 by John Wiley & Sons, Inc.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The right of Michel Aliani & Michael N. A. Eskin to be identified as the authors of this work has been asserted in accordance with law.
Registered Offices
John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA
Editorial Office
9600 Garsington Road, Oxford, OX4 2DQ, UK
For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com.
Wiley also publishes its books in a variety of electronic formats and by print-on-demand. Some content that appears in standard print versions of this book may not be available in other formats.
Limit of Liability/Disclaimer of Warranty
While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.
Library of Congress Cataloging-in-Publication Data
Names: Aliani, Michel, editor. | Eskin, N. A. M. (Neason Akivah Michael), editor.
Title: Bitterness : perception, chemistry and food processing / edited by Michel Aliani, Manitoba, Canada; Michael N. A. Eskin, Manitoba, Canada.
Other titles: Bitterness (John Wiley & Sons)
Description: First edition. | Hoboken : John Wiley & Sons, Inc., 2017. | Series: Institute of food technologists series | Includes bibliographical references and index.
Identifiers: LCCN 2016049478 (print) | LCCN 2016049871 (ebook) | ISBN 9781118590294 (hardback) | ISBN 9781118590317 (pdf) | ISBN 9781118590232 (epub)
Subjects: LCSH: Bitterness (Taste) | BISAC: TECHNOLOGY & ENGINEERING / Food Science.
Classification: LCC TX546 .B58 2017 (print) | LCC TX546 (ebook) | DDC 664/.07–dc23
LC record available at https://lccn.loc.gov/2016049478
Cover Design: Wiley
Cover Images: Background Image: fcafotodigital/Gettyimages; Inset Images (from top to bottom): luckyraccoon/shutterstock; ansonsaw/Gettyimages; Brent Hofacker/shutterstock
Titles in the series
Natural Food Flavors and Colorants
by Mathew Attokaran, PhD
Food Carotenoids: Chemistry, Biology and Technology
by Delia B. Rodriguez-Amaya
Biofilms in the Food Environment, 2nd Edition
by Anthony L. Pometto III, Ali Demirci
Spray Drying Techniques for Food Ingredient Encapsulation
by C. Anandharamakrishnan, Padma Ishwarya S.
Trait-Modified Oils in Foods
by Frank T. Orthoefer (Editor), Gary R. List (Editor)
Membrane Processing for Dairy Ingredient Separation
by Kang Hu (Editor), James Dickson (Editor)
Nanotechnology and Functional Foods: Effective Delivery of Bioactive Ingredients
by Cristina Sabliov (Editor), Hongda Chen (Editor), Rickey Yada (Editor)
Anti-Ageing Nutrients: Evidence-Based Prevention of Age-Associated Diseases
by Deliminda Neves (Editor)
Food Industry Design, Technology and Innovation
by Helmut Traitler, Birgit Coleman, Karen Hofmann
Food Oligosaccharides: Production, Analysis and Bioactivity
by F. Javier Moreno (Editor), María Luz Sanz (Editor)
Food Texture Design and Optimization
by Yadunandan Lal Dar (Editor), Joseph M. Light (Editor)
Mathematical and Statistical Methods in Food Science and Technology
by Daniel Granato (Editor), Gaston Ares (Editor)
Bioactive Compounds from Marine Foods: Plant and Animal Sources
by Blanca Hernandez-Ledesma (Editor), Miguel Herrero (Editor)
Resistant Starch: Sources, Applications and Health Benefits
by Yong-Cheng Shi (Editor), Clodualdo C. Maningat (Editor)
Processing and Nutrition of Fats and Oils
by Ernesto M. Hernandez (Original Author), Afaf Kamal-Eldin (Original Author)
Practical Ethics for Food Professionals: Ethics in Research, Education and the Workplace
by J. Peter Clark (Editor), Christopher Ritson (Editor)
Advances in Dairy Ingredients
by Geoffrey W. Smithers (Editor), Mary Ann Augustin (Editor)
Improving Import Food Safety
by Wayne Ellefson (Editor), Lorna Zach (Editor), Darryl Sullivan (Editor)
Food Irradiation Research and Technology, 2nd Edition
by Xuetong Fan (Editor), Christopher H. Sommers (Editor)
Coffee: Emerging Health Effects and Disease Prevention
List of Contributors
Dr. Ayyappan Appukutan Aachary
Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Canada
Dr. Michel Aliani
Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Manitoba, Canada
Dr. Rotimi Aluko
Department of Human Nutritional Sciences, U of Manitoba, Winnipeg, Canada
Dr. Daniel Baumgartner
Agroscope, Institute for Food science IFS, Schloss, Switzerland
Dr. Malik Behrens
Department of Molecular Genetics, German Institute of Human Nutrition, Potsdam, Rebbruecke, Nuthenal, Germany.
Raj Bhullar
Department of Oral Biology, Faculty of Dentistry, University of Manitoba, Winnipeg, Canada
Dr. Prashen Chelikani
Department of Oral Biology, Faculty of Dentistry, University of Manitoba, Winnipeg, Canada
Dr. Geraldine Dowling
Department of Pharmacology and Therapeutics, School of Medicine, Trinity Centre for Health Sciences, St. James's Hospital, James's Street, Dublin, Ireland
Dr. Michael N. A. Eskin
Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Canada
Dr. Erin Goldberg
Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Manitoba, Canada
Jennifer Grant
Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Canada
Dr. Ernst Hoehn
Swiss Federal Research Station
Dr. Wolfgang Meyerhof
Department of Molecular Genetics, German Institute of Human Nutrition, Potsdam, Rebbruecke, Nuthenal, Germany
Donna Ryland
Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Manitoba, Canada
Dr. Peng Shi
State Key Laboratory of Genetic Resources and Evolution, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, China.
Nisha Singh
Department of Oral Biology, Faculty of Dentistry, University of Manitoba, Winnipeg, Canada
Jasbir Upadhyaya
Department of Oral Biology, Faculty of Dentistry, University of Manitoba, Winnipeg, Canada
Preface
Bitterness is one of the most interesting and least studied/understood of all the human tastes. It produces aversive reactions because it was originally associated with the plant source being poisonous. In fact, it was considered a defence mechanism for avoiding the ingestion of such harmful substances so that early human survival was based on the knowledge and ability to discriminate between edible plants particularly those with potentially harmful effects. With the advent of modern technology our understanding of bitterness is far more sophisticated and that we now know that not all bitter compounds are poisonous. In fact there are many foods in which bitterness is quite acceptable such as in some cheeses and beverages. In this book we have attempted to provide a comprehensive review of bitterness, from the novel genes in humans responsible for the expression of bitterness to methods used to remove or reduce bitterness in functional foods and nutraceuticals.
