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 Mg(2+) into porphyrins or Fe(2+) 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 K(m) of the other. The suggested sequential mechanism for the reaction is supported by derivative plots of slopes and intercepts. The K(m) for deuteroporphyrin was 21.3mum and that for Co(2+) was 6.13mum. The enzyme incorporated Co(2+), Fe(2+), Zn(2+), Ni(2+) and Mn(2+); Cd(2+) was not incorporated and was an inhibitor, competitive with respect to Co(2+), non-competitive with respect to deuteroporphyrin. The K(i) for Cd(2+) was 0.73mum. Ferrochelatase was inhibited by protohaem, non-competitively with respect to Co(2+) or with respect to deuteroporphyrin. Inhibition by magnesium protoporphyrin was non-competitive with respect to deuteroporphyrin, uncompetitive with respect to Co(2+). The inhibitory concentrations of the metalloporphyrins are lower than those required for the inhibition of delta-aminolaevulate synthetase by protohaem. Fe(2+) 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 Fe(3+) incorporation increases as anaerobic conditions are achieved.