The disulfide bridge in the head domain of rhodobacter sphaeroides cytochrome c1 is needed to maintain its structural integrity

Cytochrome c(1) of Rhodobacter sphaeroides ubiquinol-cytochrome c oxidoreductase contains several insertions and deletions that distinguish it from the complex of other higher organisms. Additionally, this bacterial cytochrome c(1) contains two nonconserved cysteines, C145 and C169, with the latter included in the second long insertion located upstream of the sixth heme ligand, M185. The orientation of the insertions and the state of these non-heme binding cysteines remain unknown. Mutating one or both cysteines is found to have comparable effects on the functionality of the cytochrome bc(1) complex. Mutants show an electron transfer activity decreased to a rate that is still high enough to support delayed photosynthetic growth. The mutated cytochrome c(1) has a decreased E(m) without any alteration in the heme ligation environment since none of the mutants binds carbon monoxide. The low E(m) is believed to be caused by a structural modification in the head domain of cytochrome c(1). Analysis of the mutants reveals that the two cysteines form a disulfide bridge. Cleavage of cytochrome c(1) between the two cysteines followed by gel electrophoresis shows two fragments only under reducing conditions, confirming the existence of a disulfide bridge. The disulfide bridge is essential in maintaining the structural integrity of cytochrome c(1) and thus the functionality of the cytochrome bc(1) complex.