The book is organized into four sections. The first section covers the biology of bitterness perception with chapter 1 discussing the biochemistry of the 25 human bitter taste receptors of the TAS2R gene family. Chapter 2 examines the physiological aspects of bitterness while chapter 3 discusses human bitterness from an evolutionary perspective.
Section II covers the chemistry of bitterness with chapter 4 detailing those secondary plant metabolites responsible for the bitterness of selected fruits and vegetables. The compounds responsible for the bitterness of such beverages as tea, coffee, cocoa, wine and cider are reviewed in chapter 5, whereas ‘food protein-derived bitter peptides’ is the subject of chapter 6.
The analysis of bitterness, both sensory and chemical, is detailed in section III. Chapter 7 is a comprehensive review of sensory methods for assessing the bitterness of foods and beverages while chapter 8 is focused on the application of mass spectrometry for identifying bitter compounds. The final chapter in this section, chapter 9, discusses the ability of the electronic tongue to analyze bitterness and its correlation with sensory analysis.
The final section, section IV, covers the physical and chemical methods available for removing or masking bitterness in functional foods and nutraceuticals. The recent development of bitter blockers is also discussed as it provides a healthy alternative to adding sugar or salt for masking bitterness.
We hope this book will provide useful information to food scientists as well as those working in the food and flavor industries. We are grateful to colleagues from around the world for their important contributions to this book and acknowledge the excellent editorial assistance provided by the staff of Wiley.
Michel Aliani and Michael N. A. Eskin
Section I
The Biology of Bitterness Perception
Chapter 1Biochemistry of Human Bitter Taste Receptors
Jasbir Upadhyaya, Nisha Singh, Raj Bhullar and Prashen Chelikani
1.1 Introduction
The gustatory system has been selected during evolution to detect nutritive and beneficial compounds as well as harmful substances. Humans, and probably other mammals, can taste many compounds but distinguish between five basic tastes which are sweet, bitter, sour, salt and umami. Sour and salt tastes are thought to be perceived via cation channels (Heck et al., 1984; Kinnamon et al., 1988; Ugawa et al., 1998). In contrast, sensation of bitter, sweet and umami tastes is initiated by the interaction of taste molecules with G protein-coupled receptors (GPCRs) (Adler et al., 2000; Gilbertson et al., 2000; Sainz et al., 2001). Bitter taste, among all tastes, is believed to have evolved as a central warning signal against the ingestion of potentially toxic substances. The molecular events in the perception of taste start at the apical surface of taste receptor cells (TRCs) found in taste buds in the mouth. Taste buds are found in taste papillae located on the tongue, the palate, and to a lesser extent the epiglottis, pharynx and larynx, and each taste bud is formed of 50-100 TRCs (Lalonde and Eglitis, 1961; Miller, 1986; Brouwer and Wiersma, 1978). The interaction of tastants with taste receptors, located in the membrane of TRCs, initiates signaling cascades which are transmitted to the brain through sensory afferents and perceived as taste (Chen et al., 2011).
1.2 Bitter Taste Receptors: T2Rs
In humans, bitter taste is perceived by 25 members of the GPCR superfamily, referred to as T2Rs, which are 291 to 334 amino acids long (Adler et al., 2000, Chandrashekar et al., 2000, Matsunami et al., 2000). These taste receptors, discovered a little more than a decade ago, encode for intronless genes which are referred to as TAS2Rs. The HUGO gene nomenclature of TAS2R is used wherever the gene is mentioned. Except for the TAS2R1 gene, which is localized on chromosome 5p, all other TAS2Rs are organized in the genome in clusters on human chromosomes 7q and 12p, and are genetically linked to loci that influence bitter perception (Conte et al., 2002). Additionally, there are a large number of TAS2R pseudogenes and more than 80 single nucleotide polymorphisms (SNPs) among individual TAS2R genes (Conte et al., 2002; Kim et al., 2005). The classification of T2Rs within the GPCR family is unclear, with some describing them as a separate family (Horn et al., 2003), whereas other classification systems have grouped them with the frizzled receptors (Fredriksson et al., 2003). The International Union of Basic and Clinical Pharmacology (IUPHAR) list Frizzled receptors as a separate GPCR family, Class F, and this class does not include T2Rs (Sharman et al., 2013). T2Rs are relatively divergent, showing ∼25–90% amino acid identity (Adler et al., 2000; Matsunami et al., 2000). This variability corresponds well with an ability to interact with chemically diverse ligands associated with bitter tastes. A single bitter compound is capable of activating multiple T2Rs and each T2R can be activated by multiple bitter compounds (Meyerhof et al., 2010). Like all GPCRs, T2Rs contain seven transmembranes (TMs), three extracellular loops (ECLs) and three intracellular loops (ICLs), with a short extracellular N- and an intracellular C-terminus (Fig. 1.1). The other class of taste GPCRs, which codes for sweet and umami receptors (T1Rs), belongs to the class C GPCR family (Lagerstrom and Schioth, 2008). Sweet and umami tastes are mediated by three GPCRs that combine to form two heterodimeric receptors, T1R1/T1R3 for umami and T1R2/T1R3 for sweet-tasting compounds (Li et al., 2002; Nelson et al., 2001, 2002; Zhao et al., 2003). In contrast to the short N-terminus of T2Rs, T1Rs are characterized by a long N-terminus, also known as Venus flytrap, which forms the primary or orthosteric ligand binding site (Pin et al., 2003). Differences in ligand specificity between species has been reported for the sweet and umami receptors (Xu et al., 2004; Li et al., 2002; Nelson et al., 2002). Human T1R1/T1R3 specifically responds to L-Glu, whereas mouse T1R1/T1R3 responds more strongly to other L-amino acids than to L-Glu. In a recent study, the residues in the extracellular Venus flytrap domain of T1R1 which are crucial for amino acid recognition in the human- and mouse-type responses were identified (Toda et al., 2013). In contrast to the low amino acid identity in the N- and C-termini and the ECLs, sequence conservation is more in the TMs and ICLs of T2Rs. The TMs and ECLs are the predicted regions of ligand binding in T2Rs and ICLs are the regions for G-protein interaction (Adler et al., 2000).
