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Properties of the Mg-Protoporphyrin IX Monomethyl Ester (Oxidative) Cyclase System ¹

Department of Botany, University of California, Davis, California 95616

Mg-protoporphyrin IX monomethyl ester (oxidative) cyclase, the enzyme system responsible for the formation of the chlorophyll isocyclic ring, exhibits requirements for both essential sulfhydryls and essential disulfides. It is inhibited by N-ethylmaleimide, dithiothreitol, and -mercaptoethanol, but not by sodium arsenite. This enzyme system shows some substrate specificity: (a) the 6-side-chain of the macrocycle can either be a methyl propionate ester, or its -hydroxy or -keto derivatives; (b) the 7-side-chain can either be a propionic acid or a methyl propionate ester; (c) both the 4-vinyl and the 4-ethyl series can serve as substrates, at least at the -keto ester level; (d) the activity appears to be lost if the side-chain in the 2-position is reduced from a vinyl to an ethyl.