The biosynthesis of vitamin B12 involves enzymic C-methylation of uroporphyrinogen-lII at 8 sites. Earlier work had established that the first 3 methyl groups are introduced at positions 2,7, and 20 in that order. Pulse-labelling experiments with 13C-labelled and unlabelled S-adenosylmethionine show that the remaining 5 C-methyl groups are added in the order 17, 12, 1, 15, 5. The pulse labelling approach depends on assaying the biosynthetic product for extent of 13C-labelling by n.m.r.
Department of Chemistry
University of Cambridge