Figure 1.1 Predicted secondary structure model of the bitter taste receptor T2R4. The coding region is 299 amino acids long, has a short extracellular N-terminus, three extracellular loops, seven transmembrane (TM1-TM7) helices, three intracellular loops and a short C-terminus.
1.3 T2R Signal Transduction
Long before the discovery of T2Rs, the involvement of taste-specific Gα protein, Gα-gustducin, in bitter receptor mediated transduction mechanism was demonstrated (Wong et al., 1996). The generation of α-gustducin knock-out mice resulted in dramatic reduction of their bitter tasting abilities. Moreover, T2Rs were shown to functionally couple to transducin (He et al., 2002) in vivo as well as to other Gi/Go proteins in vitro (Ozeck et al., 2004). The mechanism involved in the perception of bitter taste and the second messengers or other downstream components of T2R signaling pathway were also known before the T2Rs were discovered in 2000 (Kurihara et al., 1994; Spielman et al., 1996; Chandrashekar et al., 2000). A cation channel, transient receptor potential melastatin subtype 5 channel (TRPM5), was found coexpressed with other taste signaling molecules in taste tissue (Perez et al., 2002).
The canonical T2R signal transduction pathway is described below. The binding of a bitter-tasting compound, also referred to as an agonist, on the extracellular surface of a T2R causes conformational changes in the receptor, and this in turn activates the heterotrimeric G-protein complex, α-gustducin, β1/3 and γ13 on the intracellular surface of the receptor. The βγ-subunits activate the enzyme phospholipase Cβ2 (PLC β2) which hydrolyzes inositol phospholipid (PIP2) resulting in the production of 1,4,5-triphosphate (IP3) and diacylglycerol (DAG). Generation of IP3 activates IP3 receptors on the membrane of endoplasmic reticulum (ER), thus opening the calcium release channels and causing transient increase in intracellular calcium. This opens the monovalent selective TRPM5 channels, leading to sodium influx, membrane depolarization and thus release of ATP as a neurotransmitter to activate the gustatory afferents (Finger et al., 2005) (Fig. 1.2). Gα-gustducin activates phosphodiesterases (PDEs) which lead to a reduction in cAMP production (McLaughlin et al., 1992; Spielman, 1998).
Figure 1.2 Bitter taste signaling pathway (IP3 pathway) Abbreviations: PLCβ2, phospholipase C β2; PIP2, phospatidyl-inositol-biphosphate; DAG, diacylglycerol; IP3, inositol triphosphate; ER, endoplasmic reticulum; Ca2+, calcium; Na+, sodium.
1.4 Bitter Taste Perception and T2R Polymorphisms
The sensitivity of humans to the perception of some bitter compounds varies greatly (Bartoshuk, 2000a, 2000b). This variable bitter taste perception is the best-known example of genetic variation in oral sensation. A vast number of structurally diverse compounds elicit bitter taste in humans and many bitter substances can be detected at concentrations roughly 1000-fold lower than substances that stimulate other basic tastes (Meyerhof, 2005). Studies on the genetics of taste perception for phenylthiocarbamide (PTC) began in the early 1930s with the accidental finding by A. L. Fox that crystals of PTC tasted very bitter to some people but not to others (Fox, 1932). Thus, 6-n-propyl-2-thiouracil (PROP) and PTC, which share thiocyanate (NC=S) moiety, taste bitter to some people but are tasteless to others (Fox, 1932).
Sensitivity to PTC/PROP is an inherited trait, and PROP sensitivity was linked with lower acceptability of other bitter compounds and lower reported liking for some bitter foods. Based on the detection thresholds for PTC/PROP solutions, people were categorized into supertasters, tasters and non-tasters. Similarly, inbred mouse strains differ in their ability to detect certain bitter taste stimuli, such as sucrose octaacetate (SOA) and cycloheximide. Genetic studies in humans have demonstrated that the ability to detect PROP is determined by a locus on chromosome 5p15 (Reed et al., 1999).
How humans respond to different bitter tasting compounds is an important question in the field of bitter taste research. Missense mutations were found in the sequences of T2R5 in mouse strains deficient to cycloheximide sensitivity (Chandrashekar et al., 2000). These genetic variants, found in bitter-insensitive mouse strains, also were less responsive in cell-based assays compared with alleles from bitter-sensitive strains. This demonstrated that alleles of a taste receptor can change both behavioral and cellular responses to bitter compounds. A similar discovery was made in humans when naturally occurring alleles of the TAS2R38 gene, which is localized to chromosome 7q, were reported to be responsible for individual differences in the ability of humans to taste PTC and PROP (Mennella et al., 2005). Three polymorphic variants in T2R38 (proline or alanine at position 49, alanine or valine at position 262, and valine or isoleucine at position 296) gave rise to five common haplotypes that accounted for 55-85% of the variance in PTC sensitivity (Bufe et al., 2005). The taster haplotype or PROP-sensitive individuals possess one or two dominant alleles (proline-alanine-valine; PAV/PAV), or PAV/AVI (alanine-valine-isoleucine), whereas insensitive individuals are recessive for the trait, AVI/AVI (Bufe et al., 2005). The ability to taste PTC/PROP may protect against cigarette smoking and has also been linked to decreased alcohol consumption (Cannon et al., 2005; Duffy et al., 2004).
Until recently, TAS2R38 was considered the only bitter taste gene that exhibits prominent phenotypic variation in humans. But variation in bitter receptor sequence is not confined to the TAS2R38 locus. Human TAS2Rs have more genetic variation within and between populations than do most other genes (Kim et al., 2005). One possible explanation is that genes adapt to local conditions especially to the bitter toxins in food. SNPs in some other TAS2R genes have recently been identified. For example, a missense mutation in the TAS2R16 gene, which encodes the β-glucopyranoside receptor or T2R16, reduces sensitivity of the receptor to bitter-taste stimuli which has been associated with risk for alcohol dependence (Bufe et al., 2002; Wang et al., 2007). Polymorphism in the TAS2R43 gene allele makes people very sensitive to bitterness of the natural plant compounds aloin and aristolochic acid (Pronin et al., 2007). TAS2R43 and TAS2R44 gene alleles are also related to the bitterness perception of artificial sweetener, saccharin. Recently an SNP in the cluster of T2Rs on chromosome 12, which contributes to the variation in human bitterness perception of caffeine, was identified (Ledda et al., 2014). Thus, it seems likely that the examination of multiple taste phenotypes might provide a more complete understanding of human eating behavior than a single taste phenotype.
