RVHS 6. Proteins - Enzymes

River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes 1 River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes Name: ( ) Class: 25J__ Date: References Title Authors Biology (9th Edition) Campbell and Reece Longman A-Level Course in Biology: Core Syllabus Volume 1 Hoh Biological Science 1. Organisms, Energy and Environment (3 rd Edition) Taylor, Green, Stout and Soper Harper’s Illustrated Biochemistry (27th Edition) Murray, Granner and Rodwell H2 Biology Syllabus 9477 (2025) Candidates should be able to use the knowledge gained in the following section(s) in new situations or to solve related problems. Related Topics Content Proteins Mode of action of enzymes Enzyme inhibitors Learning Outcomes 1C. Proteins p. 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 lock -and-key and induced -fit hypotheses. q. Investigate and explain the effects of temperature, pH, enzyme concentration and substrate concentration of an enzyme -catalysed reaction by measuring rates of formation of products (e.g. measuring gas produced using catalase) or rate of disappearance of substrate (e.g. using amylase, starch, and iodine). r. Describe the structure of competitive and non -competitive inhibitors with reference to the binding sites of the inhibitor. s. Explain the effects of competitive and non -competitive inhibitors (including allosteric inhibitors) on the rate of enzyme activity. Lecture Outline I. Importance of Enzymes II. Nature of Enzymes A. Terminology B. Structure C. Property III. Mode of Enzyme Action A. Activation Energy B. Mechanism of Enzyme Action C. Enzyme Specificity

River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes 2 IV. Measuring Rate of Enzyme-Catalysed Reaction V. Factors Affecting Rate of Enzyme-Catalysed Reactions A. Enzyme Concentration B. Substrate Concentration C. Temperature D. pH VI. Enzyme Inhibition A. Competitive Inhibition B. Non-competitive Inhibition C. Allosteric Regulation VII. Enzyme Cofactors A. Inorganic ions B. Coenzymes Websites URL Description https://www.youtube.com/watch?v=qgVFkRn8f10 Summary video on enzymes https://www.youtube.com/watch?v=6EDBlowVST0 Summary video on enzyme reactions

River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes 3 I. Importance of Enzymes ⬧ Industrial processes use catalysts to speed up the production of important chemicals, reducing the need for high temperatures and pressures. ⬧ The reactions inside living organisms must take place at atmospheric pressure and much lower temperatures when compared with industrial reactions. At the human body temperature of 37°C, reactions would be too slow to sustain life without catalysts. ⬧ Extracellular enzymes are secreted by some cells to catalyse reactions. Digestive enzymes (e.g. amylase, peptidase, lipase) secreted by the pancreatic acinar cells into the duodenum catalyse the breakdown of food into smaller molecules. ⬧ All cells contain intracellular enzymes to catalyse biochemical reactions taking place within the cell. ⬧ The chemical reactions of life are collectively known as metabolism, which comprises: o Anabolic reactions, in which complex compounds are synthesised from simpler ones, usually with energy absorption. E.g. Condensation reactions such as synthesis of starch from glucose o Catabolic reactions, in which complex compounds are broken down into simpler ones, usually with energy release. E.g. Hydrolytic reactions and oxidative reactions such as cellular respiration. Figure showing Metabolism = anabolism + catabolism Source: Advanced Biology Principles and Applications (2nd Edn). pp.170 ⬧ A sequence of metabolic reactions makes a metabolic pathway. The molecules involved in the steps in the pathway are known as metabolites or intermediates. E.g. Enzyme 1, 2, 3, 4 and 5 are enzymes involved in the metabolic pathway below. A is the precursor, B, C, D are the 1

River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes 4 In a metabolic pathway, o a specific enzyme catalyses each step o an intermediate can be used as a starting point for many other pathways. E.g. Intermediate C can be used to produce D and F, where both pathways use different enzymes o enzymes control the chemical reactions that occur within cells to ensure that they proceed in an orderly manner and at a sufficient rate.

River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes 5 II. Nature of Enzymes A. Terminology ⬧ Enzymes are biological catalysts because they are protein / RNA molecules synthesised by living cells. As catalysts, enzymes speed up chemical reactions but remain unchanged at the end. ⬧ The chemical(s) which an enzyme works on is called its substrate(s). ⬧ An enzyme combines with its substrate to form a short -lived enzyme-substrate complex. This proximity of the substrates in the complex greatly increases the probability / chances of a reaction occurring. ⬧ Once a reaction has occurred, the complex breaks up into product(s) and enzyme. ⬧ The enzyme remains unchanged at the end of the reaction and is free to interact again with more substrates. B. Structure ⬧ Only a small portion of the enzyme molecule (ranging from 3 to 12 amino acid residues) comes into direct contact with the substrate. This region is called the active site of the enzyme. ⬧ The amino acid residues at the active site can be divided into two groups, o contact residues , which are responsible for the specificity of the enzyme and form a shape complementary to that of the substrate. o catalytic residues, which are responsible for the ability of the enzyme to catalyse a particular chemical reaction. They are responsible for the breakage and formation of bonds. ⬧ The remaining amino acid residues, which make up the bulk of the enzyme, function to maintain the globular shape of the enzyme. This is essential for the optimal function of the active site.

River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes 6 C. Property ⬧ Most enzymes are globular proteins. Other enzymes are RNA in nature (e.g. ribozyme). o Being proteins or RNA in nature, enzymes are coded for by DNA. (KIV: Transcription & Translation) ⬧ Enzymes are catalysts. o They lower the activation energy of reactions they catalyse. ⬧ Enzymes are not changed or consumed by the reaction; hence they can be used over and over again. ⬧ The catalysed reaction is reversible. ⬧ Enzymes are highly efficient. o A very small amount of catalyst brings about the change of a large amount of substrate. E.g. one molecule of the enzyme catalase can catalyse the decomposition of 600 000 molecules of H2O2 per second at body temperature. ⬧ Enzymes are highly specific in action. E.g. catalase will only catalyse the decomposition of H 2O2. o The specificity of the enzyme is attributed to a compatible fit between the shape of its active site and the shape of its substrate. ⬧ Enzyme activity is affected by o enzyme concentration o substrate concentration o temperature o pH

River Valley High School 2025 JC1 H2 Biology Lecture Topic 6: Proteins - Enzymes 7 III. Mode of Enzyme Action A. Activation Energy ⬧ The laws of thermodynamics predict what will and will not happen under given conditions. They do not predict the rate of these processes. However, consider a mixture of petrol and oxygen maintained at room temperature. Although a reaction between the two substances is thermodynamically po