The crystal structure of the ternary cyanide complex of P450cam and camphor was determined to 1.8 Angstrom resolution and found to be identical with the structure of the active oxygen complex [I. Schlichting et al., 2000, Science 287, 1615]. Notably, cyanide binds in a bent mode and induces the active conformation that is characterized by the presence of two water molecules and a flip of the carbonyl of the conserved Asp251. The structure of the ternary complex of cyanide, L-arginine, and the oxygenase domain of inducible nitric oxide synthase was determined to 2.4 Angstrom resolution. Cyanide binds essentially linearly, interacts with L-Arg, and induces the binding of a water molecule at the active site. This water is positioned by backbone interactions, located 2.8 Angstrom from the nitrogen atom of cyanide, and could provide a proton required for O-O bond scission in the hydroxylation reaction of nitric oxide synthase. (C) 2002 Elsevier Science (USA). All rights reserved.