The reaction of the substrate analog 2-ketoglutarate with adenosylcobalamin-dependent glutamate mutase

Glutamate mutase is one of several adenosylcobalamin-dependent enzymes that catalyze unusual rearrangements that proceed through a mechanism involving free radical intermediates. The enzyme exhibits remarkable specificity, and so far no molecules other than L-glutamate and L-threo-3-methylaspartate have been found to be substrates. Here we describe the reaction of glutamate mutase with the substrate analog, 2-ketoglutarate. Binding of 2-ketoglutarate (or its hydrate) to the holoenzyme elicits a change in the UV-visible spectrum consistent with the formation of cob(II)alamin on the enzyme. 2-ketoglutarate undergoes rapid exchange of tritium between the 5'-position of the coenzyme and C-4 of 2-ketoglutarate, consistent with the formation of a 2-ketoglutaryl radical analogous to that formed with glutamate. Under aerobic conditions this leads to the slow inactivation of the enzyme, presumably through reaction of free radical species with oxygen. Despite the formation of a substrate-like radical, no rearrangement of 2-ketoglutarate to 3-methyloxalacetate could be detected. The results indicate that formation of the C-4 radical of 2-ketoglutarate is a facile process but that it does not undergo further reactions, suggesting that this may be a useful substrate analog with which to investigate the mechanism of coenzyme homolysis.