Cobalamin biosynthesis
In molecular biology, cobalamin biosynthesis is the synthesis of cobalamin (vitamin B12).
Cobalamin (vitamin B12) is a structurally complex cofactor, consisting of a modified tetrapyrrole with a centrally chelated cobalt. Cobalamin is usually found in one of two biologically active forms: methylcobalamin and adenosylcobalamin. Most prokaryotes, as well as animals, have cobalamin-dependent enzymes, whereas plants and fungi do not appear to use it. In bacteria and archaea, these enzymes include methionine synthase, ribonucleotide reductase, glutamate and methylmalonyl-CoA mutases, ethanolamine ammonia-lyase, and diol dehydratase. In certain mammals, cobalamin is obtained through the diet, and is required for methionine synthase and methylmalonyl-CoA mutase.
There are at least two distinct cobalamin biosynthetic pathways in bacteria:
Either pathway can be divided into two parts:
There are about 30 enzymes involved in either pathway, where those involved in the aerobic pathway are prefixed Cob and those of the anaerobic pathway Cbi. Several of these enzymes are pathway-specific: CbiD, CbiG, and CbiK are specific to the anaerobic route of S. typhimurium, whereas CobE, CobF, CobG, CobN, CobS, CobT, and CobW are unique to the aerobic pathway of P. denitrificans.
The CbiB or CobD protein converts cobyric acid to cobinamide by the addition of aminopropanol on the F carboxylic group. It is part of the cob operon.
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