A researcher follows a protocol to test the activity of a mitochondrial extract containing all of the soluble enzymes of the matrix. Because the mitochondrial extract was dialyzed, the protocol lists low molecular weight cofactors that must be added to the extract in order to catalyze the oxidation of acetyl-CoA to CO2.
The list does not include lipoic acid, a known cofactor of the citric acid cycle.

Why is lipoic acid omitted from the list of cofactors to add back to the extract?

a) The added TPP can substitute for lipoic acid in the pyruvate dehydrogenase complex.

b) Lipoic acid is covalently attached to the pyruvate dehydrogenase complex.

c) The disulfide bond in lipoic acid prevents diffusion through the dialysis membrane.

d)The Kd of lipoic acid binding to pyruvate dehydrogenase is extremely low.

e)The oxidation of acetyl-CoA to CO2 does not require lipoic acid in vitro.

In this case, the correct answer is b) Lipoic acid is covalently attached to the pyruvate dehydrogenase complex since pyruvate dehydrogenase complex has the capacity to get diffused from the dialysis membrane. Such fact backs up the usage of lipoic acid as acyl carrier and electron carrier. In addition, inside the pyruvate dehydrogenase complex, the carboxyl group belonging to the lipoic acid turn out covalently linked to a lysine group of dihydrolipoamide transacetylase enzyme, which promotes the formation of the lipoamide.

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