Chapter 23: Hormonal Regulation and Integration of Mammalian Metabolism: Insulin, Glucagon, and Tissue-Specific Metabolism

Section 23.1 introduces the overall reactions of the cycle, which oxidize acetyl-CoA to CO₂ while reducing NAD⁺ and FAD to NADH and FADH₂, which will later fuel oxidative phosphorylation. Key enzymes such as citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and succinate dehydrogenase are discussed in terms of their mechanisms, regulatory roles, and contribution to energy conservation. Section 23.2 elaborates on the energetics of the cycle, showing how each acetyl-CoA yields three NADH, one FADH₂, and one GTP (or ATP, depending on the tissue), which together contribute significantly to the cell’s ATP budget. The reactions are mapped step-by-step: condensation of acetyl-CoA with oxaloacetate to form citrate; isomerization to isocitrate; oxidative decarboxylation to α-ketoglutarate; further decarboxylation to succinyl-CoA; and substrate-level phosphorylation to succinate, followed by regeneration of oxaloacetate through dehydrogenation and hydration reactions. Section 23.3 highlights regulation of the cycle at key irreversible steps, primarily via allosteric modulators such as NADH, ATP, ADP, Ca²⁺, and intermediates like succinyl-CoA. The cycle’s amphibolic nature is emphasized—it supplies not only energy but also biosynthetic precursors for amino acids, nucleotide bases, porphyrins, and more. Anaplerotic reactions, which replenish TCA intermediates, are discussed in detail, including pyruvate carboxylase and other inputs that ensure the cycle continues even when intermediates are siphoned off for biosynthesis. The chapter concludes with clinical connections, including thiamine deficiency (which impairs pyruvate dehydrogenase), mitochondrial diseases, and how cancer cells may reprogram TCA activity. It also introduces the glyoxylate cycle, a modified TCA pathway in plants and microorganisms that enables the net synthesis of glucose from fatty acids. This comprehensive review of the citric acid cycle shows its role as both an energy-producing engine and a biosynthetic powerhouse at the heart of metabolism.