Chapter 23: Tricarboxylic Acid Cycle

The tricarboxylic acid cycle, commonly referred to as the citric acid cycle or Krebs cycle, serves as the metabolic cornerstone of aerobic energy production and biosynthetic precursor synthesis. This chapter systematically examines how acetyl-CoA, generated from the breakdown of carbohydrates, lipids, and amino acids, enters the cycle by condensing with oxaloacetate through the enzyme citrate synthase, initiating a series of eight sequential enzymatic reactions. Each step of the cycle is examined in detail, including the isomerization of citrate to isocitrate via aconitase, the oxidative decarboxylation reactions catalyzed by isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase that yield reducing equivalents in the form of NADH, the substrate-level phosphorylation step mediated by succinyl-CoA synthetase that directly generates GTP, the FAD-dependent oxidation catalyzed by succinate dehydrogenase producing FADH₂, and the final regeneration of oxaloacetate through malate dehydrogenase. The chapter emphasizes that the reducing equivalents NADH and FADH₂ subsequently feed into the electron transport chain to generate the bulk of cellular ATP through oxidative phosphorylation. Critical attention is devoted to the sophisticated regulation of this cycle through multiple mechanisms including feedback inhibition by ATP and NADH, allosteric activation by calcium in muscle tissue, and substrate availability constraints. Beyond its canonical role in energy metabolism, the cycle functions as an amphibolic pathway, with its intermediates serving as precursors for gluconeogenesis, fatty acid synthesis, amino acid biosynthesis, and heme formation, thereby integrating catabolic and anabolic processes. Clinical case studies contextualize these principles, including metabolic consequences of energy overload, nutritional insufficiency effects on cycle flux, thiamine deficiency-induced impairment of pyruvate dehydrogenase leading to Wernicke-Korsakoff syndrome and lactic acidosis, inherited pyruvate dehydrogenase deficiencies manifesting as Leigh disease, toxic blockade of cycle enzymes through compounds like fluoroacetate, and metabolic adaptation during hypoxic stress mediated by hypoxia-inducible transcription factors. Understanding the cycle's regulation, enzymatic mechanisms, and clinical pathophysiology provides essential insight into how metabolic dysfunction underlies human disease.