The role of lipoic acid residues in the pyruvate dehydrogenase multienzyme complex of Escherichia coli

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Abstract
Two lipoic acid residues on each dihydrolipoamide acetyltransferase (E2; EC 2.3.1.12) chain of the pyruvate dehydrogenase multienzyme complex of E. coli underwent oxidoreduction reactions with NAD+ catalyzed by the lipoamide dehydrogenase (EC 1.6.4.3) component. Two moles of reagent/mol of E2 chain was incorporated when the complex was incubated with N-ethylmaleimide in the presence of acetyl-SCoA and NADH. Four moles of reagent/mol of E2 chain was incorporated when the complex was incubated with N-ethylmaleimide in the presence of NADH. Between 1 and 2 mol of acetyl groups/mol of E2 chain was incorporated when the complex was incubated with acetyl-SCoA plus NADH. Two moles of acetyl groups/mol of E2 chain was incorporated when the complex was incubated with pyruvate either before or after many catalytic turnovers through the overall reaction. There was no evidence to support the view that only half of the dihydrolipoic acid residues can be reoxidized by NAD+. Chemical modification of lipoic acid residues with N-ethylmaleimide proceeded faster than the accompanying loss of enzymic activity under all conditions tested, which indicates that not all the lipoyl groups are essential for activity. The most likely explanation for this result is an enzymic mechanism in which 1 lipoic acid residue can take over the function of another.

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