The Structure And Biochemistry Character Of The Oxidoreduase CcmH Of Escherichia Coli

CcmH (cytochromes c maturation protein H) is a membrane-anchored thiol oxidoreductase of Escherichia coli. The protein has been hypothesized to be involved in the recognition and reduction of apocytochrome c, which is an essential component of the assembly line necessary for the maturation of c-type cytochromes in the periplasm of Gram-negative bacteria. But the reaction mechanism remains unsolved. CcmH belongs to the thioredoxin superfamily and the N-terminal contains thioredoxin-like domain, including the conserved Cys-X-X-Cys motif in the active site. In order to work out the topology of CcmH, we developed the SCAM assay. Our data indicates the CcmH is a five transmembrane (including signal peptide) protein with the N-terminal and the C-terminal in the periplasm. The redox potential of thioredoxin reflects the capability of reduction or oxidization, indicates the direction of electron transfer. With the GSSG/GSH redox equilibrium system, we calculated the standard potential of CcmH, which is -230mV. Compared to DsbE(-186mV), CcmH is a strong reductase, which indicates electron transduction pathway in vivo: CcmH→CcmG→apocytochrome c.