Metabolite Channelling and Metabolic Complexity Substrate channelling in 2-0x0 acid dehydrogenase multienzyme complexes

Heteronuclear NMR spectroscopy and other experiments indicate that the true substrate of the E l component of 2-0x0 acid dehydrogenase complexes is not lipoic acid but the lipoyl domain of the E2 component. E l can recognize the lipoyllysine residue as such, but reductive acylation ensues only if the domain to which the lipoyl group is attached is additionally recognized by virtue of a mosaic of contacts distributed chiefly over the half of the domain that contains the lipoyl-lysine residue. The lipoyl-lysine residue may not be freely swinging, as supposed hitherto, but may adopt a preferred orientation pointing towards a nearby loop on the surface of the lipoyl domain. This in turn may facilitate the insertion of the lipoyl group into the active site of E l , where reductive acylation is to occur. The results throw new light on the concept of substrate channelling and active-site coupling in these giant multifunctional catalytic machines.