Mobility and active-site coupling in 2-oxo acid dehydrogenase complexes.

The 2-oxo acid dehydrogenase complexes consist of multiple copies of each of three enzymes, 2-oxo acid decarboxylase (E1), lipoate acetyltransferase (E2) and lipoamide dehydrogenase (E3), which catalyse successive steps in the overall reaction. The complexes are based on a structural core made up of the E2 chains, which also contain lipoic acid residues covalently attached to lysine residues. These lipoic acid residues are involved in transferring the substrate between the different active sites. A combination of limited proteolysis and 1H NMR experiments has shown that the E2 component has an unusual structure, having a substantial segment of polypeptide chain in the form of a highly flexible random coil. This flexibility allows the lipoyl-lysine residues to move rapidly over considerable distances, and provides a mechanism for the system of active-site coupling observed in these complexes.