The Citric Acid Cycle

The citric acid cycle is a closed loop of eight enzymatic reactions occurring in the mitochondrial matrix that is critical for glucose catabolism. Also called the Krebs cycle or TCA cycle, it processes acetyl CoA through a series of redox, dehydration, hydration, and decarboxylation reactions. The cycle regenerates oxaloacetate, allowing continuous energy extraction from glucose.

Acetyl CoA, a two-carbon molecule produced from pyruvate oxidation production of acetyl coa in mitochondria, donates its acetyl group to oxaloacetate, a four-carbon molecule. This combination forms citrate, a six-carbon intermediate that initiates the cycle. The CoA group is released and diffuses away to combine with another acetyl group.

Each cycle produces three NADH molecules, one FADH2 molecule, one ATP (or GTP), and releases two carbon dioxide molecules. These electron carriers, NADH and FADH2, are high-energy molecules used in the electron transport chain to generate additional ATP through oxidative phosphorylation.

Pyruvate oxidation produces two acetyl CoA molecules from each glucose molecule. Since the citric acid cycle processes one acetyl CoA per turn, the cycle must run twice to completely oxidize all carbons from a single glucose molecule and extract all available energy.

Oxidation steps reduce NAD+ to NADH and FAD to FADH2, capturing high-energy electrons. These reductions occur when isocitrate is oxidized to alpha-ketoglutarate, when alpha-ketoglutarate forms succinyl CoA, when succinate is oxidized to fumarate, and when malate is oxidized back to oxaloacetate.

During step five, a phosphate group replaces the CoA group on succinyl CoA, forming succinate. This phosphate is then transferred to GDP or ADP, producing GTP or ATP through substrate-level phosphorylation. This direct mechanism generates one ATP per cycle turn.

NADH and FADH2 produced in the citric acid cycle provide electrons to the electron transport chain, which uses these electrons to pump protons across the inner mitochondrial membrane. This creates a proton gradient that drives ATP synthesis, generating the majority of ATP from glucose catabolism.