Biosynthesis of porphyrins and related macrocycles. Part 9. Biosynthesis of the corrin macrocycle of vitamin B12 including the stereochemistry of C-methylation in ring c

Abstract

Earlier work on the biosynthesis of vitamin B12 is outlined; carbon-13 labelling coupled with n.m.r. spectroscopy is used for the current studies with Propionibacterium shermanii. Growth of this organism, administration of precursors, and isolation of vitamin B12 are described. Incorporation experiments with 5-amino[5-13C]laevulinic acid and with [methyl-13C] methionine confirm that the corrin macrocycle is constructed from eight molecules of 5-aminolaevulinic acid (one of which loses C-5) and seven methyl groups derived from methionine. Degradation of the labelled vitamin B12 derived from [methyl-13C]methionine proves that the pro-R methyl group at C-12 of ring c is the one derived from methionine. This result is confirmed by experiments with [methyl-2H3]-methionine which also establish that C-methylation at C-7 and C-12 of vitamin B12 occurs by intact transfer of the S-methyl group from methionine.