This is a five step process.
- Step A: Pyruvate is decarboxylated by pyruvate dehydrogenase with help from TPP.
- Step B: The reactive carbon (between the N and the S of the five membered ring) of the TPP is oxidized and transferred as the acetyl group to lipoamide (which is the prosthetic group of the dihydrolipoyl transacetylase). This forms hydroxyethyl-TPP. An H+ ion is required for the intermediate to give off CO2.
- Step C: E2 (dihydrolipoyl transacetylase with cofactor lipoamide) oxidizes hydroxyethyl- to acetyl- and then transfers acetyl- to CoA, forming acetyl-CoA.
- Step D: Acetyl CoA was made in the previous step. However, the process is incomplete. The E2is still attached to the acetyl CoA molecule. So, E3 (dihydrolipoyl dehydrogenase) oxidizes the thiol groups of the dihydrolipoamide back to lipoamide.
- Step E: As a side reaction, NAD+ becomes reduced to NADH.