Chapter 9: Regulation of Enzymes

Substrate concentration effects are analyzed through Michaelis-Menten kinetics, which defines enzyme velocity in terms of Km and Vmax parameters; clinical relevance emerges through conditions like maturity-onset diabetes of the young where glucokinase mutations compromise both kinetic properties and glucose-sensing capacity for insulin secretion. The chapter then systematically explores reversible inhibition mechanisms, including competitive inhibition where inhibitors compete with substrate for the active site, noncompetitive inhibition affecting Vmax through allosteric mechanisms, and product inhibition that provides negative feedback; practical application appears in using ethanol to competitively block methanol metabolism in poisoning cases. Allosteric regulation receives detailed treatment, emphasizing how cooperativity allows enzymes to respond sensitively to metabolic signals through conformational transitions, with phosphofructokinase-1 and isocitrate dehydrogenase exemplifying pathways regulated by energy-status indicators like ATP, ADP, and AMP. Covalent modification through phosphorylation cascades provides rapid response to hormonal signals, with protein kinase A and calcium-calmodulin mediating epinephrine and glucagon effects on glycogen mobilization. Proteolytic activation of zymogens demonstrates how irreversible cleavage enables explosive metabolic responses in blood clotting and digestion. Long-term regulation operates through changes in enzyme abundance via transcriptional control and ubiquitin-mediated degradation, exemplified by MEOS pathway induction during chronic ethanol exposure and protein loss during fasting or stress. The chapter synthesizes these mechanisms into integrated regulatory principles including feedback inhibition, feed-forward activation, tissue-specific isozyme distribution, and compartmentalization, illustrating how multilayered enzyme control achieves metabolic precision in health and adapts during disease.