12.2.5—Krebs cycle carbon compounds

The Krebs cycle is a series of enzyme-controlled reactions in the mitochondrial matrix. Acetyl-CoA supplies a two-carbon acetyl group to a four-carbon oxaloacetate acceptor, forming a six-carbon citrate that is converted back to oxaloacetate while releasing carbon dioxide and capturing transferable reducing power.

1. Enter the cycle: Acetyl-CoA carries a two-carbon acetyl group into the mitochondrial matrix. Oxaloacetate, a four-carbon acceptor, combines with it to form the six-carbon citrate.
2. Process the carbon compound: A sequence of enzyme-controlled reactions changes citrate through intermediate compounds. The carbon skeleton is progressively oxidised and carbon is removed as carbon dioxide.
3. Capture reducing power: Dehydrogenation transfers hydrogen/electrons to coenzymes, producing reduced coenzymes that can carry energy to later respiration stages. Detailed NAD/FAD carrier accounting belongs to the next card.
4. Make a small direct ATP return: A phosphate group is transferred from an intermediate to ADP, giving a small amount of ATP by substrate-level phosphorylation.
5. Regenerate the acceptor: The sequence returns to four-carbon oxaloacetate. Regeneration matters because the same acceptor can combine with the next acetyl-CoA, so the pathway is a cycle rather than a one-way chain.

Counting basis: the cycle turns once for each acetyl-CoA entering. One glucose produces two pyruvate and therefore two acetyl-CoA for aerobic processing, so the cycle turns twice per glucose. Keep this doubling separate from the chemical events of one turn.

The Krebs cycle occurs in the mitochondrial matrix and does not directly use oxygen as a reactant. It releases carbon dioxide and produces reduced coenzymes plus a small direct ATP return; the later respiratory chain explains how most reducing power is used.