Chapter 6: Mechanisms of Enzymes

The foundation rests on understanding transition states and activation energy: enzymes lower the energy barrier that reactants must overcome by stabilizing the transition state to a far greater degree than they stabilize the substrate itself. Enzymatic catalysis operates through two complementary modes. Chemical catalysis, mediated by reactive amino acid residues in the active site such as histidine, aspartate, and serine, accounts for modest rate increases through acid-base catalysis and covalent intermediates. Binding catalysis, however, provides the dominant source of catalytic power through several interconnected mechanisms. The proximity effect concentrates mobile reactants from dilute solution into close proximity, dramatically reducing entropy penalties. Critically, enzymes bind substrates with relatively weak affinity to avoid thermodynamic traps, while simultaneously achieving extraordinarily tight binding to the reaction transition state—sometimes up to fifteen orders of magnitude stronger than substrate binding. This transition-state stabilization is the defining characteristic of enzyme catalysis. The enzyme active site undergoes induced fit upon substrate binding, ensuring substrate specificity and preventing unproductive side reactions. The chapter illustrates these principles through detailed examination of mechanistic models including serine proteases, which employ a catalytic triad to activate a nucleophilic serine residue for peptide bond cleavage; lysozyme, which uses substrate distortion and general acid catalysis to hydrolyze glycosidic bonds; and triose phosphate isomerase, a diffusion-limited enzyme that approaches the physical limits of catalytic velocity. Specialized enzymes like superoxide dismutase employ electrostatic guidance to accelerate substrate encounter rates beyond normal diffusion limits. Throughout, the chapter emphasizes that enzyme mechanisms represent elegant solutions to thermodynamic and kinetic challenges, combining multiple catalytic strategies to achieve both extraordinary specificity and unprecedented reaction rates.