Cobalamin biosynthesis

Cobalamin biosynthesis
Symbol CobD_Cbib
Pfam PF03186
InterPro IPR004485
Symbol CobS
Pfam PF02654
InterPro IPR003805
Symbol CobT
Pfam PF06213
InterPro IPR006538
PDB 1cbu EBI.jpg
adenosylcobinamide kinase/adenosylcobinamide phosphate guanylyltransferase (cobu) from salmonella typhimurium
Symbol CobU
Pfam PF02283
Pfam clan CL0023
InterPro IPR003203
SCOP 1cbu
PDB 1nij EBI.jpg
yjia protein
Symbol cobW
Pfam PF02492
Pfam clan CL0023
InterPro IPR003495
SCOP 1nij
CobW C terminal
PDB 1nij EBI.jpg
yjia protein
Symbol CobW_C
Pfam PF07683
InterPro IPR011629
SCOP 1nij
PDB 1dah EBI.jpg
dethiobiotin synthetase complexed with 7,8-diamino-nonanoic acid, 5'-adenosyl-methylene-triphosphate, and manganese
Symbol CbiA
Pfam PF01656
Pfam clan CL0023
InterPro IPR002586
SCOP 1dts
PDB 1sr8 EBI.jpg
structural genomics, 1.9a crystal structure of cobalamin biosynthesis protein (cbid) from archaeoglobus fulgidus
Symbol CbiD
Pfam PF01888
InterPro IPR002748
CbiG N terminus
Symbol CbiG_N
Pfam PF11760
InterPro IPR021744
CbiG central region
Symbol CbiG_mid
Pfam PF11761
InterPro IPR021745
CbiG C terminus
Symbol CbiG_C
Pfam PF01890
InterPro IPR002750
Symbol CbiJ
Pfam PF02571
InterPro IPR003723
Symbol CbiM
Pfam PF01891
Pfam clan CL0315
InterPro IPR002751
Symbol CbiN
Pfam PF02553
InterPro IPR003705
TCDB 3.A.1
Symbol CbiQ
Pfam PF02361
InterPro IPR003339
TCDB 3.A.1
PDB 1tjn EBI.jpg
crystal structure of hypothetical protein af0721 from archaeoglobus fulgidus
Symbol CbiX
Pfam PF01903
Pfam clan CL0043
InterPro IPR002762
Symbol CbiZ
Pfam PF01955
InterPro IPR002808

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 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]

Pathways of cobalamin biosynthesis

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

  • Anaerobic pathway in which cobalt insertion is the first committed step towards cobalamin synthesis;[5] found in Salmonella typhimurium, Bacillus megaterium, and Propionibacterium freudenreichii subsp. shermanii.

Either pathway can be divided into two parts:

  • Corrin ring synthesis (differs in aerobic and anaerobic pathways)
  • Adenosylation of corrin ring, attachment of aminopropanol arm, and assembly of the nucleotide loop (common to both pathways).[6]

Proteins involved in cobalamin biosynthesis

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.[7]


Aerobic cobalt chelatase consists of three subunits, CobT, CobN and CobS. Cobalamin (vitamin B12) can be complexed with metal via the ATP-dependent reactions (aerobic pathway) (e.g., in P. denitrificans) or via ATP-independent reactions (anaerobic pathway) (e.g., in Salmonella typhimurium).[8][9] The corresponding cobalt chelatases are not homologous. However, aerobic cobalt chelatase subunits CobN and CobS are homologous to Mg-chelatase subunits BchH and BchI, respectively.[9] CobT, too, has been found to be remotely related to the third subunit of Mg-chelatase, BchD (involved in bacteriochlorophyll synthesis, e.g., in Rhodobacter capsulatus).[9]

The CobS protein is a cobalamin-5-phosphate synthase that catyalzes the reactions:

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

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.[10][11]


CobU proteins are bifunctional cobalbumin biosynthesis enzymes which display cobinamide kinase and cobinamide phosphate guanyltransferase activity. The crystal structure of the enzyme reveals the molecule to be a trimer with a propeller-like shape.[12]


CobW proteins are generally found proximal to the trimeric cobaltochelatase subunit CobN, which is essential for vitamin B12 (cobalamin) biosynthesis.[1] They contain a P-loop nucleotide-binding loop in the N-terminal domain and a histidine-rich region in the C-terminal portion suggesting a role in metal binding, possibly as an intermediary between the cobalt transport and chelation systems. CobW might be involved in cobalt reduction leading to cobalt(I) corrinoids. CobW-like proteins include P47K, a Pseudomonas chlororaphis protein needed for nitrile hydratase expression,[13] and urease accessory protein UreG, which acts as a chaperone in the activation of urease upon insertion of nickel into the active site.[14]


