ENZYMES

1 EJC H2 Biology T1W10 Enzymes Biomolecules of Life (Part 2) - Enzymes 1. Overview of Topic Many enzymes are made up of proteins. Enzymes are biological catalysts which speed up the rate of reactions. In this topic, we will examine the mode of action of enzymes in terms of their catalytic action via their activity sites. We will also study how en zyme-substrate complexes are formed through the proposed models of enzyme substrate interactions. Various factors such as temperature, pH, enzyme concentrations & substrate concentrations affect enzyme catalysed reactions. Enzyme can also be influenced by inhibitors and activators which can regulate the rates of enzymatic reactions. 2. Learning Outcomes a. Explain the mode of action of enzymes in terms of an active site, enzyme -substrate complex, lowering of activation energy and enzyme specificity using the l ock-and-key and induced -fit hypothesis. b. Investigate and explain the effects of temperature, pH, enzyme concentra tion and substrate concentration of an enzyme-catalysed reaction by measuring rates of formation of products (eg measuring gas produced using c atalase) or rate of disappearance of sub strate (e.g. using amylase, starch and iodine). c. Describe the structure of competitive and non-competitive inhibitors with reference to the binding sites of the inhibitor. d. Explain the effects of competitive and non-competitive inhibitors (including allosteric inhibitors) on the rate of enzyme activity. 3. References Campbell, N.A. and Reece, J.B. (2011), Biology (9th edition), Pearson Benjamin-Cummings, San Francisco Brooker, R.J., Widmaier, E.P., Graham, L. and Stiling, P. (2008), Biology, Mc-Graw-Hill, New York

2 EJC H2 Biology T1W10 Enzymes Contents 1. Overview of Topic ...................................................................................................... 1 2. Learning Outcomes.................................................................................................... 1 3. References ................................................................................................................. 1 4. Introduction ............................................................................................................... 3 5. General Information on Enzymes .............................................................................. 4 A. Definition ............................................................................................................... 4 B. General Characteristics of Enzymes ...................................................................... 4 6. The Active Site of Enzymes ........................................................................................ 6 7. Enzyme co-factors ..................................................................................................... 9 8. Models of Enzyme Action ........................................................................................ 10 A. The ‘Lock & Key’ Hypothesis ................................................................................ 10 B. The ‘Induced-Fit’ model....................................................................................... 11 9. Energy Profile of Enzymatic Reactions .................................................................... 12 A. Lowering the Activation Energy (EA) barriers ..................................................... 12 B. Molecular Basis of Enzyme Action....................................................................... 14 10. Investigation of Enzyme-catalysed Reactions ..................................................... 15 A. Measuring the rate of product formed over time ............................................... 15 B. Measuring the rate of disappearance of substrate ............................................. 18 11. Factors Affecting The Rate of Enzyme Catalysed Reactions ............................... 21 A. Temperature ........................................................................................................ 21 B. pH ........................................................................................................................ 23 C. Enzyme Concentration ........................................................................................ 25 D. Substrate concentration ...................................................................................... 26 12. Enzyme inhibition ................................................................................................ 28 A. Competitive Inhibition ......................................................................................... 28 B. Non-Competitive Inhibition ................................................................................. 29 C. Allosteric Inhibition ................................................................................................. 31

3 EJC H2 Biology T1W10 Enzymes 4. Introduction In living cells, hundreds of different biochemical reactions take place rapidly and simultaneously. How is it possible for there to be such orderliness in what must be a potentially chaotic situation? How can reactions take place so rapidly at such modest temperatures? The answers to these questions come from a study of enzymes. Enzymes act as highly specific biological catalysts that speed up the rate of metabolic reactions. Enzymes also provide a means by which individual reactions can be controlled. The mechanism of these regulatory processes includes allosteric control, competitive inhibition, non -competitive inhibition, coval ent modification of enzyme and variation in the amount of enzymes synthesized (which we will cover later or in other topics). Enzymes are vitally important, because in their absence, reactions in the cell would be too slow to sustain life. Fig.4.1: An illustration of how enzymes catalyse biological reactions Notes to self

4 EJC H2 Biology T1W10 Enzymes 5. General Information on Enzymes A. Definition Enzymes are biological catalysts that increase the rate of a reaction and are chemically unaltered at the end of the reaction and thus can be reused. They are effective in small amounts. B. General Characteristics of Enzymes (I) Enzymes are mostly globular proteins They consist of one or more polypeptide chains coiled and folded to form a globular unit. (ref. tertiary or quaternary level of protein organisation). As globular proteins, they are extremely complex molecules with intricate 3 - dimensional contours and distinct surface geometries. The action of enzymes depends on their 3-dimensional structure/ conformation. There are exceptions because some enzymes can be composed of ribonucleic acids (RNA), e.g. ribozyme or complexes of RNA and protein, e.g telomerase. Fig.5.1: Telomerase is an example of a globular enzyme which is consists both protein and RNA (in the form a RNA template). The substrates for telomerase are DNA nucleotides and a DNA template strand. Notes to self Notes to self

5 EJC H2 Biology T1W10 Enzymes (II) Enzymes increase the rates of reactions Rate of enzymatically catalysed reactions are typically 10 6 to 1012 times greater than those of corresponding uncatalysed reaction. (III) Enzymes operate at milder reaction conditions Enzyme-catalysed reactions can occur under relatively milder conditions i.e. temperatures below 100oC, atmospheric pressure and nearly neutral pH normally encountered in the organism. In contrast, efficient chemical catalysts often requires elevated temperatures and pressures as well as extreme pH. (IV) Enzymes exhibit substrate specificity An enzyme or a particular type of enzyme will usually catalyze a specific chemical reaction. o Absolute specificity (When an e