Surface-enhanced resonance Raman (SERR) spectroscopy was employed to study the potential-dependent processes of the electron-transferring heme protein cytochrome of Thermus thermophilus adsorbed on a silver c 552 (Cyt-c 552 ) electrode. In the reduced state, the SERR spectrum of is very similar to the resonance Raman (RR) Cyt-c 552 spectrum of the dissolved species, ruling out substantial conformational changes due to adsorption. The adsorbed oxidized form, however, exists in di †erent conformational states including species in a Ðve-coordinated high-spin state and a six-coordinated low-spin state which is di †erent from that of the dissolved species. Based on the SERR spectra measured in the potential range between 0.0 and Ô0.2 V (vs. a saturated calomel electrode), an apparent redox potential of Ô0.097 V was obtained which is signiÐcantly 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 Ðndings, which are closely related to those obtained previously for mitochondrial cytochrome c, are discussed on the basis of the speciÐc structural properties of 1998 Cyt-c 552 . ( John Wiley & Sons. Ltd.