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).[7]
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.
CbiB/CobD
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.[8]
CobT/CobN/CobS
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).[9][10] 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.[10] 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).[10]
The CobS protein is a cobalamin-5-phosphate synthase that catyalzes the reactions:
The proteinproduct 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.[11][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,[14] and urease accessory protein UreG, which acts as a chaperone in the activation of urease upon insertion of nickel into the active site.[15]
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.[20] CbiG is responsible for the opening of the delta-lactone ring and extrusion of the C2-unit.[21] 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.[21]
CbiJ
The CbiJ family of proteins includes the CobK and CbiJ precorrin-6x reductases EC1.3.1.54. 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.[22][23]
The cobalttransportprotein CbiN is part of the active cobalt transport system involved in uptake of cobalt into the cell involved with cobalaminbiosynthesis (vitamin B12). It has been suggested that CbiN may function as the periplasmic binding protein component of the active cobalt transport system.[17]
CbiQ
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.[24]
CbiX
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.[25] The CbiX proteins found in archaea appear to be shorter than those found in eubacteria.[26]
CbiZ
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.[27]
References
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^Banerjee R (April 2006). "B12 trafficking in mammals: A for coenzyme escort service". ACS Chem. Biol. 1 (3): 149–59. doi:10.1021/cb6001174. PMID17163662.
^Roessner CA, Santander PJ, Scott AI (2001). "Multiple biosynthetic pathways for vitamin B12: variations on a central theme". Vitam. Horm. 61: 267–97. doi:10.1016/s0083-6729(01)61009-4. PMID11153269.
^Heldt D, Lawrence AD, Lindenmeyer M, Deery E, Heathcote P, Rigby SE, Warren MJ (August 2005). "Aerobic synthesis of vitamin B12: ring contraction and cobalt chelation". Biochem. Soc. Trans. 33 (Pt 4): 815–9. doi:10.1042/BST0330815. PMID16042605.
^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. 148 (Pt 6): 1845–53. doi:10.1099/00221287-148-6-1845. PMID12055304.
^Roth JR, Lawrence JG, Bobik TA (1996). "Cobalamin (coenzyme B12): synthesis and biological significance". Annu. Rev. Microbiol. 50: 137–81. doi:10.1146/annurev.micro.50.1.137. PMID8905078.
^ abcFodje 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. PMID11469861.
^Maggio-Hall LA, Claas KR, Escalante-Semerena JC (May 2004). "The last step in coenzyme B(12) synthesis is localized to the cell membrane in bacteria and archaea". Microbiology. 150 (Pt 5): 1385–95. doi:10.1099/mic.0.26952-0. PMID15133100.
^Thompson TB, Thomas MG, Escalante-Semerena JC, Rayment I (May 1998). "Three-dimensional structure of adenosylcobinamide kinase/adenosylcobinamide phosphate guanylyltransferase from Salmonella typhimurium determined to 2.3 A resolution,". Biochemistry. 37 (21): 7686–95. doi:10.1021/bi973178f. PMID9601028.
^Hashimoto Y, Nishiyama M, Horinouchi S, Beppu T (October 1994). "Nitrile hydratase gene from Rhodococcus sp. N-774 requirement for its downstream region for efficient expression". Biosci. Biotechnol. Biochem. 58 (10): 1859–65. doi:10.1271/bbb.58.1859. PMID7765511.
^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. PMID17309280.
^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. PMID15741157.{{cite journal}}: CS1 maint: unflagged free DOI (link)
^Scott AI, Roessner CA (August 2002). "Biosynthesis of cobalamin (vitamin B(12))". Biochem. Soc. Trans. 30 (4): 613–20. doi:10.1042/bst0300613. PMID12196148.
^ abKajiwara 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. PMID16866557.
^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. PMID16835730.
^Brindley AA, Raux E, Leech HK, Schubert HL, Warren MJ (June 2003). "A story of chelatase evolution: identification and characterization of a small 13-15-kDa "ancestral" cobaltochelatase (CbiXS) in the archaea". J. Biol. Chem. 278 (25): 22388–95. doi:10.1074/jbc.M302468200. PMID12686546.{{cite journal}}: CS1 maint: unflagged free DOI (link)
