STUDIES ON THE BIOSYNTHESIS OF PORPHYRIN AND BACTERIOCHLOROPHYLL BY RHODOPSEUDOMONAS SPHEROIDES. 4. S-ADENOSYLMETHIONINE-MAGNESIUM PROTOPORPHYRIN METHYLTRANSFERASE

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Abstract
An enzyme, S-adenosyl-methlonlne-magnesium-protoporphyrin-methyltransferase, in Rhodopseudomonas spheroides catalyzes the transfer of the methyl group from S-adenosylmethlonine to magnesium-protoporphyrin to form a magnesium-protoporphyrln monomethyl-ester. It is confined to the chromatophores, to which it is firmly bound. It also occurs in chroma-tophores from R. rubrum. No activity could be detected in extracts from R. spheroides grown under high aeration in the dark. Zinc-protoporphyrln, calcium-protoporphyrln, magnesium-mesoporphyrln and magnesium-deuteroporphyrln are also substrates for the enzyme. Ferrous-, ferric-, manganous- and manganic-protoporphyrins are not substrates but strong inhibitors of the methylation of magnesium-protoporphyrin. Metal-free porphyrins are not substrates, but some of them are inhibitors. Both S-adenosylhomocysteine and S-adeno-sylethionine inhibit the reaction competitively.