Biosynthesis of vitamin B12 in anaerobic bacteria: Experiments with Eubacterium limosum of the transformation of 5-hydroxy-6-mehtyl-benimidazole, its nucleoside, its cobamides, and of 5-hydroxybenzimidazolylcobamide into vitamin B12

Publication Type

Journal contribution

Authors

Schulze, B.; Vogler, B.; Renz, P.

Year of publication

1998

Published in

Eur. J. Biochem.

Band/Volume

3/254

Page (from - to)

620-5

In anaerobic bacteria 5-hydroxybenzimidazole and 5-hydroxy-6-methylbenzimidazole are precursors of the 5,6-dimethylbenzimidazole moiety of vitamin B12. In order to elucidate the pathway from these bases to vitamin B12, expts. on the transformation of 5-hydroxy-6-methylbenzimidazole, of 5-hydroxy-6-methylbenzimidazole-.alpha.-D-ribofuranoside, of 5-hydroxybenzimidazolylcobamide and of 5-hydroxy-6-methylbenzimidazolycobamide into vitamin B12 were carried out. The vitamin B12 synthesized by the anaerobe Eubacterium limosum in the presence of 5-hydroxy-6- methylbenzimidazole and L-[methyl-13C]methionine was subjected to NMR spectroscopy. It revealed that the Me group at C5 of the 5,6-dimethylbenzimidazole moiety was 13C labeled, whereas the Me group at C6 was unlabeled. This shows that the transformation of 5-hydroxy-6-methylbenzimidazole into the base moiety of vitamin B12 occurs regiospecifically. 5-Hydroxy-6-methylbenzimidazole-.alpha.-D-ribofuranoside as well as 5-hydroxybenzimidazolylcobamide and 5-hydroxy-6-methylbenzimidazolylcobamide were also transformed into vitamin B12 by E. limosum. When 5-hydroxy-6-methylbenzimidazolylcobamide 13C labeled at C2 of the base part and 14C labeled in the ribose was used for this expt., the vitamin B12 obtained from this cobamide was 13C and 14C labeled in the same positions. This demonstrates that the .alpha.-glycosidic bond of the precursor cobamide is not split during the formation of vitamin B12. It can be deduced from these results that the precursor bases are transformed regiospecifically into their .alpha.-nucleotides, and partially into their cobamides. The .alpha.-nucleotides are then transformed into .alpha.-ribazole-5'-phosphate and, subsequently, into vitamin B12. Most likely the cobamides are degraded to the .alpha.-nucleotides before being used for the biosynthesis of vitamin B12. A pathway for the latter process is suggested.