1.5 Ligand Binding and Activation Mechanisms of T2Rs
Bitter compounds are not only numerous but also structurally diverse. They include hydroxy fatty acids, peptides, amino acids, amines, N-heterocyclic compounds, ureas, thioureas, carbamides, esters, lactones, phenols, alkaloids, glycosides and many more. In contrast, only 25 T2Rs have been identified, raising the question as to how the vast array of bitter compounds can be detected by such a limited number of receptors. While many T2Rs remain poorly characterized, the ligand specificity of several T2Rs was explored in the past decade (Chandrashekar et al., 2000; Bufe et al., 2002, Kim et al., 2003; Behrens et al., 2004; Kuhn et al., 2004; Pronin et al., 2004; Brockhoff et al., 2007; Sainz et al., 2007; Dotson et al., 2008; Maehashi et al., 2008; Upadhyaya et al., 2010; Meyerhof et al., 2010). Whereas some receptors recognize only a single or few compounds, others respond to multiple compounds. The affinity of T2Rs for their respective bitter ligands is low, with EC50 values in the high micromolar to low millimolar range (Meyerhof et al., 2010). Thus, bitter compounds activate various T2Rs in different concentration ranges, differences usually being in the range of 10- to 100-fold. However, knowledge of the structural determinants of T2Rs is crucial to provide insights into the molecular basis of bitter sensing and to design new taste modifiers. Molecular modeling and site-directed mutagenesis studies were pursued to characterize the ligand-binding pocket of some T2Rs. The 3D structure of T2R38, also referred to as the PTC receptor, was predicted using computational method MembStruk and homology modeling (Floriano et al., 2006). Hierdock and ScanBindSite computational tools were then used to generate models of PTC bound to T2R38 to predict the binding site and binding energy. According to these models, PTC binds at a site distant from the variant amino acids P49A, A262V and V296I (Floriano et al., 2006). It is also suggested that the inability of humans to taste PTC is due to a failure of G-protein activation rather than decreased binding affinity of the receptor for PTC. This study emphasizes the role of TM6 and TM7 in PTC receptor function. The introduction of bulkier side chains in the nontaster variant alters the packing of TMs 6 and 7, which might render the movement of TM6 more difficult (Biarnes et al., 2010). A recent study predicted the 3D structure of T2R38 using BiHelix and SuperBiHelix Monte Carlo methods (Tan et al., 2012). This study suggests that the residue 262 is involved in interhelical hydrogen bond network which stabilizes the receptor in tasters (hTAS2R38PAV, hTAS2R38AAI, and hTAS2R38PVV), but not in the non-tasters (hTAS2R38AVI) (Tan et al., 2012).
In a study by Pronin et al., chimeric receptors for T2R43 and T2R44 were generated in an effort to identify the residues involved in ligand recognition (Pronin et al., 2004). T2R43 is activated by 6-nitrosaccharin and N-isopropyl-2-methyl-5-nitrobenzenesulfonamide (IMNB), a bitter derivative of saccharin. Whereas, T2R44 is activated by denatonium and 6-nitrosaccharin. The amino acid sequences of T2R43 and T2R44 are 89% identical and 15 of the 34 amino acid differences among them are concentrated in ECL1 and ECL2, while ECL3 is completely conserved. T2R43 and T2R44 chimeras were generated by swapping their ECLs–1 and –2. There are only four amino acid differences between T2R43 and T2R44 in ECL1. Functional studies revealed that ECL1 is very important for receptor activation, as replacing these residues of T2R43 with those of T2R44 is sufficient to render T2R43 insensitive to IMNB. On the other hand, replacing both ECL1 and ECL2 in T2R43 with T2R44 loops eliminated most of the activation by 6-nitrosaccharin. Recently, ligand docking simulations and functional analysis using point mutants of T2R16 were performed to identify binding sites of the receptor to β-glucopyranosides (Sakurai et al., 2010). Seven amino acid residues in TMs 3, 5 and 6 were involved in ligand recognition. Amino acid residues Glu86, Trp94 and Gln177 were involved in salicin recognition, whereas His181 and hydrophobic residues, Phe93, Phe240 and Ile243 likely contributed to formation of the binding site. With the generation of chimeric and mutant receptors, followed by functional analysis, the amino acid residues critical for the activation of T2R46, T2R43 and T2R31 were identified (Brockhoff et al., 2010). The construction of receptor chimeras demonstrated that agonist selectivity was predominantly determined by TM7 region of the receptors. Exchange of two residues within TM7 between T2R46, activated by strychnine, and T2R31, activated by aristolochic acid, was sufficient to invert the agonist selectivity.
