FIGURE 9.

CcmI-apocytochrome c₂ interactions in the absence and presence of hemin during the Ccm process. A, shows the Ccm machinery and its two substrates, apocytochrome c₂ and heme, forming holocytochrome c₂. B, depicts the interactions between CcmI and apocytochrome c₂ in the absence and presence of hemin (thought to be provided by CcmE). This hypothetical scheme takes into account all available data to show that the CcmI-2 portion of CcmI binds tightly the C-terminal helix of apocytochrome c₂, bringing its heme-binding site near CcmE (left panel). Upon the availability of heme via CcmE, apocytochrome c₂ binds heme noncovalently to form a b-type cytochrome intermediate that interacts less tightly with CcmI-2 (middle two panels). The subsequent formation of covalent thioether bonds between heme and apocytochrome c₂, via currently unknown steps catalyzed by the remaining components of Ccm System I, yields holocytochrome c₂ (right panel).