Enzymes in Fruit and Vegetable Processing: Chemistry and Engineering Applications 1420094335, 9781420094336, 9781420094343

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Table of contents :
Enzymes in Fruit and Vegetable Processing - Chemistry and Engineering Applications......Page 4
Contents......Page 6
Preface......Page 8
The Editor......Page 10
List of Contributors......Page 12
1.1 Nature of enzymes......Page 16
1.2 Enzyme classification and nomenclature......Page 17
1.3 Enzyme kinetics......Page 18
1.4 Factors affecting enzyme activity......Page 22
1.5 Enzyme inactivation......Page 25
1.6 Enzymes in fruit and vegetable processing......Page 28
References......Page 31
Contents......Page 34
2.2.1 Phenolic oxidation......Page 35
2.2.1.1 Phenolic oxidation in fruits......Page 37
2.2.1.2 Phenolic oxidation in vegetables......Page 42
2.2.2.1 Garlic greening......Page 45
2.2.2.3 Radish pickle yellowing......Page 46
2.2.3.1 Potato browning......Page 47
2.3.1.1 Blueberry browning......Page 48
2.3.2 Betalain degradation......Page 49
2.3.4 Chlorophyll degradation......Page 50
2.3.4.1 Broccoli yellowing......Page 51
2.4 Conclusions......Page 52
References......Page 53
3.1 Introduction......Page 60
3.2 Aroma volatile compounds in fruits......Page 61
3.3.1 Alcohol acetyl transferase (AAT)......Page 63
3.3.2 Lipoxygenase......Page 68
3.3.3 Alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC)......Page 70
3.3.4 Other enzymes......Page 72
3.4 Enzymes of flavor volatiles production and regulation in vegetables—carrot......Page 75
3.5 Conclusions......Page 76
Abbreviations......Page 77
References......Page 78
Contents......Page 86
4.1.1 Describing texture......Page 87
4.1.2 Textural Properties of fruits and vegetables......Page 88
4.2.1.1 Representations of the primary cell wall......Page 90
4.2.1.2 Pectic matrix......Page 92
4.2.1.3 Hemicellulose-cellulose network......Page 94
4.2.2 Changes in cell wall structure and composition......Page 95
4.2.3.1 Enzymes that act on the pectic network......Page 99
4.2.3.2 Enzymes that act on the hemicellulose-cellulose network......Page 106
4.2.3.3 Molecular, biochemical, and hormonal regulation......Page 108
4.2.4.1 Water loss......Page 111
4.2.4.2 Loss of membrane integrity......Page 112
4.2.5 Wounding and the case of fresh-cut produce......Page 113
4.3.1 Conventional breeding......Page 114
4.3.2 Genetic engineering......Page 116
4.3.3 Additives to interfere with texture-related enzymes......Page 117
4.4 Conclusion......Page 119
Abbreviations......Page 120
References......Page 121
5.1 Introduction......Page 138
5.2 Blanching systems......Page 140
5.3 Enzymes responsible for quality deterioration in vegetables......Page 141
5.4.1 Selection of blanching indicator enzyme......Page 142
5.4.2 Thermal stability of indicator enzymes......Page 144
5.4.2.1 Loglinear (monophasic) model......Page 146
5.4.2.2 Biphasic model......Page 147
5.5.1.1 Color and pigments......Page 149
5.5.1.2 Vitamins......Page 153
References......Page 156
6.1 Introduction......Page 160
6.2 Effects of morphological and physiological characteristics of citrus fruits on enzymatic peeling......Page 163
6.3 Citrus species used in enzymatic peeling studies......Page 165
6.4 Treatments prior to enzymatic peeling of citrus fruits......Page 168
6.5 Effect of the pattern of flavedo cuts on peeling efficiency......Page 170
6.6 Effect of vacuum conditions (pressure and time) on citrus fruit peeling efficiency......Page 173
6.7 Enzymatic preparations for the enzymatic peeling of citrus fruits......Page 177
6.8 Influence of temperature and pH on enzymatic peeling......Page 182
