Chapter 5: Properties of Enzymes

Enzymes are remarkable biological catalysts that accelerate metabolic reactions by reducing activation energy while maintaining equilibrium conditions and exhibiting exceptional substrate specificity and stereoselectivity. The International Union of Biochemistry and Molecular Biology classifies enzymes into six functional categories based on reaction type: oxidoreductases catalyze electron transfer, transferases mediate group transfer typically requiring coenzymes, hydrolases use water to break bonds, lyases eliminate groups to form double bonds nonoxidatively, isomerases rearrange molecular structure, and ligases join substrates using ATP energy. Enzyme kinetics quantifies catalytic behavior through the Michaelis-Menten framework, where initial velocity varies hyperbolically with substrate concentration. Critical kinetic parameters include maximum velocity reflecting full enzyme saturation, the Michaelis constant measuring substrate affinity and representing the concentration at half-maximal velocity, the catalytic turnover number quantifying molecules converted per active site per second, and catalytic efficiency describing reaction rates under physiological conditions. The Lineweaver-Burk transformation linearizes kinetic data for graphical analysis. Enzyme inhibition occurs through reversible mechanisms including competitive inhibition that competes for the active site, uncompetitive inhibition targeting the enzyme-substrate complex, noncompetitive inhibition at allosteric sites, and mixed inhibition affecting both parameters differently, or through irreversible mechanisms involving covalent active site modification. Metabolic regulation employs allosteric mechanisms where regulatory molecules induce conformational transitions between active and inactive enzyme states through concerted or sequential models, exemplified by phosphofructokinase responding to multiple metabolic signals. Covalent modification through phosphorylation by kinases and dephosphorylation by phosphatases provides rapid activity switching. Multienzyme complexes organize sequential pathway enzymes through metabolite channeling, enabling direct substrate transfer between active sites to enhance reaction velocity, protect labile intermediates, and concentrate reactants locally.