Potential-Dependent Surface Enhanced Resonance Raman Spectroscopy of Cytochrome C₅₅₂ from Thermus thermophilus

Surface-enhanced resonance Raman (SERR) spectroscopy was employed to study the potential-dependent processes of the electron-transferring heme protein cytochrome C₅₅₂ (Cyt-c₅₅₂) of Thermus thermophilus adsorbed on a silver electrode. In the reduced state, the SERR spectrum of Cyt-C₅₅₂ is very similar to the resonance Raman (RR) spectrum of the dissolved species, ruling out substantial conformational changes due to adsorption. The adsorbed oxidized form, however, exists in different conformational states including species in a five-coordinated high-spin state and a six-coordinated low-spin state which is different from that of the dissolved species. Based on the SERR spectra measured in the potential range between 0.0 and -0.2 V (vs. a saturated calomel electrode), an apparent redox potential of -0.097 V was obtained which is significantly more negative than the value determined in solution. This discrepancy and the non-Nernstian behavior could be attributed to the coupling of electron-transfer reactions and conformational transitions. These findings, which are closely related to those obtained previously for mitochondrial cytochrome c, are discussed on the basis of the specific structural properties of Cyt-c₅₅₂.