6.9 Reuse of the enzyme preparation in an industrial peeling process......Page 184
6.10 Conclusions......Page 186
References......Page 187
7.1 Introduction......Page 190
7.1.1 Non-citrus fruit juice production: World market......Page 191
7.2.1 Extraction methods......Page 192
7.2.2 Clarification and fining......Page 194
7.3 Enzymes in the non-citrus fruit industry......Page 196
7.3.1 Mash enzymatic treatment......Page 197
7.4.1 Temperature dependence on the pectic enzyme activities......Page 198
7.4.2 pH dependence on the pectic enzymes activities......Page 199
7.4.3 Enzymatic hydrolysis of starch in fruit juices......Page 201
7.4.4 pH and temperature dependences on amylases activities......Page 203
7.5.2 Application of immobilized enzymes in fruit juice ultrafiltration......Page 204
7.6 Conclusions......Page 207
References......Page 208
8.1 Introduction......Page 212
8.2 Overview of citrus processing technologies......Page 213
8.3 Pectin methylesterase (PME) in citrus juice......Page 216
8.4 Kinetic parameters of PME thermal inactivation in citrus juices......Page 219
8.5 Clear citrus juice processing......Page 222
8.7 Conclusions......Page 225
References......Page 226
9.1 Introduction......Page 230
9.2 Use of pectinases in winemaking......Page 231
9.2.1 The increase of yield......Page 232
9.2.2 Clarification and filterability of musts and wines......Page 233
9.2.3 Maceration of grapes for red wine vinification......Page 235
9.3 β-Glucosidases......Page 240
9.4 The presence of cinnamyl esterase activity in enzyme preparations......Page 242
9.5 Glucanases......Page 246
9.6 Ureases......Page 247
9.7 Utilization of lysozyme......Page 248
9.8 Conclusions......Page 250
Abbreviations......Page 251
References......Page 252
10.1 Introduction......Page 260
10.2.1 High hydrostatic pressure (HP) processing......Page 262
10.2.2 High-intensity pulsed electric field (PEF) processing......Page 263
10.3.1 Understanding of HP processing effect on stability of enzyme as a protein molecule......Page 264
10.3.2 Effect of HP processing on stability of fruit and vegetable enzymes......Page 267
10.3.3 Understanding of pressure effect on enzymatic reactions in fruit and vegetables......Page 287
10.3.3.1 Case study on texture improvement of fruit and vegetables under pressure......Page 295
10.3.3.2 Case study on health benefit enhancement of Brassicaceae vegetables under pressure......Page 298
10.4.1 Understanding of PEF processing effect on stability of enzyme as a protein molecule......Page 299
10.4.2 Effect of PEF on stability of fruit and vegetable enzymes......Page 300
References......Page 320
11.1 Introduction......Page 328
11.2.1 Biosensor recognition elements......Page 330
11.2.2 Immobilization procedures......Page 331
11.2.3 Transducers......Page 334
11.3 Use of biosensors as analytical tools for fruit and vegetable processing......Page 337
Abbreviations......Page 348
References......Page 349
Contents......Page 356
12.1 Introduction......Page 357
12.2.1 Genome sequencing......Page 358
12.2.2 Omics-facilitated enzyme discovery......Page 359
12.3.1 Structure-function relation......Page 361
12.3.3 High-throughput screening for improved functionality......Page 362
12.4.1 Developing high-producing cell lines......Page 363
12.4.3 Enzyme production and purification......Page 365
12.5.2 Whole fruits......Page 367
12.5.6 Wine mouthfeel......Page 368
12.5.8 Synergistic applications in white wine making......Page 369
12.6 Conclusions......Page 370
References......Page 371
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Enzymes in Fruit and Vegetable Processing Chemistry and Engineering Applications

