Biology – Respiration | e-Consult

Oxaloacetate (OAA) plays a critical role as the initial acceptor molecule in the Krebs cycle. It combines with acetyl-CoA (2C) to form citrate (6C), initiating the cycle. This reaction is catalyzed by citrate synthase.

Acetyl-CoA Entry: Acetyl-CoA enters the cycle by reacting with oxaloacetate. This reaction is highly regulated and dependent on the availability of both substrates and the energy status of the cell. The formation of citrate is irreversible under cellular conditions.

Subsequent Reactions and Regeneration: Following the formation of citrate, a series of enzymatic reactions occur. These reactions involve oxidation, decarboxylation, and hydration steps. The key steps include:

1. Citrate is isomerized to isocitrate.
2. Isocitrate is oxidatively decarboxylated to α-ketoglutarate, releasing CO2 and producing NADH.
3. α-ketoglutarate is oxidatively decarboxylated to succinyl-CoA, releasing CO2 and producing NADH.
4. Succinyl-CoA is converted to succinate, coupled with the production of GTP.
5. Succinate is oxidized to fumarate, producing FADH2.
6. Fumarate is hydrated to malate.
7. Malate is oxidized to oxaloacetate, regenerating the starting molecule and producing NADH. This is the final step in the cycle.

The regeneration of oxaloacetate is essential for the cycle to continue, allowing for the repeated oxidation of acetyl-CoA and the production of energy-rich molecules (NADH, FADH2, and GTP).