Molecular Organization of Cytochrome c(2) near the Binding Domain of Cytochrome bc(1) Studied by Electron Spin–Lattice Relaxation Enhancement

[Image: see text] Measurements of specific interactions between proteins are challenging. In redox systems, interactions involve surfaces near the attachment sites of cofactors engaged in interprotein electron transfer (ET). Here we analyzed binding of cytochrome c(2) to cytochrome bc(1) by measuring paramagnetic relaxation enhancement (PRE) of spin label (SL) attached to cytochrome c(2). PRE was exclusively induced by the iron atom of heme c(1) of cytochrome bc(1), which guaranteed that only the configurations with SL to heme c(1) distances up to ∼30 Å were detected. Changes in PRE were used to qualitatively and quantitatively characterize the binding. Our data suggest that at low ionic strength and under an excess of cytochrome c(2) over cytochrome bc(1), several cytochrome c(2) molecules gather near the binding domain forming a “cloud” of molecules. When the cytochrome bc(1) concentration increases, the cloud disperses to populate additional available binding domains. An increase in ionic strength weakens the attractive forces and the average distance between cytochrome c(2) and cytochrome bc(1) increases. The spatial arrangement of the protein complex at various ionic strengths is different. Above 150 mM NaCl the lifetime of the complexes becomes so short that they are undetectable. All together the results indicate that cytochrome c(2) molecules, over the range of salt concentration encompassing physiological ionic strength, do not form stable, long-lived complexes but rather constantly collide with the surface of cytochrome bc(1) and ET takes place coincidentally with one of these collisions.