Krebs cycle intermediates, NADH generation, and anaplerosis
The citric acid cycle (Krebs cycle) oxidizes two-carbon acetyl units from acetyl-CoA through a series of eight enzymatic steps, regenerating oxaloacetate as the entry point acceptor. Each turn produces 3 NADH, 1 FADH₂, 1 GTP, and releases 2 CO₂. The cycle is regulated at three irreversible steps: citrate synthase (inhibited by ATP/NADH), isocitrate dehydrogenase (activated by ADP), and α-ketoglutarate dehydrogenase (inhibited by NADH/succinyl-CoA). Anaplerotic reactions, especially glutamate→α-ketoglutarate, replenish intermediates drained for biosynthesis.