Metabolic Pathways Reference

The universal pathway for glucose catabolism. Each glucose molecule (6 carbons) is split into two 3-carbon pyruvate molecules through a sequence of 10 enzyme-catalyzed reactions. The pathway invests 2 ATP in the preparatory phase and produces 4 ATP in the payoff phase, for a net gain of 2 ATP per glucose.

The central metabolic hub that oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins. Each turn of the cycle completely oxidizes one acetyl group (2 carbons) to 2 CO₂, generating high-energy electron carriers (NADH, FADH₂) that feed the electron transport chain.

The final stage of aerobic respiration. Electrons from NADH and FADH₂ are passed through a series of protein complexes (I–IV), each containing metal cofactors. The energy released pumps H⁺ ions across the inner mitochondrial membrane, creating a proton gradient that drives ATP synthase. Iron is found in cytochrome heme groups and iron-sulfur clusters across Complexes I–IV. Copper is essential in Complex IV (cytochrome c oxidase), where it helps catalyze the final electron transfer to O₂.

Light-dependent reactions capture solar energy and convert it to chemical energy. Photosystem II splits water molecules (photolysis), releasing O₂ and feeding electrons through an electron transport chain to Photosystem I. Magnesium sits at the center of every chlorophyll molecule, essential for light absorption. Manganese forms the oxygen-evolving complex (OEC) in PS II, a Mn₄CaO₅ cluster that catalyzes water oxidation.

The primary pathway for fatty acid catabolism. Each cycle shortens the fatty acid chain by 2 carbons, releasing one acetyl-CoA unit. For a 16-carbon palmitate, 7 cycles produce 8 acetyl-CoA, 7 NADH, and 7 FADH₂, yielding ~106 ATP total after all products enter the citric acid cycle and ETC.

The body’s primary mechanism for ammonia detoxification. Toxic NH₃ from amino acid catabolism is converted to non-toxic, water-soluble urea for renal excretion. The cycle spans two cellular compartments: the first reaction occurs in the mitochondrial matrix, while the remaining steps take place in the cytoplasm. Key amino acid intermediates include ornithine, citrulline, and arginine.