Resonance Raman studies of Escherichia coli sulfite reductase hemoprotein. 2. Fe4S4 cluster vibrational modes

Resonance Raman (RR) spectra from the hemoprotein subunit of Escherichia coli sulfite reductase (SiR-HP) are examined in the low-frequency (200-500 cm-1) region where Fe-S stretching modes are expected. In spectra obtained with excitation in the siroheme Soret or Q bands, this region is dominated by siroheme modes. Modes assignable to the Fe4S4 cluster are selectively enhanced, however, with excitation at 488.0 or 457.9 nm. The assignments are confirmed by observation of the expected frequency shifts in SiR-HP extracted from E. coli grown on 34S-labeled sulfate. The mode frequencies and isotopic shifts resemble those seen in RR spectra of other Fe4S4 proteins and analogues, but the breathing mode of the cluster at 342 cm-1 is higher than that observed in the other species. Spectra of various ligand complexes of SiR-HP reveal only slight sensitivity of the cluster terminal ligand modes to the presence of exogenous heme ligands, at variance with a model of ligand binding in a bridged mode between heme and cluster. Close examination of RR spectra obtained with siroheme Soret-band excitation reveals additional 34S-sensitive features at 352 and 393 cm-1. These may be attributed to a bridging thiolate ligand.