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2.4.2.21

Accepted name:

nicotinate-nucleotide—dimethylbenzimidazole phosphoribosyltransferase

Reaction:

β-nicotinate D-ribonucleotide + 5,6-dimethylbenzimidazole = nicotinate + α-ribazole 5′-phosphate

For diagram of the enzyme’s role in corrin biosynthesis, click here

Glossary:

α-ribazole 5′-phosphate = N¹-(5-phospho-α-D-ribosyl)-5,6-dimethylbenzimidazole

Other name(s):

nicotinate mononucleotide-dimethylbenzimidazole phosphoribosyltransferase; nicotinate ribonucleotide:benzimidazole (adenine) phosphoribosyltransferase; nicotinate-nucleotide:dimethylbenzimidazole phospho-D-ribosyltransferase; CobT; nicotinate mononucleotide (NaMN):5,6-dimethylbenzimidazole phosphoribosyltransferase

Systematic name:

nicotinate-nucleotide:5,6-dimethylbenzimidazole phospho-D-ribosyltransferase

Comments:

Also acts on benzimidazole, and the clostridial enzyme acts on adenine to form 7-α-D-ribosyladenine 5′-phosphate. The product of the reaction, α-ribazole 5′-phosphate, forms part of the corrin-biosynthesis pathway and is a substrate for EC 2.7.8.26, adenosylcobinamide-GDP ribazoletransferase [4]. It can also be dephosphorylated to form α-ribazole by the action of EC 3.1.3.73, α-ribazole phosphatase.

Links to other databases:

BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 37277-76-2

References:

1.	Friedmann, H.C. Partial purification and properties of a single displacement trans-N-glycosidase. J. Biol. Chem. 240 (1965) 413–418. [PMID: 14253445]
2.	Friedmann, H.C. and Fyfe, J.A. Pseudovitamin B₁₂ biosynthesis. Enzymatic formation of a new adenylic acid, 7-α-D-ribofuranosyladenine 5′-phosphate. J. Biol. Chem. 244 (1969) 1667–1671. [PMID: 5780835]
3.	Fyfe, J.A. and Friedmann, H.C. Vitamin B₁₂ biosynthesis. Enzyme studies on the formation of the α-glycosidic nucleotide precursor. J. Biol. Chem. 244 (1969) 1659–1666. [PMID: 4238408]
4.	Cameron, B., Blanche, F., Rouyez, M.C., Bisch, D., Famechon, A., Couder, M., Cauchois, L., Thibaut, D., Debussche, L. and Crouzet, J. Genetic analysis, nucleotide sequence, and products of two Pseudomonas denitrificans cob genes encoding nicotinate-nucleotide: dimethylbenzimidazole phosphoribosyltransferase and cobalamin (5′-phosphate) synthase. J. Bacteriol. 173 (1991) 6066–6073. [DOI] [PMID: 1917841]
5.	Cheong, C.G., Escalante-Semerena, J.C. and Rayment, I. Structural investigation of the biosynthesis of alternative lower ligands for cobamides by nicotinate mononucleotide: 5,6-dimethylbenzimidazole phosphoribosyltransferase from Salmonella enterica. J. Biol. Chem. 276 (2001) 37612–37620. [DOI] [PMID: 11441022]
6.	Cheong, C.G., Escalante-Semerena, J.C. and Rayment, I. Capture of a labile substrate by expulsion of water molecules from the active site of nicotinate mononucleotide:5,6-dimethylbenzimidazole phosphoribosyltransferase (CobT) from Salmonella enterica. J. Biol. Chem. 277 (2002) 41120–41127. [DOI] [PMID: 12101181]

[EC 2.4.2.21 created 1972]

2.7.8.26

Accepted name:

adenosylcobinamide-GDP ribazoletransferase

Reaction:

(1) adenosylcobinamide-GDP + α-ribazole = GMP + adenosylcobalamin
(2) adenosylcobinamide-GDP + α-ribazole 5′-phosphate = GMP + adenosylcobalamin 5′-phosphate

For diagram of the enzyme's role in corrin biosynthesis, click here

Other name(s):