Fermentation of protein-rich foods results in the formation of bitter peptides, which are responsible for the bitter taste of fermented food. Bitter casein digests were able to activate T2R1, T2R4, T2R14 and T2R16 in a heterologous expression system (Maehashi et al., 2008). Two bitter dipeptides, Gly-Phe (glycine-phenylalanine) and Gly-Leu (glycine-leucine), activated T2R1 more strongly, whereas they evoked no or weak responses in other receptors. The ability of bitter di- and tri-peptides to activate T2R1 was tested further (Upadhyaya et al., 2010). Results revealed that bitter tri-peptides also activated T2R1 and were more potent than the tested di-peptides. Among all the tested peptides, Phe-Phe-Phe (phenylalanine-phenylalanine-phenylalanine) activated T2R1-expressing cells the most, at concentrations of 0.125–1 mM that humans also perceive as bitter, with an EC50 value of 370 μM. Phe-Phe-Phe consists of hydrophobic amino acids and the bitter taste of a peptide is more apparent when the hydrophobic amino acid is located at the C-terminus. For the tri-peptides, the middle amino acid residue is considered more important than both the C- and N-terminal amino acids (Wu and Aluko, 2007). In addition, some peptides with ACE (angiotensin-converting enzyme)-inhibitory activity were also able to activate T2R1. Homology modeling and docking studies showed that amino acid residues from TMs 1-3, TM 7 and from ECL1 and ECL2 contributed in forming the ligand binding pocket of T2R1 for the peptide ligands (Upadhyaya et al., 2010). In another study of T2R1, molecular modeling and site-directed mutagenesis studies revealed that two asparagines, Asn66 and the highly conserved Asn24, are important for dextromethorphan (DXM)-induced receptor signaling (Singh et al., 2011). Asn24 plays a crucial role in receptor activation by mediating a hydrogen-bond network connecting TM1-TM2-TM7, whereas Asn66 is essential for bonding to DXM. There is a unique signature sequence of T2Rs, the LXXSL motif. It plays a predominantly structural role in stabilizing the helical conformation of TM5 at the cytoplasmic end and a functional role by influencing the conformation of ICL3. Replacement of the conserved residues in this motif with bulky β-branched amino acids results in protein misfolding and/or non-functional receptor (Singh et al., 2011).
Recently, the role of ICL3 in quinine-mediated activation of bitter taste receptor T2R4 was demonstrated using alanine scan mutagenesis (Pydi et al., 2013). ICL3 of T2R4 consists of 23 amino acid residues which were mutated to alanine. Only 14 of the 23 mutants displayed quinine-induced signaling in a concentration-dependent manner. Three mutants, Q216A, T230A and V234A, showed an increased response to quinine. Six mutants, R213A, Q219A, K220A, Q229A, E231A and H233A, showed no detectable or statistically significant increase in intracellular calcium mobilization, suggesting that they may have an important role in receptor activation. Whereas mutants I215A, F225A and P228A displayed altered receptor activation and/or defective ligand binding. Some mutants showed statistically significant basal signaling or constitutive activity. H214A, which is present in 24 of the 25 human T2Rs, showed the highest constitutive activity (i.e., in the absence of agonist). A recent study identified a conserved KLK/R motif in the C-terminus of T2Rs. This KLK motif was suggested to perform a critical functional role involving trafficking and activation in T2R4 (Upadhyaya et al., 2015).
1.6 Nutrigenomics of Taste
The PROP phenotype serves as a general marker for bitterness perception which influences general food preferences and dietary behavior with subsequent links to body weight and chronic disease risk. Strong bitter taste is closely associated with the presence of toxins and is aversive. However, moderate bitter taste is appealing and expected in a variety of foods including beer, wine, chocolates and many cheeses. Fischer and colleagues noted that PTC tasters tended to manifest a thin and angular body type, whereas non-tasters tended to have generous body proportions (Fischer et al., 1966). Studies in overweight middle-aged women have provided convincing evidence linking PROP status with body weight (Goldstein et al., 2005). Goldstein et al. showed that non-taster women were heavier than supertaster women by ∼6 BMI (body mass index) units. Anatomical evidence demonstrates that individuals who differ in taste sensitivity to PTC/PROP also differ in the density of fungiform taste papillae on the anterior surface of tongue (Bartoshuk et al., 1994; Essick et al., 2003; Tepper and Nurse, 1997). Non-tasters have the lowest density of fungiform papillae, whereas supertasters have the highest density.
Isothiocyanates, the breakdown products of glucosinolates that are widely distributed in plants, interfere with the uptake of iodine by the thyroid gland, leading to goiter, and cretinism in its extreme form. Although iodine deficiency is the primary cause of this disease, goitrogens in the food supply can play a contributing role particularly when dietary iodine is low. It was shown that a large percentage of athyroidic cretins in a clinical population in the United States were PTC non-tasters (Shepard, 1961). Investigation of the role of PROP status in children's selection and consumption of vegetables showed that non-taster children consumed more bitter vegetables overall than taster children (Bell and Tepper, 2006). PROP status has also been linked to sweet taste preference in children. Taster preschool children showed greater preferences for sweets than non-taster children (Keller and Tepper, 2004). The perception of oral irritation from capsaicin (chili pepper), cinnamaldehyde (from cinnamon), and carbonation is influenced by PROP sensitivity (Karrer and Bartoshuk, 1991; Prescott and Swain-Campbell, 2000; Prescott et al., 2004). Individual differences in fat perception have been linked to PROP taster status and taste bud density. A study in college students revealed that medium and supertasters reliably discriminated a high-fat from a low-fat dressing, whereas non-tasters could not distinguish the two samples (Tepper and Nurse, 1997). Study by Keller and coworkers in preschool children demonstrated that this phenotype might have greater influence on preferences for fats in females than males (Keller et al., 2002). Discretionary fat intake did not differ between taster and non-taster boys.
Few studies have examined associations between PROP status and disease risk, though the data addressing this issue is scarce. No associations were reported between T2R38 polymorphisms and cardiovascular risk in the elderly women or between PROP status and lipid profiles in the breast cancer patients (Timpson et al., 2005; Drewnowski et al., 2007). However, a modest association between greater sensitivity to PROP and a higher number of colon polyps was found in older men undergoing routine screening for colon pathology (Basson et al., 2005).
Dental caries is the most common chronic disease of childhood that is neither self-limiting nor amenable to short-term pharmacological management (Edelstein and Douglass, 1995). Effective dentistry requires early identification of children at higher risk for caries so they may receive early and intense preventive intervention. The individual differences in PROP sensitivity have been linked to dental caries and can be used as an important tool to determine the taster status in relation to caries experience in children (Rupesh and Nayak, 2006; Verma et al., 2006; Pidamale et al., 2012; Hedge and Sharma, 2008). A comprehensive review of the role of diet and dental caries reaffirmed that sucrose is the most important dietary item associated with dental caries (Habibian et al., 2001). Non-taster children may have higher concentration and frequencies of sugar intake compared to children who are medium or supertasters and are therefore more susceptible to dental caries (Anliker et al., 1991). Whereas supertasters and medium tasters are more likely to avoid sweet food, thus making them less prone to dental decay. Streptococcus mutans levels were also shown to increase from tasters to non-tasters, thus placing them at higher risk of developing caries (Verma et al., 2006).
