The copper atom in FVIII and its ligands. A single copper atom (gold) is shown with ligands H99 and H1957, whereas an oxygen heteroatom (not shown) of water or hydroxide acts as a third ligand and is hydrogen bonded to the A100 main chain carbonyl and the sidechain hydroxyl of Y105. H161 is too distant to act as a copper ligand. Neighboring residues are displayed as α-carbon ribbons: A1 (red) and A3 (green).