Fig. 5. CrmK structures and proposed mechanism. (A) Cartoon representation of CrmK subunit. The F domain (colored in blue) and S domain (colored in green) are responsible for the binding of FAD and CRM P 7, respectively. FAD and CRM P 7 molecules are shown in stick mode. (B) FAD is bicovalently bound to CrmK via residues His64 and Cys124. In all the subunits of CrmK, the FAD isoalloxazine ring is not planar with a bend of ∼10°. The substrate molecule CRM P 7 (carbon in cyan) was observed in three (A, B, and C) out of four subunits whereas in the subunit D, the product CRM M 4 (carbon in green) was observed in the binding pocket. The electron density 2Fo-Fc map (colored in orange) is contoured at 1.2σ and, for clarity, it covers only His64, Cys124, the isoalloxazine ring, and the bound ligands. (C) The CrmK active site. The residues lining the active site pocket are shown in stick mode. The H-bonds are indicated by black dashed lines. No water molecules were found to mediate the interactions between CRM P 7 and the protein. (D) E376 as an alternative catalytic base (to abstract a proton from the substrate). The potential conformation of the C7 hydroxyl group of CRM P 7 shown here allows for the formation of an H-bond with E376. (E) The proposed catalytic mechanism of CrmK.