1.7 Bitter Taste Blockers
The sense of taste has a significant impact on food selection, nutrition and health. It is, therefore, highly desirable to modulate bitter taste perception and bitter taste receptors so that beneficial food and medicines may be rendered more palatable. In addition to having an important role in food and nutraceutical industries, bitter taste blockers could be beneficial as chemical probes to examine the role of T2R function in gustatory and non-gustatory tissues. The T2R antagonist, GIV3727, was able to inhibit the activation of T2R44 by saccharin and acesulfame-K (Slack et al., 2010). This compound also inhibited five additional T2Rs, including the closely related T2R43. It appears the –COOH moiety is essential for antagonist activity of GIV3727 since replacement of this group with an ester or corresponding alcohol abolished its activity. Two residues in TM7 are important for antagonist activity in T2R43/T2R44. Shortly after this study, probenecid, an approved inhibitor of Multidrug Resistance Protein 1 (MRP1) transporter, was shown to inhibit T2R16, T2R38 and T2R43 in a non-competetive (allosteric) mechanism (Greene et al., 2011). And, two natural sesquiterpene lactones from edible plants, 3β-Hydroxydihydrocostunolide (3HDC) and 3β-Hydroxypelenolide (3HP), were identified which blocked the responses of T2R46 receptor (Brockhoff et al., 2011). Besides T2R46, 3HDC also inhibited T2R30 and T2R40, and 3HP inhibited T2R30, T2R43 and T2R44. Recent studies characterized few novel bitter blockers, γ-aminobutyric acid (GABA), abscisic acid and Na,Na-bis(carboxymethyl)-L-lysine (BCML). These acted as competitive inhibitors of quinine-activated human T2R4, sharing the same orthosteric site as agonist quinine (Pydi et al., 2014, Pydi et al., 2015). Though there is a vast number of bitter agonists known for T2Rs, the knowledge of bitter taste blockers or T2R antagonists and inverse agonists is limited. Hence there is an urgent need to discover more natural or synthetic blockers for T2Rs to increase the consumption of healthy bitter foods and for drug compliance.
1.8 Expression of T2Rs in Extraoral tissues
With the molecular identification of taste GPCRs, it has become clear that taste signaling is not limited to taste buds, but occurs in many extraoral tissues and has additional functions apart from taste. Shortly after the discovery of T2Rs in taste tissue, their expression was demonstrated in the gastrointestinal tract (GIT) and enteroendocrine STC-1 cells of rodents and humans (Wu et al., 2002; Rozengurt, 2006), where they are involved in the chemosensation of nutrients. Gα-gustducin and Gα-transducin were also expressed in these tissues, suggesting that a taste-sensing mechanism may also exist in the GIT. Addition of bitter compounds like denatonium, PTC, PROP, caffeine and cycloheximide to STC-1 cell cultures promoted rapid [Ca2+]i responses (Wu et al., 2002; Masuho et al., 2005). In addition, activation of T2Rs stimulated the secretion of hunger hormone ghrelin (Janssen et al., 2011) via the gustatory G-protein, α-gustducin.
In the airway epithelium, expression of T2Rs was revealed in chemosensory receptor cells of the nasal epithelium and in ciliated epithelial cells (Shah et al., 2009; Tizzano et al., 2011; Masuho et al., 2005). Application of bitter substances to the nasal epithelium activated the trigeminal nerve and elicited protective reflexes like apnea to prevent inhalation of bacteria further into the respiratory system or sneezing and coughing to expel bacteria from the nasal cavity. Exposure of T2Rs in motile cilia of human airway epithelial cells with bitter compounds stimulated ciliary beat frequency (Shah et al., 2009), thus initiating a defensive mechanical mechanism to eliminate the offending compound. In the human airway smooth muscle (ASM), T2Rs lead to ASM relaxation and bronchodilation (Deshpande et al., 2010). Bitter tastants like denatonium, saccharin and chloroquine caused relaxation of mouse isolated ASM preparations, and dilation of airways that was three-fold greater than the presently used β-agonists. This relaxation by T2Rs was due to increased [Ca2+]i that was suggested to activate large conductance potassium channels (BKCa) and result in hyperpolarization of the cell membrane. Additional studies showed that bronchodilatory effects of T2R agonists were not impeded by β2-AR desensitization (An et al., 2012). These findings have reinforced the role of T2Rs as potential novel targets in asthma pharmacotherapy. Expression of the 25 human TAS2Rs was revealed in the pulmonary artery smooth muscle cells where they are functional and activated by bitter compounds (Upadhyaya et al., 2014). This study suggests that T2Rs in the vasculature might be involved in regulating the vascular tone (Upadhyaya et al., 2014).
Regulation of the mucosal innate defense of human and mouse upper respiratory epithelium by activation of T2R38 was recently demonstrated (Lee et al., 2012, 2014). Gram-negative respiratory pathogens like Pseudomonas aeruginosa produce acyl-homoserine lactones (AHLs) as signals for their population density (quorum sensing). AHLs are chemically related to bitter sesquiterpene lactones and activate T2R38 in upper respiratory epithelium. Receptor activation causes calcium and nitric oxide (NO) signaling resulting in stimulation of mucociliary clearance, the major physical respiratory defense against inhaled pathogens. Genetic variation in T2R38 has also been linked to individual differences in susceptibility to respiratory infection.
1.9 Conclusion
With the deorphanization of T2Rs, studies of the mechanisms of their interaction with bitter agonists have started revealing how these receptors are able to sense such a vast array of bitter compounds. Knowledge of their ligand bound structure would further help in the identification or design of taste modulators like bitter blockers. T2R blockers could have widespread utility in antioxidant and/or nutrient-fortified food and beverages, and in pharmaceutical and nutraceutical industry.
