| Research Abstract |
Sulfate-reducing bacteria belonging to the genus Desulfovibrio, are included among the domain eubacteria and considered to be closely related phylogenetically to the early living Organisms on earth. The metabolic pathways in Desulfovibrio may represent the early evolution of life. In 1993, Akutsu, Park and Sano discovered that the methyl groups at the C-2 and C-7 positions of heam c in cytochrome c_3 from D.vulgaris Miyazaki F arise, not from C-2 of 5-aminolaevulinic acid, but from the methyl group of L-methionine. This new finding strongly supports the possibility that an alternative pathway from uroporphyrinogen III to protohaem IX via an intermediate, dihydrosirohydrochlorin, could have existed long before aerobic organisms evolved. As shown in this report, culture of D.vulgaris in medium supplemented with 5-aminolaevulinic acid and L-methionine-d_3 resulted in the formation of porphyrins in which the methyl groups at the C-2 and C-7 positions were deuterated, which was confirmed by
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mass spectral analysis. These porphyrins include sirohydrochlorin, coproporphyrin III,protoporphyrin IX.A new compound with hexacarboxylic acid, i.e., (12,18-decarboxy) dihydrosirohydrochlorin which is a putative intermediate between dihydrosirohydrochlorin and coproporphyrinogen III was isolated. This structure was confirmed by high resolution liquid secondary ionization mass spectrometry and ^1H-NMR.These results indicate that D.vulgaris takes the following alternative pathway in heam biosynthesis ; (1) the usual pathway from 5-aminolaevulinic acid to uroporphyrinogen III,(2) di-methylation, which derives from L-methionine, at the C-2 and C-7 positions of uroporphyrinogen III to from dihyd rosirohydrochlorin, (3) decarboxylation of acetate groups at the C-12 and C-18 positions of dihydrosirohydrochlorin to from (12,18-decarboxy) dihydrosirohydrochlorin, (4) elimination of acetate groups from the C-2 and C-7 positions of (12,18-decarboxy) dihydrosirohydrochlorin to from copropohyrinogen III,(5) protoporphyrinogen IX formation, (6) oxidation to form protoporphyrin IX and finally chelation of Fe (II). Recently we succeeded in isolating from the lysate of cultured bacteria three enzymes (uroporphyrinogen-SAM-methyl transferase, precorrin-2 decarboxylase and acetate eliminase) which are involved in this alternative pathway. Less
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