InterPro domain: IPR003805

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 adocobalamin. 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 include methionine synthase, ribonucleotide reductase, glutamate and methylmalonyl-CoA mutases, ethanolamine ammonia lyase, and diol dehydratase [ 1 ]. In mammals, cobalamin is obtained through the diet, and is required for methionine synthase and methylmalonyl-CoA mutase [ 2 ].

There are at least two distinct cobalamin biosynthetic pathways in bacteria [ 3 ]:

• Aerobic pathway that requires oxygen and in which cobalt is inserted late in the pathway [ 4 ]; found in Pseudomonas denitrificans and Rhodobacter capsulatus.
• Anaerobic pathway in which cobalt insertion is the first committed step towards cobalamin synthesis [ 5 , 6 ]; found in Salmonella typhimurium, Bacillus megaterium, and Propionibacterium freudenreichii subsp. shermanii.

Either pathway can be divided into two parts: (1) corrin ring synthesis (differs in aerobic and anaerobic pathways) and (2) adenosylation of corrin ring, attachment of aminopropanol arm, and assembly of the nucleotide loop (common to both pathways) [ 7 ]. 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.

This entry represents the CobS protein, which joins adenosylcobinamide-GDP and alpha-ribazole to generate adenosylcobalamin (Ado-cobalamin) and also synthesizes adenosylcobalamin 5'-phosphate from adenosylcobinamide-GDP and alpha-ribazole 5'-phosphate [ 8 ]. It catalyses the reactions:

• Adenosylcobinamide-GDP + alpha-ribazole = GMP + adenosylcobalamin
• Adenosylcobinamide-GDP + alpha-ribazole 5'-phosphate = GMP + adenosylcobalamin 5'-phosphate

The protein product from these catalyses is associated with a large complex of proteins and is induced by cobinamide. CobS is involved in part III of cobalamin biosynthesis, one of the late steps in adenosylcobalamin synthesis that, together with CobU, CobT, and CobC proteins, defines the nucleotide loop assembly pathway [ 9 , 10 ].

1. Comparative genomics of the vitamin B12 metabolism and regulation in prokaryotes. J. Biol. Chem. 278, 41148-59
2. B12 trafficking in mammals: A for coenzyme escort service. ACS Chem. Biol. 1, 149-59
3. Multiple biosynthetic pathways for vitamin B12: variations on a central theme. Vitam. Horm. 61, 267-97
4. Aerobic synthesis of vitamin B12: ring contraction and cobalt chelation. Biochem. Soc. Trans. 33, 815-9
5. Isolation and characterization of 14 additional genes specifying the anaerobic biosynthesis of cobalamin (vitamin B12) in Propionibacterium freudenreichii (P. shermanii). Microbiology (Reading, Engl.) 148, 1845-53
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7. Biosynthesis of cobalamin (vitamin B12): a bacterial conundrum. Cell. Mol. Life Sci. 57, 1880-93
8. Reassessment of the late steps of coenzyme B12 synthesis in Salmonella enterica: evidence that dephosphorylation of adenosylcobalamin-5'-phosphate by the CobC phosphatase is the last step of the pathway. J. Bacteriol. 189, 2210-8
9. In vitro synthesis of the nucleotide loop of cobalamin by Salmonella typhimurium enzymes. Proc. Natl. Acad. Sci. U.S.A. 96, 11798-803
10. The last step in coenzyme B(12) synthesis is localized to the cell membrane in bacteria and archaea. Microbiology (Reading, Engl.) 150, 1385-95