Anaerobic porphyrin biosynthesis

In the experiments of specific deuteration of cytochrome c(3) (D. vulgaris, Miyazaki), it was found that the deuterated methyl group of methionine-methyl-da in the medium was incorporated not only into the methionine residues of the polypeptide but also into the C-2 and C-7 positions of the heme groups. It shows that methionine is involved in the biosynthesis of porphyrin in this sulfate-reducing bacterium. In the known porphyrin biosynthetic pathway, all four methyl groups of protoporphyrin IX are transformed from acetate groups by uroporphyrinogen decarboxylase. This pathway cannot explain the incorporation of methyl groups into the C-2 and C-7 positions from methionine. Therefore, our result is the strong evidence for the existence of previously unknown pathway of anaerobic porphyrin biosynthesis in D. vulgaris. Intermediates from uroporphyrinogen III to coproporphyrinogen III would be formed through methylation at the C-2 and C-7 positions, followed by decarboxylation of the acetate groups at the C and D rings.