[Tuition Notes - Overmugged] Chapter 3 Enzymes

Chapter 3: Enzymes H2 Biology

TABLE OF CONTENTS Introduction01 Enzyme Kinetics02 Factors Affecting Enzyme Activity 03 Enzyme Inhibition04

Introduction Enzymes, are also known as biological catalysts, since they catalyse biological reactions. While most enzymes are globular proteins, some are RNA molecules known as ribozymes. They are important as many metabolic reactions, though spontaneous, occur at a very slow rate. Thus, enzymes are needed to speed up these reactions. Enzymes are usually classified according to the type of reaction they catalyse and they can be named according to their substrates (i.e. reactants that enzymes act on).

Introduction Enzymes are defined as biological catalysts which speed up the rate of metabolic reactions (both catabolic and anabolic) while remaining chemically unchanged at the end of the reaction. Catabolic Reactions – The breaking down of complex molecules into simpler ones. Anabolic Reactions – The building up of complex molecules from simpler ones. Common Enzyme Properties: 1) Highly specific 2) Effective in small amounts with high turnover rates 3) Remain chemically unchanged at the end of the reaction 4) Affected by certain factors, e.g. temperature, pH, substrate and enzyme concentration 5) May require cofactors to function 6) Activity is tightly regulated 7) Allow reactions to reach equilibrium in a shorter time

Introduction Enzymes have a unique three-dimensional conformation with an active site (i.e. catalytic site and substrate-binding site) of the enzyme. The active site is formed by 3 to 12 amino acids from different parts of a single polypeptide chain held together by hydrogen bonds, ionic bonds, disulfide bonds and/or hydrophobic interactions. Other amino acids are involved in maintaining the overall three-dimensional structure of the enzyme.

Introduction The enzyme specificity is determined by the fit between the shape of the enzyme’s active site and its substrate. The active site of an enzyme is complementary to its substrate in terms of shape, size, charge and orientation. Enzymes are specific to only one particular substrate or one group of similar substrates e.g. lipases hydrolyses only lipids.

Introduction Substrate binding to the enzyme active site results in the formation of the enzyme-substrate complex (E-S complex). The substrates are held in the active site by weak bonds such as hydrogen bonds, ionic bonds and hydrophobic interactions. The active site (catalytic site) catalyses the conversion of substrate to product. Once the products are formed, they are no longer complementary to the active site and thus, will leave the enzyme. The enzyme is then available to act on other substrates.

Introduction Two hypotheses explain how enzymes function. 1) Lock & Key Hypothesis: The enzyme acts as a lock and the substrate acts as a key, which fits precisely. The active site of the enzyme is perfectly complementary to the substrate in terms of shape, size, charge and orientation. The substrate binds to enzyme’s active site to form the enzyme-substrate complex. This mode of activation is more probable for enzymes that work on only one type of substrate.

Introduction 2) Induced Fit Hypothesis: Enzymes may work in a more flexible manner. The active site is not perfectly complementary to the substrate in terms of shape, size and orientation. However, upon forming some bonds with the substrate, the enzyme changes its shape, which leads to a precise fit to form the enzyme-substrate complex. This mode of action is more probable for enzymes that work on a group of closely-related substrates, e.g. lipases.

Introduction Activation energy (EA) is the initial investment of energy that reactant molecules must possess to overcome an energy barrier, in order for a reaction to begin. Enzymes speed up biological reactions as they provide an alternative pathway, which has a lower activation energy (EA) as compared to the uncatalysed reaction. Thus, more reactant molecules possess energy equal or more than the activation energy required for the catalysed reaction. As such, the reactions occur at a faster rate and a high temperature is not required.