Anaerobic protoporphyrin biosynthesis does not require incorporation of methyl groups from methionine.
It was recently reported (H. Akutsu, J.-S. Park, and S. Sano, 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 c3. It was suggested that under anaerobic conditions, protoporphyrin IX biosynthesis proceeds via a novel pathway that does not involve coproporphyrinogen III as a precursor but instead may use precorrin-2 (1,3-dimethyluroporphyrinogen III), a siroheme and vitamin B12 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 [14C]methyl-L-methionine under anaerobic conditions in a strict anaerobe (Chlorobium vibrioforme) and a facultative anaerobe (Rhodobacter capsulatus). In both organisms, 14C 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 [14C]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-deprived methyl groups into porphyrins or their precursors is not generally necessary for the anaerobic biosynthesis of protoporphyrin IX-based tetrapyrroles.
Bollivar
D W
DW
Division of Biology and Medicine, Brown University, Providence, Rhode Island 02912, USA.
Elliott
T
T
Beale
S I
SI
D000332:Aerobiosis; D001419:Bacteria / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D002855:Chromatography, Thin Layer; D008715:Methionine / Q000378:metabolism*; D008780:Methyltransferases / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D011524:Protoporphyrins / Q000096:biosynthesis*; D011758:Pyrroles / Q000378:metabolism; D016354:Rhodobacter capsulatus / Q000378:metabolism; D012486:Salmonella typhimurium / Q000235:genetics / Q000378:metabolism; D013050:Spectrometry, Fluorescence; D045725:Tetrapyrroles
D011524:Protoporphyrins; D011758:Pyrroles; D045725:Tetrapyrroles; C033434:protoporphyrin IX monomethyl ester; D008715:Methionine; C028025:protoporphyrin IX; D008780:Methyltransferases; C060508:uroporphyrin-III C-methyltransferase