Paper No.: 01. Paper Title: FOOD CHEMISTRY. Module 22: Enzymes: General nature and Kinetics of. enzyme reactions

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1 Paper No.: 01 Paper Title: FOOD CHEMISTRY Module 22: Enzymes: General nature and Kinetics of enzyme reactions

2 Enzymes: General nature and kinetics of enzyme reactions

3 INTRODUCTION Enzymes are defined as biocatalysts synthesized by living cells which are protein in nature, colloidal and thermolabile in character, and specific in their action. The presence and maintenance of a complete and balanced set of enzymes is essential for the breakdown of nutrients to supply energy and chemical building blocks. In 1897 Eduard Buchner discovered that yeast extracts could ferment sugar to alcohol, proving that fermentation was promoted by molecules that continued to function when removed from cells. Frederick W. Kühne coined the name enzymes (Greek- in yeast). In 1926, James Sumner - For the first time isolated and crystallized the enzyme urease.

4 General Nature Enzymes are powerful and highly specific catalysts Enzymes that catalyze the conversion of one or more substrates into one or more different products enhance the rates of the reaction by factors of at least 10 6 compared to the corresponding noncatalyzed reaction E.g. Carbonic anhydrase enzyme catalyzes a reaction which is 10 7 times as fast as the uncatalyzed one Enzymes are extremely selective i.e., they are specific both for the type of reaction catalyzed and for a single substrate or a small set of closely related substrates. E.g. Stereospecificity of enzymes

5 Cont., The catalytic activity of many enzymes depends on the presence of small molecules termed cofactors. An enzyme without its cofactor is referred to as an apoenzyme; the complete, catalytically active enzyme is called a holoenzyme. Cofactors are two types - Metal ions E.g. Carbonic anhydrase Zinc ion (Zn 2+ ) Glutathione peroxidase - Selenium - Coenzymes - Coenzymes can be bound either tightly or loosely to the enzyme. Tightly bound coenzymes are called prosthetic groups. E.g. Pyruvate dehydrogenase Thiamine pyrophosphate (TPP) Lactate dehydrogenase-nicotinamide adenine dinulceotide (NAD)

6 Classification of enzymes Many enzymes are named by adding the suffix "-ase" to the name of their substrate or to a word or phrase describing their activity. E.g. Urease - hydrolysis of urea DNA polymerase - the polymerization of nucleotides to form DNA International Union of Biochemists (IUB) have adopted a system for classification of enzymes and accordingly enzymes are divided into 6 classes based on the type of reaction catalyzed and the substrates involved. Six IUB classes of enzymes are 1. Oxidoreductases catalyze oxidations and reductions. 2. Transferases catalyze transfer of groups such as methyl or glycosyl groups from a donor molecule to an acceptor molecule.

7 Cont., 3. Hydrolases catalyze the hydrolytic cleavage of C C, C O, C N, P O, and certain other bonds, including acid anhydride bonds. 4. Lyases catalyze cleavage of C C, C O, C N, and other bonds by elimination, leaving double bonds, and also add groups to double bonds. 5. Isomerases catalyze geometric or structural changes within a single molecule. 6. Ligases catalyze the joining together of two molecules, coupled to the hydrolysis of a pyrophosphoryl group in ATP or a similar nucleoside triphosphate.

8 Enzyme Kinetics How enzymes work An enzyme-catalyzed reaction takes place within the confines of a pocket on the enzyme called the active site. The molecule that is bound in the active site and acted upon by the enzyme is called the substrate. The formation of an enzyme-substrate complex is the first step in enzymatic catalysis. An enzyme alters only the reaction rate but not the laws of thermodynamics and consequently can not alter the equilibrium of a chemical transformation. Enzymes accelerate reactions by facilitating the formation of the transition states. The free energy difference between transition state and the substrate is called the activation energy. Enzymes decrease this activation energy andaccelerate reaction rates.

9 Kinetics of enzyme reaction It is mainly concerned with the quantitative measurement of the rates of enzyme-catalyzed reactions and the systematic study of factors that affect these rates. Increase in the substrate concentration gradually increases the velocity of enzyme reaction within the limited range of substrate levels. A rectangular hyperbola is obtained when velocity is plotted against the substrate concentration Effect of substrate concentration on enzyme velocity

10 The equilibrium for the formation of an unstable enzyme-substrate complex is given by [E] [S] K m = [ES] The equilibrium constant, Km can be expressed in the form of the Michaelis-Menten equation, as follows v = V max [S] K m + [S] v = measured velocity [S] = substrate concentration V max = maximum velocity K m = Michaelis-Menten constant The above equation becomes K m = [S], if the measured velocity (v) is equal to ½ V max The Michaelis-Menten constant or K m is defined as the substrate concentration to produce half-maximum velocity in an enzyme catalysed reaction

of Michaelis-Menten equation, V max and K m can be accurately determined 1 1 K m 1 = + v V max V max

11 Lineweaver-Burk plot or double-reciprocal plot As V max is approached asymptotically it is impossible to determine an accurate value of V max and therefore K m However, taking the reciprocal of both sides of Michaelis-Menten equation, V max and K m can be accurately determined 1 1 K m 1 = + v V max V max [S] A plot of 1/v versus 1/[S], called a Lineweaver-Burk or double reciprocal plot, yields a straight line

12 Factors influencing enzyme activity The catalytic properties of enzymes, and consequently their activity, are influenced by numerous factors like concentration of enzyme, concentration of substrate, temperature, ph, presence of inhibitors, radiation etc. 1. As the concentration of the enzyme is increased, the velocity of the reaction proportionately increases Enzyme activity The influence of other important factors have been discussed in detail in the next module. Enzyme concentration [E]

BIOLOGY NOTES. CHAPTER 5 : BIOCATALYSIS SUBTOPIC : 5.1 Properties of enzymes and mechanism of actions

BIOLOGY NOTES. CHAPTER 5 : BIOCATALYSIS SUBTOPIC : 5.1 Properties of enzymes and mechanism of actions BIOLOGY CHAPTER 5 : BIOCATALYSIS SUBTOPIC : 5.1 Properties of enzymes and mechanism of actions LEARNING OUTCOMES: a) State the properties of enzymes. b) State the six classes of enzyme according to IUB