Biology – Respiration | e-Consult

When oxygen is available, pyruvate, which is produced during glycolysis in the cytoplasm, cannot directly enter the mitochondria. Therefore, it must first be converted into acetyl-CoA. This crucial conversion occurs in the mitochondrial matrix and is catalyzed by the pyruvate dehydrogenase complex.

Pyruvate Dehydrogenase Complex (PDC) Reaction:

• Pyruvate is decarboxylated (carbon dioxide is removed), yielding a two-carbon molecule, acetyl-CoA.
• The remaining two-carbon fragment is oxidized, and the electrons are transferred to NAD+, reducing it to NADH.
• Coenzyme A (CoA) is attached to the acetyl group, forming acetyl-CoA.

The acetyl-CoA then enters the mitochondria. It participates in the link reaction (also known as the transition reaction), which is the initial step of Krebs cycle (Citric Acid Cycle). The link reaction involves the combination of acetyl-CoA with oxaloacetate to form citrate. This reaction is irreversible and is catalyzed by acetyl-CoA synthase.

Key Enzymes:

• Pyruvate Dehydrogenase Complex: Catalyzes the conversion of pyruvate to acetyl-CoA. Requires thiamine pyrophosphate (TPP), lipoamide, FAD, and NAD+.
• Acetyl-CoA Synthase: Catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate.

Products of the Link Reaction:

• Acetyl-CoA: Enters the Krebs cycle.
• CO2: Released during decarboxylation.
• NADH: Carries high-energy electrons to the electron transport chain.