Dynamics of the Heterogeneous Electron-Transfer Reaction of Cytochrome c552 from Thermus thermophilus. A Time-Resolved Surface-Enhanced Resonance Raman Spectroscopic Study

Abstract

The heterogeneous electron-transfer process of cytochrome c552 (Cyt-c552) adsorbed on an Ag electrode was studied by surface-enhanced resonance Raman (SERR) spectroscopy. Upon adsorption the heme protein Cyt-c552, which acts as an electron carrier in the respiratory chain of Thermus thermophilus, exists in a potential-dependent equilibrium between two conformational states (B1, B2) similar to cytochrome c (Cyt-c) (Wackerbarth et al. Appl. Spectrosc. 1999, 53, 283). From the stationary SERR spectra measured as a function of the potential, the apparent redox potential of state B1 was determined to be −0.044 V (versus saturated calomel electrode) which is 31 mV more negative than the redox potential of Cyt-c552 in solution. On the basis of a model for the interfacial potential distribution that accounts for the potential drop at the redox site of the adsorbed protein, it is concluded that the true redox potential of the adsorbed state B1 is the same as that for the heme protein in solution. This conclusion is consistent with the structural identity of state B1 and Cyt-c552 in solution that is derived from the comparison of the SERR and resonance Raman spectra. On the other hand, the formation of B2 is associated with substantial structural changes of the heme pocket and a large negative shift of the redox potential. The dynamics of the heterogeneous reduction of B1 was studied by time-resolved SERR spectroscopy which combines the spectroscopic measurements with the potential jump technique. In contrast to Cyt-c, the electron transfer was found to be much faster than the conformational transition to B2 so that the data could be analyzed on the basis of a one-step relaxation process. For the formal unimolecular electron-transfer rate constant a value of 4.6 s-1 was obtained. On the basis of the rate constants measured as a function of the overpotential, the reorganization energy was determined to be 0.15 eV. This unusually low value may be due to a strongly diminished contribution of the solvent reorientation for the adsorbed species and the specific heme pocket structure of Cyt-c552 optimized for the electron-transfer reaction. The differences in the electron-transfer mechanism and dynamics compared to Cyt-c are obviously related to the unique structural properties of Cyt-c552 which may be the consequence of the adaptation to the extreme living conditions of the thermophilic bacterium.