Resonance Raman study of sirohydrochlorin and siroheme in sulfite reductases from sulfate reducing bacteria

Soret-excited resonance Raman (RR) spectra are reported for the sirohemes in the oxidized and Cr^II(EDTA)-reduced forms of both desulforubidin from D. baculatus (DSR) and the low molecular weight sulfite reductase from D. vulgaris (lSIR) and for sirohydrochlorin in the oxidized form of desulfoviridin from D. gigas (DSV). Several patterns in the RR spectra of the enzymes can be utilized as signatures for the siroheme/sirohydrochlorin moiety. The active site for DSR and lSIR consists of a siroheme exchange-coupled to a [4Fe-4S]²⁺ cluster. Upon addition of Cr^II(EDTA), the active center of DSR and lSIR undergoes a one-electron and two-electron reduction, respectively. The RR spectra of DSR suggest that the siroheme iron is high spin and 5-coordinate in the oxidized enzyme and probably remains high spin and 5-coordinate upon reduction. The iron in the siroheme of oxidized lSIR changes from a low spin and probably 6-coordinate configuration to a high spin, 5-coordinate complex upon two-electron reduction of the ative site. Close similarities between the RR spectral features of the two-electron-reduced assimilatory sulfite reductases from E. coli and from D. vulgaris (lSIR) are discussed.