Resonance Raman spectroscopic studies of cytochrome c at charged interfaces

Abstract

The structural changes of cytochrome c bound to charged surfaces have been analyzed by resonance Raman and surface enhanced resonance Raman spectroscopy. These surfaces, which are provided by a silver electrode, heteropolytungstates, and phospholipid vesicles, simulate the binding domain of cytochrome oxidase, the physiological redox partner of cytochrome c. In all model systems, binding of both the oxidated and reduced cytochrome c leads to the formation of two conformational states (I,II). It is shown that the equilibria between these states as well as their individual conformations depend on electrostatic interactions. While state I exhibits the same structure as the uncomplexed cytochrome c, in state II the heme crevice assumes an open structure and the iron-methionine bond is weakened. This structural change is ascribed to the rupture of the Lys-13-Glu-90 salt bridge so that the loop 80–90 swings away from the heme edge and the structure of the heme pocked is loosened. This modification of the heme crevice is accompanied by a negative shift of the redox potential. From the detailed Raman spectroscopic studies of cytochrome c bound to these model systems, characteristic spectral properties of the states I and II are identified. Thus it is possible to analyze the resonance Raman spectra of the cytochrome c/ cytochrome oxidase complex, where the state II can also be detected. These findings suggest that the opening of the heme crevice plays a functional role for the physiological electron transfer process.