Question

(5 points): The alpha-ketogluterate dehydrogenase complex uses three proteins and 5 different co-factors to convert pyruvate to acetyl-CoA. Identify these three proteins and five co-factors and describe the chemical role that each plays in the reaction mechanism of the a-KGDH complex. Write the balanced equation for the overall reaction catalyzed by this enzyme.

Answer 1

The alpha-ketogluterate dehydrogenase complex uses three proteins and 5 different co-factors to convert pyruvate to acetyl-CoA. Identify these three proteins and five co-factors and describe the chemical role that each plays in the reaction mechanism of the a-KGDH complex. Write the balanced equation for the overall reaction catalyzed by this enzyme.

Answer:

The pyruvate dehydrogenase complex is involved in attachment of glycolysis to the TCA cycle. It is composed of three enzymes involving five cofactors. The pyruvate dehydrogenase complex oxidizes down pyruvate to generate acetyl-coA.

Pyruvate dehydrogenase is a multi-enzyme complex in which three enzymes are involved:

1. Enzyme1: Pyruvate dehydrogenase which uses thiamine pyrophosphate (TPP) as its prosthetic group.( Molecule covalently attached to the enzyme)
2. Enzyme2: Dihydrolipoyl transacetylase which uses lipoamide and coenzyme A as its prosthetic groups.
3. Enzyme3: Dihydrolipoyl dehydrogenase which uses FAD and NAD+ as its cofactors.

This is a five step process.

1. Step A: Pyruvate is decarboxylated by pyruvate dehydrogenase with help from thiamine pyrophosphate.
2. Step B: The reactive carbon of the thiamine pyrophosphate is oxidized and transferred as the acetyl group to lipoamide (which is the prosthetic group of the dihydrolipoyl transacetylase). Formation of hydroxyethyl-TPP occurs. for this reation to occur an H+ ion is required for the intermediate to give off CO2.
3. Step C: Enzyme2 (dihydrolipoyl transacetylase with cofactor lipoamide) oxidizes hydroxyethyl- to acetyl- and then transfers acetyl- to CoA, forming acetyl-CoA.
4. Step D: Acetyl CoA was made in the previous step but the process is incomplete. The Enzyme2 is still attached to the acetyl CoA molecule. So, all the thiol groups of the dihydrolipoamide are oxidized back to lipoamide by Enzyme3 (dihydrolipoyl dehydrogenase).
5. Step E: As a side reaction, NAD+ is reduced to NADH.

C02 COO HO RH H2R CoA-SH FADH2 NAD FAD SH SH NADHH

Balanced Equation:  2Pyruvate + 2NAD+ +2coA → 2acetyl coA + 2NADH + 2CO2