An iron tetrahydroporphyrin prosthetic group common to both assimilatory and dissimilatory sulfite reductases

The heme² chromophore of the “assimilatory” E. coli sulfite reductase is an iron-octacarboxylic tetrahydroporphyrin of the isobacteriochlorin type (1). Although the two “dissimilatory” sulfite reductases, desulfoviridin and desulforubidin, from the sulfate reducing bacteria Desulfovibrio gigas and Desulfovibrio desulfuricans (Norway strain), have absorption spectra and reaction products which differ from those of E. coli sulfite reductase, the present studies indicate that they contain prosthetic groups with an organic structure closely similar or identical to that of the E. coli sulfite reductase heme. EPR spectra show high-spin ferriheme in all three enzymes. It is clear, however, that the prosthetic groups must reside in substantially different environments within their respective proteins.