Glutamate and 2-methyleneglutarate mutase: From microbial curiosities to paradigms for coenzyme B-12-dependent enzymes

Glutamate mutase and 2-methyleneglutarate mutase are coenzyme B-12-dependent enzymes that catalyse carbon-skeleton rearrangements of their substrates. These reactions are initiated by homolysis of the coenzyme's cobalt-carbon sigma-bond. This gives cob(II)alamin and 5'-deoxyadenosyl radical, which abstracts a hydrogen atom from a substrate molecule. The resulting substrate-derived radical rearranges to a product-related radical, possibly by a fragmentation-recombination mechanism involving, for glutamate mutase, an acrylate molecule and glycinyl radical as intermediates. Evidence for this remarkable process derives from spectroscopic investigations (EPR), isotopic labelling and model studies.