Studies on the biosynthesis of porphyrin and bacteriochlorophyll by Rhodopseudomonas spheroides. 5. Zinc-protoporphyrin chelatase

Abstract

Zinc-protoporphyrin chelatase, was detected in chromatophores of R. spheroides. It catalyzes the formation of zinc protoporphyrin from ZnCl2 and protoporphyrin. Zinc-protoporphyrin chelatases are present in mitochondria from guinea-pig liver and from rabbit liver, heart and kidney. The rate of the reaction is about 5 times as high in a water-ether emulsion as in an aqueous medium. Methanol and Tween-80 stimulate to a smaller extent. The enzyme is specific for Zn2+ions. Of the salts tested as inhibitors only those with Cu2+, Co2+, Mn2+, and Ca2+ ions are active. When ascorbic acid is present in the assay mixture, ferrochelatase activity can be demonstrated in all the extracts. Under these conditons Fe2+ ions competitively inhibit zinc protoporphyrin formation and Zn2+ ions are a non-competitive inhibitor of heme formation. The specificity of the zinc-protoporphyrin chelatase towards other porphyrins was tested. Unlike protoporphyrin, some porphyrins readily form a Zn complex in the absence of enzyme.