Biosynthesis of vitamin B₁₂ in anaerobic bacteria

Abstract

In anaerobic bacteria 5-hydroxybenzimidazole and 5-hydroxy-6-methylbenzimidazole are precursors of the 5,6-dimethylbenzimidazole moiety of vitamin B 12 . In order to elucidate the pathway from these bases to vitamin B 12, experiments on the transformation of 5-hydroxy-6-methylbenzimidazole, of 5-hydroxy-6-methylbenzimidazole-A-D-ribofuranoside, of 5-hydroxybenzimidazolylcobamide and of 5-hydroxy-6-methylbenzimidazolylcobamide into vitamin B 12 were carried out.The vitamin B 12 synthesized by the anaerobe Eubacterium limosum in the presence of 5-hydroxy-6-methylbenzimidazole and L-[methyl-13 C]methionine was subjected to NMR spectroscopy. It revealed that the methyl group at C5 of the 5,6-dimethylbenzimidazole moiety was 13 C labeled, whereas the methyl group at C6 was unlabeled. This shows that the transformation of 5-hydroxy-6-methylbenzimidazole into the base moiety of vitamin B 12 occurs regiospecifically.5-Hydroxy-6-methylbenzimidazole-A-D-ribofuranoside as well as 5-hydroxybenzimidazolylcobamide and 5-hydroxy-6-methylbenzimidazolylcobamide were also transformed into vitamin B 12 by E. limosum. When 5-hydroxy-6-methylbenzimidazolylcobamide 13 C labeled at C2 of the base part and 14 C labeled in the ribose was used for this experiment, the vitamin B 12 obtained from this cobamide was 13 C and 14 C labeled in the same positions. This demonstrates that the A-glycosidic bond of the precursor cobamide is not split during the formation of vitamin B 12 .It can be deduced from these results that the precursor bases are transformed regiospecifically into their A-nucleotides, and partially into their cobamides. The A-nucleotides are then transformed into A-ribazole-5′-phosphate and, subsequently, into vitamin B 12 . Most likely the cobamides are degraded to the A-nucleotides before being used for the biosynthesis of vitamin B 12 . A pathway for the latter process is suggested.

Phylogenetic Characterization of Microbial Communities That Reductively Dechlorinate TCE Based upon a Combination of Molecular Techniques

“…Defined co-culture studies reported by Drzyzga et al (61) provide evidence of such syntrophic interspecies hydrogen transfer from a Desulfovibrio species to the dehalorespirer Desulfitobacterium frappieri TCE1 during the conversion of PCE to cDCE (with lactate serving as the source of electrons and in the absence of sulfate, as in ANAS). Certain LGCs such as Eubacterium limosum (a major clone type in the ANAS I library) have been reported to generate cofactors including Vitamin B12 (62), which is required by D. ethenogenes strain 195 for dechlorination. Additional studies tracking biochemicals (H2, B12, and organic acids) and employing selective physiological inhibitors will help elucidate how populations interact.…”

CobT and BzaC catalyze the regiospecific activation and methylation of the 5-hydroxybenzimidazole lower ligand in anaerobic cobamide biosynthesis

“…Intriguingly, even though DMB is a symmetric molecule, the nucleotide loop is attached specifically through the nitrogen atom derived from glutamine (58,60). This puzzling result was justified by the hypothesis that an asymmetric intermediate from benzimidazole biosynthesis must undergo regiospecific phosphoribosylation, and the resulting intermediate would be a substrate for subsequent methylations (12,14,61). Because the M. thermoacetica bza operon produces the asymmetric benzimidazoles 5-OHBza and 5-OMeBza, the activities of the MtCobT and MtBzaC provided significant insights into this long-standing puzzle.…”