Chapter 8: Enzymes as Catalysts

Two major models explain this binding process: the lock-and-key model demonstrates the importance of structural complementarity between enzyme and substrate, while the induced-fit model reveals how enzymes undergo conformational changes to optimize binding specificity and catalytic efficiency. Enzymes achieve dramatic reaction acceleration by lowering the activation energy barrier, employing multiple catalytic strategies that operate individually or in combination. Acid-base catalysis uses ionizable groups to facilitate proton transfer, covalent catalysis forms temporary covalent bonds with substrates, metal-ion catalysis harnesses metallic cofactors like zinc, magnesium, and iron for electron stabilization and transfer, catalysis by approximation positions reactants optimally, and cofactor catalysis engages organic coenzymes in electron transfer and group modifications. The chapter integrates functional groups from amino acid side chains, coenzymes derived from vitamins including thiamine, biotin, and pyridoxine, and essential metal ions that participate directly in catalytic mechanisms. Oxidation-reduction coenzymes such as NAD+ and FAD mediate electron transfer in metabolic pathways, with alcohol dehydrogenase serving as a key example demonstrating zinc's role in substrate oxidation and intermediate stabilization. Enzyme inhibition represents a critical regulatory and pathological mechanism, encompassing reversible competitive inhibition, irreversible covalent modifications like aspirin's acetylation of cyclooxygenase, and suicide inhibitors exemplified by penicillin's mechanism against bacterial peptidoglycan synthesis. Clinical applications connect abstract concepts to medical practice through cases including acetylcholinesterase inhibition in insecticide poisoning, xanthine oxidase inhibition in gout management, and thiamine deficiency-related beriberi. The Enzyme Commission classification system organizes all enzymes into six functional classes—oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases—providing a universal framework for understanding enzymatic diversity and metabolic regulation.