Elucidation of the biochemistry of bitter taste signal transduction plays an important role in understanding how humans perceive bitter taste. The next step includes deciphering how the taste signal is terminated. A study by Robinette et al. has demonstrated a 30% desensitization of T2R function with quinine pretreatment and subsequent exposure in airway smooth muscle (Robinett et al., 2011). Another study, using molecular and pharmacological techniques, showed that T2R4 does not get internalized upon agonist exposure (Upadhyaya et al., 2016). Instead, treatment with bitter compound quinine caused a two-fold increase in surface expression of T2R4 which was Brefeldin A-sensitive. Quinine pretreatment led to a reduction in subsequent calcium responses to 35 ± 5% compared to the control untreated cells. This study thus, discovered a novel pharmacochaperone role of quinine and provides insights into the possible mechanism of T2R desensitization (Upadhyaya et al., 2016). However, data of T2R desensitization is very scarce, and the potential molecular mechanisms involved in desensitization like receptor internalization, phosphorylation by the respective kinases, β-arrestin binding, leading to uncoupling of receptor-G protein complex, remain poorly characterized. Before the introduction of T2Rs as novel therapeutic targets, it is very crucial that their desensitization mechanisms be probed in detail. T2Rs have a low affinity for their respective ligands. In recent studies, T2R agonists were used at a concentration 50-100 times higher than β-agonists (An et al., 2012; Pulkkinen et al., 2012). Thus, elucidation of the signaling mechanisms utilized downstream of T2Rs, may allow the synthesis of more specific and potent bitter compounds and/or blockers.
Acknowledgement
This work was supported by a Discovery grant from the Natural Sciences and Engineering Research Council of Canada (RGPIN 356285) and the MMSF Allen Rouse Career Award to PC.
References
Adler, E., Hoon, M. A., Mueller, K. L., Chandrashekar, J., Ryba, N. J. & Zuker, C. S. (2000). A novel family of mammalian taste receptors.
Cell
100,
693–702.
An, S. S., Wang, W. C., Koziol-White, C. J., Ahn, K., Lee, D. Y., Kurten, R. C., Panettieri, R. A., JR . & Liggett, S. B. (2012). TAS2R activation promotes airway smooth muscle relaxation despite beta(2)-adrenergic receptor tachyphylaxis.
Am J Physiol Lung Cell Mol Physiol
303,
L304–L311.
Anliker, J. A., Bartoshuk, L., Ferris, A. M. & Hooks, L. D. (1991). Children's food preferences and genetic sensitivity to the bitter taste of 6-n-propylthiouracil (PROP).
Am J Clin Nutr
54,
316–320.
Bartoshuk, L. M. (2000a). Comparing sensory experiences across individuals: recent psychophysical advances illuminate genetic variation in taste perception.
Chem Senses
25,
447–460.
Bartoshuk, L. M. (2000b). Psychophysical advances aid the study of genetic variation in taste.
Appetite
34,
105.
Bartoshuk, L. M., Duffy, V. B. & Miller, I. J. (1994). PTC/PROP tasting: anatomy, psychophysics, and sex effects.
Physiol Behav
56,
1165–1171.
Basson, M. D., Bartoshuk, L. M., Dichello, S. Z., Panzini, L., Weiffenbach, J. M. & Duffy, V. B. (2005). Association between 6-n-propylthiouracil (PROP) bitterness and colonic neoplasms.
Dig Dis Sci
50,
483–489.
Behrens, M., Brockhoff, A., Kuhn, C., Bufe, B., Winnig, M. & Meyerhof, W. (2004). The human taste receptor hTAS2R14 responds to a variety of different bitter compounds.
Biochem Biophys Res Commun
319,
479–485.
Bell, K. I. & Tepper, B. J. (2006). Short-term vegetable intake by young children classified by 6-n-propylthoiuracil bitter-taste phenotype.
Am J Clin Nutr
84,
245–251.
Biarnes, X., Marchiori, A., Giorgetti, A., Lanzara, C., Gasparini, P., Carloni, P., Born, S., Brockhoff, A., Behrens, M. & Meyerhof, W. (2010). Insights into the binding of Phenyltiocarbamide (PTC) agonist to its target human TAS2R38 bitter receptor.
PLoS One
5,
e12394.
Brockhoff, A., Behrens, M., Massarotti, A., Appendino, G. & Meyerhof, W. (2007). Broad tuning of the human bitter taste receptor hTAS2R46 to various sesquiterpene lactones, clerodane and labdane diterpenoids, strychnine, and denatonium.
J Agric Food Chem
55,
6236–6243.
Brockhoff, A., Behrens, M., Niv, M. Y. & Meyerhof, W. (2010). Structural requirements of bitter taste receptor activation.
Proc Natl Acad Sci U S A
107,
11110–11115.
Brockhoff, A., Behrens, M., Roudnitzky, N., Appendino, G., Avonto, C. & Meyerhof, W. (2011). Receptor agonism and antagonism of dietary bitter compounds.
J Neurosci
31,
14775–14782.
Brouwer, J. N. & Wiersma, A. (1978). Location of taste buds in intact taste papillae by a selective staining method.
Histochemistry
58,
145–151.
Bufe, B., Breslin, P. A., Kuhn, C., Reed, D. R., Tharp, C. D., Slack, J. P., Kim, U. K., Drayna, D. & Meyerhof, W. (2005). The molecular basis of individual differences in phenylthiocarbamide and propylthiouracil bitterness perception.
Curr Biol
15,
322–327.
Bufe, B., Hofmann, T., Krautwurst, D., Raguse, J. D. & Meyerhof, W. (2002). The human TAS2R16 receptor mediates bitter taste in response to beta-glucopyranosides.
Nat Genet
32,
397–401.
Cannon, D. S., Baker, T. B., Piper, M. E., Scholand, M. B., Lawrence, D. L., Drayna, D. T., Mcmahon, W. M., Villegas, G. M., Caton, T. C., Coon, H. & Leppert, M. F. (2005). Associations between phenylthiocarbamide gene polymorphisms and cigarette smoking.
Nicotine Tob Res
7,
853–858.
Chandrashekar, J., Mueller, K. L., Hoon, M. A., Adler, E., Feng, L., Guo, W., Zuker, C. S. & Ryba, N. J. (2000). T2Rs function as bitter taste receptors.
Cell
100,
703–711.
Chen, X., Gabitto, M., Peng, Y., Ryba, N. J. & Zuker, C. S. (2011). A gustotopic map of taste qualities in the mammalian brain.
Science
333,
1262–1266.
Conte, C., Ebeling, M., Marcuz, A., Nef, P. & Andres-Barquin, P. J. (2002). Identification and characterization of human taste receptor genes belonging to the TAS2R family.
