Important enzymatic processes that involve free radicals include the actions of superoxide dismutase and ribonucleotide reductase, as well as enzyme processes involving vitamin B12. The coenzyme, vitamin B12, is a really interesting case. B12 contains a coordinated cobalt atom. An unusual aspect is that one of the coordination positions is occupied by carbon, which is the only known carbon-metal bond in a biomolecule. This carbon metal bond is weak and is homeolytically cleaved to generate a methyl radical, which then abstracts a hydrogen atom as a prelude to intramolecular rearrangement, methylation, and reduction in certain biochemical transformations such as the addition of a carbon to the carbon skeleton of L-methylmalonyl CoA to form succinyl CoA, a portion of the pathway of the degradation of the amino acids methionine, isoleucine, and valine.
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