Biosynthesis of vitamin B12: structure of precorrin-3B, the trimethylated substrate of the enzyme catalysing ring contraction

Abstract

FAB-MS, FTIR spectroscopy, ¹³C labelling and NMR experiments establish the structure of precorrin-3B, a further trimethylated intermediate along the vitamin B₁₂ pathway, thus demonstrating that precorrin-3B is synthesised from precorrin-3 (called precorrin-3A from now) by a complex oxidative reaction involving C-20 hydroxylation and γ-lactone formation from ring-A acetate to C-1, catalysed by the CobG protein in Pseudomonas denitrificans.