© 2010 Taylor and Francis Group, LLC

© 2010 Taylor and Francis Group, LLC

Enzymes in Fruit and Vegetable Processing Chemistry and Engineering Applications

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

© 2010 Taylor and Francis Group, LLC

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2010 by Taylor and Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-9434-3 (Ebook-PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright. com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface ............................................................................................................... vii The Editor ...........................................................................................................ix List of Contributors ...........................................................................................xi Chapter 1

Introduction to Enzymes ........................................................... 1 Alev Bayındırlı

Chapter 2

Effect of Enzymatic Reactions on Color of Fruits and Vegetables........................................................................... 19 J. Brian Adams

Chapter 3

Major Enzymes of Flavor Volatiles Production and Regulation in Fresh Fruits and Vegetables ......................... 45 Jun Song

Chapter 4

Effect of Enzymatic Reactions on Texture of Fruits and Vegetables ........................................................................... 71 Luis F. Goulao, Domingos P. F. Almeida, and Cristina M. Oliveira

Chapter 5

Selection of the Indicator Enzyme for Blanching of Vegetables ............................................................................ 123 Vural Gökmen

Chapter 6

Enzymatic Peeling of Citrus Fruits ..................................... 145 Maria Teresa Pretel, Paloma Sánchez-Bel, Isabel Egea, and Felix Romojaro

Chapter 7

Use of Enzymes for Non-Citrus Fruit Juice Production ................................................................................ 175 Liliana N. Ceci and Jorge E. Lozano

v

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vi

Contents

Chapter 8

Enzymes in Citrus Juice Processing ................................... 197 Domenico Cautela, Domenico Castaldo, Luigi Servillo, and Alfonso Giovane

Chapter 9

Use of Enzymes for Wine Production................................. 215 Encarna Gómez-Plaza, Inmaculada Romero-Cascales, and Ana Belén Bautista-Ortín

Chapter 10 Effect of Novel Food Processing on Fruit and Vegetable Enzymes ......................................................... 245 Indrawati Oey Chapter 11 Biosensors for Fruit and Vegetable Processing ................ 313 Danielle Cristhina Melo Ferreira, Lucilene Dornelles Mello, Renata Kelly Mendes, and Lauro Tatsuo Kubota Chapter 12 Enzymes in Fruit and Vegetable Processing: Future Trends in Enzyme Discovery, Design, Production, and Application ....................................................................... 341 Marco A. van den Berg, Johannes A. Roubos, and Lucie Parˇenicová Index ................................................................................................................ 359

© 2010 Taylor and Francis Group, LLC

Preface Fruits and vegetables are consumed as fresh or processed into different type of products. Some of the naturally occurring enzymes in fruits and vegetables have undesirable effects on product quality, and therefore enzyme inactivation is required during fruit and vegetable processing in order to increase the product shelf-life. Commercial enzyme preparations are also used as processing aids in fruit and vegetable processing to improve the process eficiency and product quality, because enzymes show activity on speciic substrates under mild processing conditions. Therefore, there has been a striking growth in the enzyme market for the fruit and vegetable industry. While fruit and vegetable processing is the subject of many books and other publications, the purpose of this book is to give detailed information about enzymes in fruit and vegetable processing from chemistry to engineering applications. Chapters are well written by an authoritative author(s) and follow a consistent style. There are 12 chapters in this book, and the chapters provide a comprehensive review of the chapter title important to the ield of enzymes and fruit and vegetable processing by focusing on the most promising new international research developments and their current and potential industry applications. Fundamental aspects of enzymes are given in Chapter 1. Color, lavor, and texture are important post-harvest quality parameters of fruits and vegetables. There are a number of product-speciic details, dependent on the morphology, composition, and character of the individual produce. Chapters 2, 3, and 4 describe in detail the effect of enzymes on color, lavor, and texture of selected fruits and vegetables. Selection of the indicator enzyme for blanching of vegetables is summarized in Chapter 5. For enzymes as processing aids, Chapter 6 describes in detail the enzymatic peeling of citrus fruits and Chapter 7 presents the importance of enzymes for juice production from pome, stone, and berry fruits. Inactivation of enzymes is required to obtain cloudy juice from citrus fruits. Chapter 8 is related to citrus juices; orange juice receives particular attention. Enzymes also play an important role in winemaking. The application of industrial enzyme preparations in the wine industry is a common practice. The use of enzymes for vii

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viii

Preface

wine production is the focus of Chapter 9. Chapter 10 provides serious review of the inactivation effect of novel technologies on fruit and vegetable enzymes to maximize product quality. Chapter 11 presents both chemical and technological information on enzyme-based biosensors for fruit and vegetable processing. The literature reported in each chapter highlights the current status of knowledge in the related area. Future trends for industrial use of enzymes are discussed in Chapter 12. The conclusion part of each chapter also presents the reader with potential research possibilities and applications. This book is a reference book to search or learn more about fruit and vegetable enzymes and enzyme-based processing of fruit and vegetables according to the latest enzyme-assisted technologies and potential applications of new approaches obtained from university and other research centers and laboratories. Such knowledge is important for the companies dealing with fruit and vegetable processing to be competitive and also for the collaboration among industry, university, and research centers. This book is also for the graduate students and young researchers who will play an important role for future perspectives of enzymes in fruit and vegetable processing. Alev Bayındırlı

