Fig. 6. (a) Proposed model for P-1 binding in Cs H-NOX. The P O unit hydrogen bonds with H102. At pH 6.5, H102 is protonated and also hydrogen bonds with the backbone carbonyl of P115, rigidifying the corrole in the haem binding pocket. (b) Proposed model for P-1 binding in Pa HasA. The P O unit hydrogen bonds with both Y75 and protonated H83 to form a small hydrogen bonding network. The H32 loop does not engage with the corrole, and FRET studies confirm that the protein adopts an apo-like structure. (c) Docking model between P-1 and the apo HasA structure (PDB ID: 3MOK). The cofactor is flanked by F51 and F78, which may serve as hydrophobic gatekeeping residues for cofactor recognition. Adapted with permission from ref. 3. Copyright 2021 American Chemical Society.