The c-type cytochromes are universal heme containing proteins that function typically as electron carriers on the p-side1 (positive) of energy-transducing membrane systems. They are distinguished by the occurrence of heme covalently attached to the polypeptide via two thioether linkages between the vinyl groups of heme and the cysteines sulfhydryls in a CXXCH2 motif in the apocytochrome c. The c-type cytochromes have been the focus of intense investigation from mechanism to structure but the details of their maturation remain poorly understood.