© 2010 Taylor and Francis Group, LLC

The Editor Alev Bayındırlı is a professor in the Department of Food Engineering, Middle East Technical University, Ankara, Turkey. She has authored or co-authored 30 journal articles. She received a BS degree (1982) from the Department of Chemical Engineering, Middle East Technical University. MS (1985) and PhD (1989) degrees are from the Department of Food Engineering, Middle East Technical University. She is working on food chemistry and technology, especially enzymes in fruit and vegetable processing.

ix

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List of Contributors J. Brian Adams Formerly of Campden & Chorleywood Food Research Association (Campden BRI) Chipping Campden Gloucestershire, UK

Domingos P.F. Almeida Faculdade de Ciências Universidade do Porto Porto, Portugal Escola Superior de Biotecnologia Universidade Católica Portuguesa Porto, Portugal

Ana Belén Bautista-Ortín Departamento de Tecnología de Alimentos, Nutrición y Bromatología Facultad de Veterinaria, Universidad de Murcia Campus de Espinardo Murcia, Spain

Alev Bayındırlı Middle East Technical University Department of Food Engineering Ankara, Turkey

Inmaculada Romero-Cascales Departamento de Tecnología de Alimentos, Nutrición y Bromatología Facultad de Veterinaria, Universidad de Murcia Campus de Espinardo Murcia, Spain Domenico Castaldo Stazione Sperimentale per le Industrie delle Essenze e dei Derivati dagli Agrumi (SSEA) Reggio Calabria, Italy Domenico Cautela Stazione Sperimentale per le Industrie delle Essenze e dei Derivati dagli Agrumi (SSEA) Reggio Calabria, Italy Liliana N. Ceci Planta Piloto de Ingeniería Química UNS-CONICET Bahía Blanca, Argentina Isabel Egea Departamento Biología del Estrés y Patología Vegetal Centro de Edafología y Biología Aplicada del Segura-CSIC Espinardo, Murcia, Spain xi

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xii

List of Contributors

Danielle Cristhina Melo Ferreira Institute of Chemistry Unicamp Campinas, São Paulo, Brazil

Lucilene Dornelles Mello UNIPAMPA Campus Bagé Bagé, RS, Brazil

Alfonso Giovane Dipartimento di Biochimica e Bioisica Seconda Università degli Studi di Napoli Napoli, Italy

Renata Kelly Mendes Institute of Chemistry Unicamp Campinas, São Paulo, Brazil

Vural Gökmen Department of Food Engineering Hacettepe University Ankara, Turkey Encarna Gómez-Plaza Departamento de Tecnología de Alimentos, Nutrición y Bromatología Facultad de Veterinaria, Universidad de Murcia Campus de Espinardo Murcia, Spain Luis F. Goulao Secção de Horticultura Instituto Superior de Agronomia Lisbon, Portugal Centro de Ecoisiologia, Bioquimica e Biotecnologia Vegetal Instituto de Investigação Cientiica Tropical Oeiras, Portugal Lauro Tatsuo Kubota Institute of Chemistry Unicamp Campinas, São Paulo, Brazil Jorge E. Lozano Planta Piloto de Ingeniería Química UNS-CONICET Bahía Blanca, Argentina

© 2010 Taylor and Francis Group, LLC

Indrawati Oey Department of Food Science University of Otago Dunedin, New Zealand Cristina M. Oliveira Secção de Horticultura Instituto Superior de Agronomia Lisbon, Portugal Lucie Parˇ enicová DSM Biotechnology Centre, DSM Delft, The Netherlands Maria Teresa Pretel Escuela Politécnica Superior de Orihuela Universidad Miguel Hernández Alicante, Spain Felix Romojaro Departamento Biología del Estrés y Patología Vegetal Centro de Edafología y Biología Aplicada del Segura-CSIC Espinardo, Murcia, Spain Johannes A. Roubos DSM Biotechnology Centre, DSM Delft, The Netherlands

List of Contributors Paloma Sánchez-Bel Departamento Biología del Estrés y Patología Vegetal Centro de Edafología y Biología Aplicada del Segura-CSIC Espinardo, Murcia, Spain Luigi Servillo Dipartimento di Biochimica e Bioisica Seconda Università degli Studi di Napoli Napoli, Italy