CobS; cobalamin synthase; cobalamin-5′-phosphate synthase; cobalamin (5′-phosphate) synthase

Systematic name:

adenosylcobinamide-GDP:α-ribazole ribazoletransferase

Comments:

In Salmonella typhimurium LT2, under anaerobic conditions, CobU (EC 2.7.7.62 and EC 2.7.1.156), CobT (EC 2.4.2.21), CobC (EC 3.1.3.73) and CobS (EC 2.7.8.26) catalyse reactions in the nucleotide loop assembly pathway, which convert adenosylcobinamide (AdoCbi) into adenosylcobalamin (AdoCbl). CobT and CobC are involved in 5,6-dimethylbenzimidazole activation whereby 5,6-dimethylbenzimidazole is converted to its riboside, α-ribazole. The second branch of the nucleotide loop assembly pathway is the cobinamide activation branch where AdoCbi or adenosylcobinamide-phosphate is converted to the activated intermediate AdoCbi-GDP by the bifunctional enzyme Cob U. CobS catalyses the final step in adenosylcobalamin biosynthesis, which is the condensation of AdoCbi-GDP with α-ribazole to yield adenosylcobalamin.

Links to other databases:

BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number: 137672-85-6

References:

1.	Maggio-Hall, L.A. and Escalante-Semerena, J.C. In vitro synthesis of the nucleotide loop of cobalamin by Salmonella typhimurium enzymes. Proc. Natl. Acad. Sci. USA 96 (1999) 11798–11803. [DOI] [PMID: 10518530]
2.	Warren, M.J., Raux, E., Schubert, H.L. and Escalante-Semerena, J.C. The biosynthesis of adenosylcobalamin (vitamin B₁₂). Nat. Prod. Rep. 19 (2002) 390–412. [PMID: 12195810]
3.	Cameron, B., Blanche, F., Rouyez, M.C., Bisch, D., Famechon, A., Couder, M., Cauchois, L., Thibaut, D., Debussche, L. and Crouzet, J. Genetic analysis, nucleotide sequence, and products of two Pseudomonas denitrificans cob genes encoding nicotinate-nucleotide: dimethylbenzimidazole phosphoribosyltransferase and cobalamin (5′-phosphate) synthase. J. Bacteriol. 173 (1991) 6066–6073. [DOI] [PMID: 1917841]

[EC 2.7.8.26 created 2004]

3.1.3.73

Accepted name:

adenosylcobalamin/α-ribazole phosphatase

Reaction:

(1) adenosylcobalamin 5′-phosphate + H₂O = adenosylcobalamin + phosphate
(2) α-ribazole 5′-phosphate + H₂O = α-ribazole + phosphate

For diagram of corrin biosynthesis (part 8), click here

Other name(s):

CobC; adenosylcobalamin phosphatase; α-ribazole phosphatase

Systematic name:

adenosylcobalamin/α-ribazole-5′-phosphate phosphohydrolase

Comments:

This enzyme catalyses the last step in the anaerobic (early cobalt insertion) pathway of adenosylcobalamin biosynthesis, characterized in Salmonella enterica [3].It also participates in a salvage pathway that recycles cobinamide into adenosylcobalamin [1].

Links to other databases:

BRENDA, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 251991-06-7

References:

1.	O'Toole, G.A., Trzebiatowski, J.R. and Escalante-Semerena, J.C. The cobC gene of Salmonella typhimurium codes for a novel phosphatase involved in the assembly of the nucleotide loop of cobalamin. J. Biol. Chem. 269 (1994) 26503–26511. [PMID: 7929373]
2.	Warren, M.J., Raux, E., Schubert, H.L. and Escalante-Semerena, J.C. The biosynthesis of adenosylcobalamin (vitamin B₁₂). Nat. Prod. Rep. 19 (2002) 390–412. [PMID: 12195810]
3.	Zayas, C.L. and Escalante-Semerena, J.C. 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 (2007) 2210–2218. [DOI] [PMID: 17209023]

[EC 3.1.3.73 created 2004, modified 2011]