Concerted movement of side chains in the haem vicinity observed on ligand binding in cytochrome c′ from Rhodobacter capsulatus

We have determined the structure of n-butylisocyanide-bound Rhodobacter capsulatus cytochrome c′. This is the first example of a ligand-bound structure of a class IIa cytochrome c. Compared with the structure of native cytochrome c′, there are significant conformational changes of amino acid residues in the haem vicinity, accompanied by a rearrangement of the hydrogen bonding pattern. The results suggest that rearrangements resulting from ligand binding could drive dimer dissociation in some species and also that the haem propionate may participate in proton transfer.