The CbiA family of proteins consists of various cobyrinic acid a,c-diamide synthases. These include CbiA and CbiP from Salmonella typhimurium.,[15] and CobQ from Rhodobacter capsulatus.[16] These amidases catalyse amidations to various side chains of hydrogenobyrinic acid or cobyrinic acid a,c-diamide in the biosynthesis of cobalamin (vitamin B12) from uroporphyrinogen III.[15]


CbiD is an essential protein for cobalamin biosynthesis in both Salmonella typhimurium and Bacillus megaterium. A deletion mutant of CbiD suggests that this enzyme is involved in C-1 methylation and deacylation reactions required during the ring contraction process in the anaerobic pathway to cobalamin (similar role as CobF).[17] The CbiD protein has a putative S-AdoMet binding site.[18] CbiD has no counterpart in the aerobic pathway.


CbiG proteins are specific for anaerobic cobalamin biosynthesis. CbiG, which shows homology with CobE of the aerobic pathway, participates in the conversion of cobalt-precorrin 5 into cobalt-precorrin 6.[19] CbiG is responsible for the opening of the delta-lactone ring and extrusion of the C2-unit.[20] The aerobic pathway uses molecular oxygen to trigger the events at C-20 leading to contraction and expulsion of the C2-unit as acetic acid from a metal-free intermediate, whereas the anaerobic route involves the internal delivery of oxygen from a carboxylic acid terminus to C-20 followed by extrusion of the C2-unit as acetaldehyde, using cobalt complexes as substrates.[20]


The CbiJ family of proteins includes the CobK and CbiJ precorrin-6x reductases EC In the aerobic pathway, CobK catalyses the reduction of the macrocycle of precorrin-6X to produce precorrin-6Y; while in the anaerobic pathway CbiJ catalyses the reduction of the macrocycle of cobalt-precorrin-6X into cobalt-precorrin-6Y.[21][22]


CbiM is an integral membrane protein which is involved in cobalamin synthesis, its exact function in unknown.


The cobalt transport protein CbiN is part of the active cobalt transport system involved in uptake of cobalt in to the cell involved with cobalamin biosynthesis (vitamin B12). It has been suggested that CbiN may function as the periplasmic binding protein component of the active cobalt transport system.[16]


The CbiQ family consists of various cobalt transport proteins Most of which are found in Cobalamin (Vitamin B12) biosynthesis operons. In Salmonella the cbiN cbiQ (product CbiQ in this family) and cbiO are likely to form an active cobalt transport system.[23]


The CbiX protein functions as a cobalt-chelatase in the anaerobic biosynthesis of cobalamin. It catalyses the insertion of cobalt into sirohydrochlorin. The structure of CbiX from Archaeoglobus fulgidus consists of a central mixed beta-sheet flanked by four alpha-helices, although it is about half the size of other Class II tetrapyrrole chelatases.[24] The CbiX proteins found in archaea appear to be shorter than those found in eubacteria.[25]


The CbiZ family of proteins includes CbiZ, which is involved in the salvage pathway of cobinamide in archaea. Archaea convert adenosylcobinamide (AdoCbi) into adenosylcobinamide phosphate (AdoCbi-P) in two steps. First, the amidohydrolase activity of CbiZ cleaves off the aminopropanol moiety of AdoCbi yielding adenosylcobyric acid (AdoCby); second, AdoCby is converted into AdoCbi-P by the action of adenosylcobinamide-phosphate synthase (CbiB). Adenosylcobyric acid is an intermediate of the de novo coenzyme B12 biosynthetic route.[26]


