Nicotinate-nucleotide—dimethylbenzimidazole phosphoribosyltransferase

Nicotinate-nucleotide-dimethylbenzimidazole phosphoribosyltransferase (EC 2.4.2.21) is an enzyme that catalyzes the chemical reaction

nicotinate mononucleotide + 5,6-dimethylbenzimidazole

⇌

{\displaystyle \rightleftharpoons }

nicotinic acid + α-ribazole 5'-phosphate

The reaction is part of the biosynthesis of cobalamin (a form of vitamin B12) in bacteria.

This enzyme is a glycosyltransferase, specifically a pentosyltransferase. The systematic name of this enzyme class is nicotinate-nucleotide:5,6-dimethylbenzimidazole phospho-D-ribosyltransferase. Other names in common use include CobT, nicotinate mononucleotide-dimethylbenzimidazole phosphoribosyltransferase, nicotinate ribonucleotide:benzimidazole (adenine) phosphoribosyltransferase, nicotinate-nucleotide:dimethylbenzimidazole phospho-D-ribosyltransferase, and nicotinate mononucleotide (NaMN):5,6-dimethylbenzimidazole phosphoribosyltransferase.

Function

This enzyme plays a central role in the synthesis of α-ribazole 5'-phosphate, an intermediate for the lower ligand of cobalamin. It is one of the enzymes in the anaerobic pathway of cobalamin biosynthesis, and one of the four proteins (CobU, CobT, CobC, and CobS) involved in the synthesis of the lower ligand and the assembly of the nucleotide loop. The reaction replaces the nicotinic acid group in nicotinate mononucleotide by reaction with 5,6-dimethylbenzimidazole (5,6-DMB), with inversion of stereochemistry.

Biosynthesis of cobalamin

Vitamin B12 (cobalamin) is used as a cofactor in a number of enzyme-catalysed reactions in bacteria, archaea and eukaryotes. The biosynthetic pathway to adenosylcobalamin from its five-carbon precursor, 5-aminolaevulinic acid, can be divided into three sections: (1) the biosynthesis of uroporphyrinogen III from 5-aminolaevulinic acid; (2) the conversion of uroporphyrinogen III into the ring-contracted, deacylated intermediate precorrin 6 or cobalt-precorrin 6; and (3) the transformation of this intermediate to form adenosylcobalamin. Cobalamin is synthesised by bacteria and archaea via two alternative routes that differ primarily in the steps of section 2 that lead to the contraction of the macrocycle and excision of the extruded carbon molecule (and its attached methyl group).