Fig. 6. E. coli cytochrome c maturation system I.

a An illustration of the Ccm I system, composed of eight proteins named CcmABCDEFGH. The system covalently attaches heme molecules to cytochrome c proteins via three functional complexes. b Two models (left and right panels) of one complex: CcmA₂B₂CD engage CcmE (left panel) and disengage CcmE (right panel). which loads a heme from CcmA₂B₂CD and chaperones it to CcmF. Insets show conserved residues implicated for heme binding in CcmC, CcmD, and CcmE, respectively. Conformational differences between these two models are shown in the middle panel, where the backbone of CcmC was used to superimpose the two models. Viewed from the periplasmic side, the two conformations of CcmA₂B₂ are displayed in blue and grey. Movement relative to CcmC is evident in CcmA₂B₂ but not in CcmD. For clarity, CcmE is omitted in this superposition plot. c A view of interactions between CcmCD and CcmE in their engaged structural model shown in the left panel of (a). CcmCD representations are transparent for clarity. The side chains of interacting residues are shown. His130 and Tyr134 of CcmE are shown in the van der Waals representation, and other interacting residues, including Trp114 and Trp119 from the heme-binding WWD domain of CcmC, are shown in the licorice representation. (d, e) Views of a heme molecule bound in the putative binding-pocket in CcmC, using the structural model in which CcmE is bound to CcmA₂B₂CD as the initial apo structure. A pore for heme access in CcmC manifests, where CcmC is rendered in a surface representation (d). The heme is displayed in van der Waals (d) and licorice (e) representations. The vinyl group expected for His130^CcmE attachment is marked in (e).