ANAEROBIC PROTOPORPHYRIN BIOSYNTHESIS DOES NOT REQUIRE INCORPORATION OF METHYL-GROUPS FROM METHIONINE

It was recently reported (H. Akutsu, J,-S, Park, and S. Sane, J, Am, Chem, Soc. 115:12185-12186, 1993) that in the strict anaerobe Desulfovibrio vulgaris methyl groups from exogenous L-methionine are incorporated specifically into the 1 and 3 positions (Fischer numbering system) on the heme groups of cytochrome c(3). It was suggested that under anaerobic conditions, protoporphyrin IX biosynthesis proceeds via a novel pathway that does not involve coproporphyrinogen III as a precursor hut instead may use precorrin-2 (1,3-dimethyluroporphyrinogen III), a siroheme and vitamin B-12 precursor which is known to be derived from uroporphyrinogen III via methyl transfer from S-adenosyl-L-methionine. We have critically tested this hypothesis by examining the production of protoporphyrin IX-based tetrapyrroles in the presence of exogenous [C-14]methyl-L-methionine under anaerobic conditions in a strict anaerobe (Chlorobium vibrioforme) and a facultative anaerobe (Rhodobacter capsulatus). In both organisms, C-14 was incorporated into the bacteriochlorophyll precursor, Mg-protoporphyrin IX monomethyl ester, However, most of the label was lost upon base hydrolysis of this compound to yield Mg-protoporphyrin IX, These results indicate that although the administered [C-14]methyl-L-methionine was taken up, converted into S-adenosyl-L-methionine, and used for methyl transfer reactions, including methylation of the 6-propionate of Mg-protoporphyrin IX, methyl groups were not transferred to the porphyrin nucleus of Mg-protoporphyrin IX, In other experiments, a cysG strain of Salmonella typhimurium, which cannot synthesize precorrin-2 because the gene encoding the enzyme that catalyzes methylation of uroporphyrinogen III at positions 1 and 3 is disrupted, was capable of heme dependent anaerobic nitrate respiration and growth on the nonfermentable substrate glycerol, indicating that anaerobic biosynthesis of protoporphyrin IX-based hemes does not require the ability to methylate uroporphyrinogen III, Together, these results indicate that incorporation of L-methionine-derived methyl groups into porphyrins or their precursors is not generally necessary for the anaerobic biosynthesis of protoporphyrin IX-based tetrapyrroles.