  1. ^ a b Rodionov DA, Vitreschak AG, Mironov AA, Gelfand MS (October 2003). "Comparative genomics of the vitamin B12 metabolism and regulation in prokaryotes". J. Biol. Chem. 278 (42): 41148–59. doi:10.1074/jbc.M305837200. PMID 12869542. 
  2. ^ Banerjee R (April 2006). "B12 trafficking in mammals: A for coenzyme escort service". ACS Chem. Biol. 1 (3): 149–59. doi:10.1021/cb6001174. PMID 17163662. 
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  5. ^ Roessner CA, Huang KX, Warren MJ, Raux E, Scott AI (June 2002). "Isolation and characterization of 14 additional genes specifying the anaerobic biosynthesis of cobalamin (vitamin B12) in Propionibacterium freudenreichii (P. shermanii)". Microbiology (Reading, Engl.) 148 (Pt 6): 1845–53. PMID 12055304. 
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  7. ^ Woodson JD, Zayas CL, Escalante-Semerena JC (December 2003). "A new pathway for salvaging the coenzyme B12 precursor cobinamide in archaea requires cobinamide-phosphate synthase (CbiB) enzyme activity". J. Bacteriol. 185 (24): 7193–201. PMC 296239. PMID 14645280. 
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  9. ^ a b c Fodje MN, Hansson A, Hansson M, Olsen JG, Gough S, Willows RD, Al-Karadaghi S (August 2001). "Interplay between an AAA module and an integrin I domain may regulate the function of magnesium chelatase". J. Mol. Biol. 311 (1): 111–22. doi:10.1006/jmbi.2001.4834. PMID 11469861. 
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  14. ^ Zambelli B, Musiani F, Savini M, Tucker P, Ciurli S (March 2007). "Biochemical studies on Mycobacterium tuberculosis UreG and comparative modeling reveal structural and functional conservation among the bacterial UreG family". Biochemistry 46 (11): 3171–82. doi:10.1021/bi6024676. PMID 17309280. 
  15. ^ a b Pollich M, Klug G (August 1995). "Identification and sequence analysis of genes involved in late steps in cobalamin (vitamin B12) synthesis in Rhodobacter capsulatus". J. Bacteriol. 177 (15): 4481–7. PMC 177200. PMID 7635831. 
  16. ^ a b Roth JR, Lawrence JG, Rubenfield M, Kieffer-Higgins S, Church GM (June 1993). "Characterization of the cobalamin (vitamin B12) biosynthetic genes of Salmonella typhimurium". J. Bacteriol. 175 (11): 3303–16. PMC 204727. PMID 8501034. 
  17. ^ Roessner CA, Williams HJ, Scott AI (April 2005). "Genetically engineered production of 1-desmethylcobyrinic acid, 1-desmethylcobyrinic acid a,c-diamide, and cobyrinic acid a,c-diamide in Escherichia coli implies a role for CbiD in C-1 methylation in the anaerobic pathway to cobalamin". J. Biol. Chem. 280 (17): 16748–53. doi:10.1074/jbc.M501805200. PMID 15741157. 
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  19. ^ Scott AI, Roessner CA (August 2002). "Biosynthesis of cobalamin (vitamin B(12))". Biochem. Soc. Trans. 30 (4): 613–20. PMID 12196148. 
  20. ^ a b Kajiwara Y, Santander PJ, Roessner CA, Pérez LM, Scott AI (August 2006). "Genetically engineered synthesis and structural characterization of cobalt-precorrin 5A and -5B, two new intermediates on the anaerobic pathway to vitamin B12: definition of the roles of the CbiF and CbiG enzymes". J. Am. Chem. Soc. 128 (30): 9971–8. doi:10.1021/ja062940a. PMID 16866557. 
  21. ^ Kim W, Major TA, Whitman WB (December 2005). "Role of the precorrin 6-X reductase gene in cobamide biosynthesis in Methanococcus maripaludis". Archaea 1 (6): 375–84. PMC 2685584. PMID 16243778. 
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  23. ^ Roth, J. R.; Lawrence, J. G.; Rubenfield, M.; Kieffer-Higgins, S.; Church, G. M. (1993). "Characterization of the cobalamin (vitamin B12) biosynthetic genes of Salmonella typhimurium". Journal of bacteriology 175 (11): 3303–3316. PMC 204727. PMID 8501034.  edit
  24. ^ Yin J, Xu LX, Cherney MM, Raux-Deery E, Bindley AA, Savchenko A, Walker JR, Cuff ME, Warren MJ, James MN (March 2006). "Crystal structure of the vitamin B12 biosynthetic cobaltochelatase, CbiXS, from Archaeoglobus fulgidus". J. Struct. Funct. Genomics 7 (1): 37–50. doi:10.1007/s10969-006-9008-x. PMID 16835730. 
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This article includes text from the public domain Pfam and InterPro IPR004485

This article includes text from the public domain Pfam and InterPro IPR003805

This article includes text from the public domain Pfam and InterPro IPR006538

This article includes text from the public domain Pfam and InterPro IPR003203

This article includes text from the public domain Pfam and InterPro IPR003495

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This article includes text from the public domain Pfam and InterPro IPR002586

This article includes text from the public domain Pfam and InterPro IPR002748

This article includes text from the public domain Pfam and InterPro IPR002750

This article includes text from the public domain Pfam and InterPro IPR003723

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This article includes text from the public domain Pfam and InterPro IPR003705

This article includes text from the public domain Pfam and InterPro IPR003339

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