© 2010 Taylor and Francis Group, LLC

xiii Jun Song Agriculture and Agri-Food Canada Atlantic Food and Horticulture Research Centre Nova Scotia, Canada Marco A. van den Berg DSM Biotechnology Centre, DSM Delft, The Netherlands

© 2010 Taylor and Francis Group, LLC

chapter one

Introduction to enzymes Alev Bayındırlı Contents 1.1 Nature of Enzymes ................................................................................... 1 1.2 Enzyme Classiication and Nomenclature............................................ 2 1.3 Enzyme Kinetics ....................................................................................... 3 1.4 Factors Affecting Enzyme Activity ........................................................ 7 1.5 Enzyme Inactivation .............................................................................. 10 1.6 Enzymes in Fruit and Vegetable Processing....................................... 13 Abbreviations ................................................................................................... 16 References.......................................................................................................... 16

1.1 Nature of enzymes Enzymes are effective protein catalysts for biochemical reactions. The structural components of proteins are L-α-amino acids with the exception of glycine, which is not chiral. The four levels of protein structure are primary, secondary, tertiary, and quaternary structures. Primary structure is related to the amino acid sequence. The amino group of one amino acid is joined to the carboxyl group of the next amino acid by covalent bonding, known as a peptide bond. The amino acid side-chain groups vary in terms of their properties such as polarity, charge, and size. The polar amino acid side groups tend to be on the outside of the protein where they interact with water, whereas the hydrophobic groups tend to be in the interior part of the protein. Secondary structure (α-helix, β-pleated sheet, and turns) is important for protein conformation. Right-handed α-helix is a regular arrangement of the polypeptide backbone by hydrogen bonding between the carbonyl oxygen of one residue (i) and the nitrogenous proton of the other residue (i+4). β-pleated sheet is a pleated structure composed of polypeptide chains linked together through interamide hydrogen bonding between adjacent strands of the sheet. Tertiary structure refers to the threedimensional structure of folded protein. Presence of disulide bridges, hydrogen bonding, ionic bonding, and hydrophobic and van der Waals interactions maintain the protein conformation. Folding the protein brings 1

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Alev Bayındırlı

together amino acid side groups from different parts of the amino acid sequence of the polypeptide chain to form the enzyme active site that consists of a few amino acid residues and occupies a relatively small portion of the total enzyme volume. The rest of the enzyme is important for the three-dimensional integrity. The quaternary structure of a protein results from the association of two or more polypeptide chains (subunits). Speciicity and catalytic power are two characteristics of an enzyme. Most enzymes can be extremely speciic for their substrates and catalyze reactions under mild conditions by lowering the free energy requirement of the transition state without altering the equilibrium condition. The enzyme speciicity depends on the conformation of the active site. The enzyme-substrate binding is generally explained by lock-and-key model (conformational perfect it) or induced it model (enzyme conformation change such as closing around the substrate). The lock-and-key model has been modiied due to the lexibility of enzymes in solution. The binding of the substrate to the enzyme results in a distortion of the substrate into the conformation of the transition state, and the enzyme itself also undergoes a change in conformation to it the substrate. Many enzymes exhibit stereochemical speciicity in that they catalyze the reactions of one conformation but not the other. Catalytic power is increased by use of binding energy, induced-it, proximity effect, and stabilization of charges in hydrophobic environment. The catalytic activity of many enzymes depends on the presence of cofactor for catalytic activity. If the organic compound as cofactor is loosely attached to enzyme, it is called a coenzyme. It is called a prosthetic group when the organic compound attaches irmly to the enzyme by covalent bond. Metal ion activators such as Ca++, Cu++, Co++, Fe++, Fe+++, Mn++, Mg++, Mo+++, and Zn++ can be cofactors. An enzyme without its cofactor is called an apoenzyme. An enzyme with a cofactor is referred as a haloenzyme. Enzymes catalyze the reactions by covalent catalysis or general acid/base catalysis.