Cytogenet Genome Res
98,
45–53.
Deshpande, D. A., Wang, W. C., Mcilmoyle, E. L., Robinett, K. S., Schillinger, R. M., An, S. S., Sham, J. S. & Liggett, S. B. (2010). Bitter taste receptors on airway smooth muscle bronchodilate by localized calcium signaling and reverse obstruction.
Nat Med
16,
1299–1304.
Dotson, C. D., Zhang, L., Xu, H., Shin, Y. K., Vigues, S., Ott, S. H., Elson, A. E., Choi, H. J., Shaw, H., Egan, J. M., Mitchell, B. D., Li, X., Steinle, N. I. & Munger, S. D. (2008). Bitter taste receptors influence glucose homeostasis.
PLoS One
3,
e3974.
Drewnowski, A., Henderson, S. A. & Cockroft, J. E. (2007). Genetic sensitivity to 6-n-propylthiouracil has no influence on dietary patterns, body mass indexes, or plasma lipid profiles of women.
J Am Diet Assoc
107,
1340–1348.
Duffy, V. B., Davidson, A. C., Kidd, J. R., Kidd, K. K., Speed, W. C., Pakstis, A. J., Reed, D. R., Snyder, D. J. & Bartoshuk, L. M. (2004). Bitter receptor gene (TAS2R38), 6-n-propylthiouracil (PROP) bitterness and alcohol intake.
Alcohol Clin Exp Res
28,
1629–1637.
Edelstein, B. L. & Douglass, C. W. (1995). Dispelling the myth that 50 percent of U.S. schoolchildren have never had a cavity.
Public Health Rep
110,
522–530; discussion 521, 531–533.
Essick, G. K., Chopra, A., Guest, S. & Mcglone, F. (2003). Lingual tactile acuity, taste perception, and the density and diameter of fungiform papillae in female subjects.
Physiol Behav
80,
289–302.
Finger, T. E., Danilova, V., Barrows, J., Bartel, D. L., Vigers, A. J., Stone, L., Hellekant, G. & Kinnamon, S. C. (2005). ATP signaling is crucial for communication from taste buds to gustatory nerves.
Science
310,
1495–1499.
Fischer, R., Griffin, F. & Rockey, M. A. (1966). Gustatory chemoreception in man: multidisciplinary aspects and perspectives.
Perspect Biol Med
9,
549–577.
Floriano, W. B., Hall, S., Vaidehi, N., Kim, U., Drayna, D. & Goddard, W. A., 3rd (2006). Modeling the human PTC bitter-taste receptor interactions with bitter tastants.
J Mol Model
12,
931–941.
Fox, A. L. (1932). The Relationship between Chemical Constitution and Taste.
Proc Natl Acad Sci U S A
18,
115–120.
Fredriksson, R., Lagerstrom, M. C., Lundin, L. G. & Schioth, H. B. (2003). The G-protein-coupled receptors in the human genome form five main families. Phylogenetic analysis, paralogon groups, and fingerprints.
Mol Pharmacol
63,
1256–1272.
Gilbertson, T. A., Damak, S. & Margolskee, R. F. (2000). The molecular physiology of taste transduction.
Curr Opin Neurobiol
10,
519–527.
Goldstein, G. L., Daun, H. & Tepper, B. J. (2005). Adiposity in middle-aged women is associated with genetic taste blindness to 6-n-propylthiouracil.
Obes Res
13,
1017–1023.
Greene, T. A., Alarcon, S., Thomas, A., Berdougo, E., Doranz, B. J., Breslin, P. A. & Rucker, J. B. (2011). Probenecid inhibits the human bitter taste receptor TAS2R16 and suppresses bitter perception of salicin.
PLoS One
6,
e20123.
Habibian, M., Roberts, G., Lawson, M., Stevenson, R. & Harris, S. (2001). Dietary habits and dental health over the first 18 months of life.
Community Dent Oral Epidemiol
29,
239–246.
He, W., Danilova, V., Zou, S., Hellekant, G., Max, M., Margolskee, R. F. & Damak, S. (2002). Partial rescue of taste responses of alpha-gustducin null mice by transgenic expression of alpha-transducin.
Chem Senses
27,
719–727.
Heck, G. L., Mierson, S. & Desimone, J. A. (1984). Salt taste transduction occurs through an amiloride-sensitive sodium transport pathway.
Science
223,
403–405.
Hedge, A. M. & Sharma, A. (2008). Genetic sensitivity to 6-n-propylthiouracil (PROP) as a screening tool for obesity and dental caries in children.
J Clin Pediatr Dent
33,
107–111.
Horn, F., Bettler, E., Oliveira, L., Campagne, F., Cohen, F. E. & Vriend, G. (2003). GPCRDB information system for G protein-coupled receptors.
Nucleic Acids Res
31,
294–297.
Janssen, S., Laermans, J., Verhulst, P. J., Thijs, T., Tack, J. & Depoortere, I. (2011). Bitter taste receptors and alpha-gustducin regulate the secretion of ghrelin with functional effects on food intake and gastric emptying.
Proc Natl Acad Sci U S A
108,
2094–2099.
Karrer, T. & Bartoshuk, L. (1991). Capsaicin desensitization and recovery on the human tongue.
Physiol Behav
49,
757–764.
Keller, K. L., Steinmann, L., Nurse, R. J. & Tepper, B. J. (2002). Genetic taste sensitivity to 6-n-propylthiouracil influences food preference and reported intake in preschool children.
Appetite
38,
3–12.
Keller, K. L. & Tepper, B. J. (2004). Inherited taste sensitivity to 6-n-propylthiouracil in diet and body weight in children.
Obes Res
12,
904–912.
Kim, U., Wooding, S., Ricci, D., Jorde, L. B. & Drayna, D. (2005). Worldwide haplotype diversity and coding sequence variation at human bitter taste receptor loci.
Hum Mutat
26,
199–204.
Kim, U. K., Jorgenson, E., Coon, H., Leppert, M., Risch, N. & Drayna, D. (2003). Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide.
Science
299,
1221–1225.
Kinnamon, S. C., Dionne, V. E. & Beam, K. G. (1988). Apical localization of K+ channels in taste cells provides the basis for sour taste transduction.
Proc Natl Acad Sci U S A
85,
7023–7027.
