Ferrochelatase of Rhodopseudomonas spheroides

Abstract

Extracts of Rhodopseudomonas spheroides contain two ferrochelatases: one is soluble and forms metalloporphyrins from deuteroporphyrin and haematoporphyrin; the other is particulate and forms metalloporphyrins from protoporphyrin, mesoporphyrin, deuteroporphyrin and haematoporphyrin. Neither enzyme incorporates Mg2+ into porphyrins or Fe2+ into porphyrin cytochrome c. By using the particulate enzyme, plots of 1/v versus 1/s when one substrate was varied and the other kept constant showed that neither substrate affected the Km of the other. The suggested sequential mechanism for the reaction is supported by derivative plots of slopes and intercepts. The Km for deuteroporphyrin was 21.3μm and that for Co2+ was 6.13μm. The enzyme incorporated Co2+, Fe2+, Zn2+, Ni2+ and Mn2+; Cd2+ was not incorporated and was an inhibitor, competitive with respect to Co2+, non-competitive with respect to deuteroporphyrin. The Ki for Cd2+ was 0.73μm. Ferrochelatase was inhibited by protohaem, non-competitively with respect to Co2+ or with respect to deuteroporphyrin. Inhibition by magnesium protoporphyrin was non-competitive with respect to deuteroporphyrin, uncompetitive with respect to Co2+. The inhibitory concentrations of the metalloporphyrins are lower than those required for the inhibition of δ-aminolaevulate synthetase by protohaem. Fe2+ is not incorporated aerobically into porphyrins unless an electron donor, succinate or NADH, is supplied; the low aerobic rate of metalloporphyrin synthesis obtained is insensitive to rotenone and antimycin. The rate of Fe3+ incorporation increases as anaerobic conditions are achieved.