Nuclear Magnetic Resonance Studies of the Biosynthesis of Vitamin B12

Biosynthesis of vitamin B12. Experiments with the anaerobe Eubacterium limosum and some labelled substrates

1. The anaerobic microorganism Eubacterium limosum (DSM 20 402), grown in the presence of L-[methyl-14C]methionine, produces vitamin B12 not only labeled in the corrin ring, but also in the 5,6-dimethylbenzimidazole moiety. The vitamin B12 obtained in a similar experiment with L-[methyl-13C]methionine showed nine signals in the 13CV NMR spectrum. Seven of these signals are due to the seven 'extra' methyl groups of the corrin ring, two signals originate from the two methyl groups of the 5,6-dimethylbenzimidazole moiety. This result is in contrast to previous experiments with aerobic and aerotolerant organisms, where L-methionine is only the precursor of the seven corrin methyl groups, and the 5,6-dimethylbenzimidazole is formed from riboflavin. 2. E. limosum transforms exogenously added benzimidazole into benzimidazolylcobamide, but with [2-14C]benzimidazole it was shown that benzimidazole is not a precursor of the 5,6-dimethylbenzimidazole moiety of vitamin B12. 3. Radioactivity from [2-14C]glycine is almost exclusively incorporated into the 5,6-dimethylbenzimidazole moiety of vitamin B12, but not into the corrin moiety. This demonstrates that in E. limosum delta-aminolevulinic acid, the precursor of the corrin moiety, is not formed as in many other organisms from succinyl-CoA and glycine.

The discovery of nature's biosynthetic pathways

Uro'gen-III is a key intermediate on the biosynthetic pathways to the vitally important natural pigments haem, chlorophyll and the cytochromes. How the unexpected structure of uro'gen-III is synthesized by living things has long been a major puzzle. Studies based on 13C-labelling are described which show a) that a single intramolecular rearrangement occurs and b) that this step occurs after the open-chain linear tetrapyrrole system has been built. A second study involves stereospecific labelling with deuterium and tritium to elucidate the absolute stereochemistry of the enzymic reaction sequence which produces the vinyl groups of haem. The third and last section of the lecture is focussed on the biosynthetic intermediates lying between uro'gen-III and cobyrinic acid on the pathway to vitamin B12. An octacarboxylic isobacteriochlorin is isolated from a vitamin B12-producing organism and this is shown to be identical with sirohydrochlorin, previously obtained by Kamin and Siegel as the metal-free prosthetic group of certain sulphite-reducing bacteria. The structure and absolute stereochemistry of sirohydrochlorin are studied and comment is made on the evolutionary interest of these findings.