Siroheme and Sirohydrochlorin

Abstract The assimilatory NADPH-sulfite reductase (EC 1.8.1.2) of Escherichia coli has been shown to contain a novel heme prosthetic group, characterized as an iron-tetrahydro-porphyrin (isobacteriochlorin type) with eight carboxylate side chains (Murphy, M. J., Siegel, L. M., Kamin, H., and Rosenthal, D. (1973) J. Biol. Chem. 248, 2801). A novel heme with identical spectral properties has now been identified as the prosthetic group of the dissimilatory sulfite reductase of Desulfotomaculum nigrificans (P-582). In contrast, extraction of the dissimilatory sulfite reductase of Desulfovibrio gigas (desulfoviridin) yields a chromophore with absorption and fluorescence spectra and chromatographic properties indistinguishable from those found with the tetrahydroporphyrin moieties obtained by demetallization of the E. coli sulfite reductase and P-582 hemes. Iron can be inserted into the methyl ester of the extracted desulfoviridin chromophore to form a heme ester; absorption spectra of this compound in two solvents are closely similar to those of E. coli sulfite reductase and P-582 hemes under the same conditions. It is concluded that both assimilatory and dissimilatory sulfite reductases contain a common type of tetrahydroporphyrin-based prosthetic group. The name has been given to this new class of tetrahydroporphyrin compound. Since the tetrapyrrolic prosthetic groups of the sulfite reductases of E. coli and Desulfotomaculum nigrificans are clearly iron-chelates of sirohydrochlorin, these compounds have been given the name siroheme. It is not clear as yet whether the tetrapyrrolic prosthetic group of desulfoviridin is present in the enzyme as the free sirohydrochlorin or a metal complex thereof.