2-ketoacid dehydrogenase complexes of Escherichia coli: stereospecificities of the three components for (R)-lipoate.

Stereospecificities of component enzymes in the pyruvate dehydrogenase complex and 2-ketoglutarate dehydrogenase complex from Escherichia coli for lipoate and dihydrolipoate are determined. Assays of the component enzymes using R,S-, R-, or S-lipoate or the enantiomers of dihydrolipoate show that only the R-enantiomers are substrates for these enzymes. Nonenzymatic reactions involving acetyl group transfer and coupled electron and acetyl group transfer between enantiomeric molecules of lipoate or/and dihydrolipoate proceed at significant rates. Coupled acetyl group and electron transfer from enzyme-bound acetyldihydrolipoyl moieties to free lipoate is also observed. The S-enantiomers are neither substrates nor inhibitors; however, products of S-enantiomers are slowly generated in enzymatic reactions owing to nonenzymatic reactions between enzyme-bound acetyldihydrolipoyl-groups and free S-lipoate or S-dihydrolipoate.