Structural Bases for Function in Cytochromes c: AN INTERPRETATION OF COMPARATIVE X-RAY AND BIOCHEMICAL DATA

Abstract

Abstract The tertiary structures of the photosynthetic cytochrome c2 of Rhodospirillum rubrum and eucaryotic mitochondrial cytochrome c are compared, together with data on their physiochemical properties and activities in physiological oxidoreduction systems. The comparison gives rise to the following observations and proposals. 1. R. rubrum cytochrome c2 does not undergo the extensive oxidoreduction-linked conformation change characteristic of eucaryotic cytochromes c, but rather is structurally constrained in a conformation in both oxidation states which is over-all most similar to that of ferrocytochrome c. 2. Upon examination and comparison of the surface structural and charge topography of cytochromes c and c2, it is apparent that they bear by far the closest similarity at their front sides (i.e. facing the heme crevice). This feature manifests itself clearly owing to the absolute invariance of many of the amino acid residues surrounding the perimeter of the heme crevice in both cytochrome c2 and 41 species of eucaryotic cytochrome c. 3. A physiological mechanism for the oxidoreduction of eucaryotic cytochrome c is proposed which is in principle similar to that previously suggested for cytochrome c2 (Sa-lemme, F.R., Freer, S.T., Xuong, Ng.H., Alden, R.A., and Kraut, J. (1973) J. Biol. Chem. 248, 3910–3921). This mechanism involves front side interaction of the cytochrome c molecule with its oxidase and reductase, and direct electron addition to and withdrawal from the heme concomitant with destabilization of the existing heme oxidation state.