Interconversion of (S)-Glutamate and (2S,3S)-3-Methylaspartate:  A Distinctive B12-Dependent Carbon-Skeleton Rearrangement

The interconversion of (S)-glutamate and (2S,3S)-3-methylaspartate catalyzed by B12-dependent glutamate mutase is discussed using results from high-level ab initio molecular orbital calculations. Evidence is presented regarding the possible role of coenzyme-B12 in substrate activation and product formation via radical generation. Calculated electron paramagnetic resonance parameters support experimental evidence for the involvement of substrate-derived radicals and will hopefully aid the future detection of other important radical intermediates. The height of the rearrangement barrier for a fragmentation−recombination pathway, calculated with a model that includes neutral amino and carboxylic acid substituents in the migrating glycyl group, supports recent experimental evidence for the interconversion of (S)-glutamate and (2S,3S)-3-methylaspartate through such a pathway. Our calculations suggest that the enzyme may facilitate the rearrangement of (S)-glutamate through (partial) proton-transfer processes that control the protonation state of substituents in the migrating group.