1.2

Enzyme classiication and nomenclature

Enzymes are classiied into six groups (Table 1.1) according to the reaction catalyzed and denoted by an EC (Enzyme Commission) number. The irst, second, and third–fourth digits of these numbers show class of the enzyme, type of the bond involved in the reaction, and speciicity of the bond, respectively. Systematic nomenclature is the addition of the sufix -ase to the enzyme-catalyzed reaction with the name of the substrate. For example, naringinase, and α-L-Rhamnoside rhamnohydrolase are trivial and systematic names of the enzyme numbered as EC 3.2.1.40, respectively. Some of the enzyme-related databases are IUBMB, International Union of Biochemistry and Molecular Biology enzyme no menclature (www.chem.qmul. ac.uk/ iubmb/enzyme/); BRENDA, comprehensive enzyme information system

© 2010 Taylor and Francis Group, LLC

Chapter one:

Introduction to enzymes

3

Table 1.1 Classiication of Enzymes EC Class

Reaction Catalyzed −



EC1: Oxidoreductases

A +B A+B

EC2: Transferases

AB + C  A + BC

EC3: Hydrolases

EC4: Lyases

AB + H 2O  AH + BOH

X

Y

A − B  A = B+ X −Y

EC5: Isomerase

X

Y

Y

X

A− B  A− B EC6: Ligases

A + B  AB

Examples Peroxidase Catalase Polyphenol oxidase Lipoxygenase Ascorbic acid oxidase Glucose oxidase Alcohol dehydrogenase Amylosucrase Dextransucrase Transglutaminase Invertase Chlorophyllase Amylase Cellulose Polygalacturonase Lipase Galactosidase Thermolysin Pectin lyase Phenylalanine ammonia lyase Cysteine sulfoxide lyase Hydroperoxide lyase Glucose isomerase Carotenoid isomerase Hydroxycinnamate CoA ligase

(www.brenda-enzymes.org); the ExPASy, Expert Protein Analysis System enzyme nomenclature (www.expasy.org/enzyme/); and EBIPDB, European Bioinformatics Institute–Protein Data Bank enzyme structures database (www.ebi.ac.uk/thornton-srv/databases/enzymes).

1.3

Enzyme kinetics

Besides the quasi-steady-state kinetics (Briggs and Haldane approach), the rate of enzyme catalyzed reactions is generally modeled by the Michaelis-Menten approach. For a simple enzymatic reaction, binding of substrate (S) with free enzyme (E) is followed by an irreversible

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Alev Bayındırlı

breakdown of enzyme-substrate complex (ES) to free enzyme and product (P). The substrate binding with E is assumed to be very fast relative to the breakdown of ES complex to E and P. Therefore, the substrate binding is assumed to be at equilibrium as shown in the following reaction scheme: 1 2   → ES k →E + P E+S ←  k

k

−1

(1.1)

The Michaelis-Menten approach concerns the initial reaction rate where there is very little product formation. It is impossible to know the enzyme concentration in enzyme preparations. Therefore, the amount of the enzyme is given as units of activity per amount of sample. One international enzyme unit is the amount of enzyme that produces 1 micromole of product per minute. According to the applied enzyme activity assay, the enzyme unit deinition must be clearly stated in research or application. The total enzyme amount (Eo) equals the sum of the amount of E and ES complex. In terms of amounts, it can be represented as follows: CEo = CE + CES

(1.2)

The dissociation constant (Km), which is also called the MichaelisMenten constant, is a measure of the afinity of enzyme for substrate: Km =

k−1 CECS = k1 CES

(1.3)

If the enzyme has high afinity for the substrate, then the reaction will occur faster and Km has a lower value. High Km value means less afinity. Km varies considerably from one enzyme to another and also with different substrates for the same enzyme. For these elementary reactions 1.1, the initial reaction rate or reaction velocity (v) is expressed as v = k2CES

(1.4)

If the enzyme is stable during the reaction, the maximum initial reaction rate (vmax) corresponds to vmax = k2CEo

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(1.5)

Chapter one:

Introduction to enzymes

5

If the initial concentration of substrate (So) is very high during the reaction (CSo >> CEo), the concentration of substrate remains constant during the initial period of reaction (CSo ≈ CS). Combining Equations 1.2–1.5, the Michaelis-Menten equation is obtained as v=

vmaxCS K m + CS

(1.6)

As an example, kinetic properties of polygalacturonase assayed in different commercial enzyme preparations were studied and the reactions in all samples followed Michaelis–Menten kinetics (Ortega et al., 2004) Michaelis-Menten expression can be simpliied as follows: zero order expression : v = vmax first order expression : v =

vmax C Km S

for C s >> K m for C s