RESONANCE RAMAN STUDIES OF RIESKE-TYPE PROTEINS

Resonance Raman (RR) spectra are reported for the [2Fe-2S] Rieske protein from Thermus thermophilus (TRP) and phthalate dioxygenase from Pseudomonas cepacia (PDO) as a function of pH and excitation wavelength. Depolarization ratio measurements are presented for the RR spectra of spinach ferredoxin (SFD), TRP, and PDO at 74 K. By comparison with previously published RR spectra of SFD, we suggest reasonable assignments for the spectra of TRP and PDO. The spectra of PDO exhibit virtually no pH dependence, while significant changes are observed in TRP spectra upon raising the pH from 7.3 to 10.1. One band near 270 cm-1, which consists of components at 266 cm-1 and 274 cm-1, is attributed to Fe(III)-N(His) stretching motions. We suggest that these two components arise from conformers having a protonated-hydrogen-bonded imidazole (266 cm-1) and deprotonated-hydrogen-bonded imidazolate (274 cm-1) coordinated to the Fe/S cluster and that the relative populations of the two species are pH-dependent; a simple structural model is proposed to account for this behavior in the respiratory-type Rieske proteins. In addition, we have identified RR peaks associated with the bridging and terminal sulfur atoms of the Fe-S-N cluster. The RR excitation profiles of peaks associated with these atoms are indistinguishable from each other in TRP (pH 7.3) and PDO and differ greatly from those of [2Fe-2S] ferredoxins. The profiles are bimodal with maxima near 490 nm and > approx. 550 nm. By contrast, bands associated with the Fe-N stretch show a somewhat different enhancement profile. Upon reduction, RR peaks assigned to Fe-N vibrations are no longer observed, with the resulting spectrum being remarkably similar to that reported for reduced adrenodoxin. This indicates that only modes associated with Fe-S bonds are observed and supports the idea that the reducing electron resides on the iron atom coordinated to the two histidine residues. Taken as a whole, the data are consistent with an S2tFeS2bFe[N(His)]2t structure for the Rieske-type cluster.