Nuclear magnetic resonance studies of the biosynthesis of vitamin B12. The origin of the methyl groups on the corrin ring.

The origin of the methyl group on carbon atom 1 of Ring A of the corrin ring of vitamin Blz was investigated by W and lH magnetic resonance spectroscopy. Recently we found by the same technique that this methyl group does not originate from the S-carbon atom of d-aminolevulinic acid. This was of considerable interest, for it has been considered that in the fusion of Rings A and D the potential &methine bridge found in porphyrins, which does arise from d-aminolevulinic acid, is converted to the methyl group found on carbon atom 1 of the corrin ring. Since the methyl group of methionine was previously found to be the source of at least six other methyl groups on the corrin ring, methionine was tested as a possible precursor of the methyl group on carbon atom 1. L-[methylliaC]Methionine was added to a growing culture of Propionibacferium shermanii and vitamin B12 was isolated. The W magnetic resonance spectra of the vitamin B12 in H20, in 0.1 M KCN, and in aqueous HCl (pH < 1) showed seven methylJaC resonances. Single frequency decoupling experiments enabled us to assign a laC resonance to the C-l methyl group. It appears, therefore, that in the synthesis of the corrin ring the aminomethyl group on Ring A or Ring D, which is the potential d-methine bridge carbon atom found in the porphyrin ring, is eliminated and that carbon atom 1 (C-l) of the corrin ring accepts a methyl group provided by L-methionine. This finding should aid in the elucidation of the mechanism of synthesis of the functional isomers of vitamin Blz and of porphyrins. Analysis of our spectra also enabled us to make other resonance assignments and to differentiate between the relative positions in space of the two geminal methyl groups on carbon atom 12 of the corrin ring which arise by different mechanisms.