Biosynthesis of vitamin B12 in anaerobic bacteria

Abstract

Eubacterium limosum transformed [2‐ 13 C]5‐hydroxybenzimidazole not only into [2‐ 13 C]5‐hy‐droxybenzimidazolylcobamide, but also into [2‐ 13 C]5‐methoxy‐6‐methylbenzimidazolylcobamide and into [2‐ 13 C]5,6‐dimethylbenzimidazolylcobamide (vitamin B12). [2‐ 13 C]5‐Hydroxy‐6‐methyl‐benzimidazole was used by this bacterium to form [2‐ 13 C]5‐hydroxy‐6‐methylbenzimidazolyl‐cobamide, [2‐ 13 C]5‐methoxy‐6‐methylbenzimidazolylcobamide and [2‐ 13 C]5,6‐dimethylbenzimid‐azolylcobamide. The 1 H‐NMR spectrum of the 5,6‐dimethylbenzimidazole isolated from the 13 Clabeled vitamin B12 preparations revealed that the externally added bases had been transformed into the vitamin B12 base almost without dilution of the label. This suggests that 5‐hydroxybenzimidazole and 5‐hydroxy‐6‐methylbenzimidazole are precursors of 5,6‐dimethylbenzimidazole. On the basis of these results, a hypothetical scheme for the biosynthesis of 5,6‐dimethylbenzimidazole via 5‐hydroxybenzimidazole and 5‐hydroxy‐6‐methylbenzimidazole is discussed. This scheme can also explain the formation of the other benzimidazole bases found in natural vitamin B12 analogs.