Protein Machineries Involved in the Attachment of Heme to Cytochrome c: Protein Structures and Molecular Mechanisms

Figure 6

: Alternative thioreduction pathways which may be operative in System I and hypothesized on the basis of structural and functional characterization of the redox-active Ccm proteins from P. aeruginosa [22, 23, 25]. Scheme 1 is a linear redox cascade whereby CcmG is the direct reductant of CcmH, which reduces oxidized apoCyt. Scheme 2 envisages a more complex scenario involving the formation of a mixed-disulfide complex between CcmH and apoCyt (Step 1). This complex is the substrate for the attack by reduced CcmG (Step 2) that liberates reduced apoCyt. The resulting disulfide bond between CcmH and CcmG is then resolved by the free Cys thiol of CcmG (probably Cys77 in Pa-CcmG). Adapted from [25].