When nitric oxide (NO) binds to heme proteins, it exerts a repulsive trans effect on the proximal ligand, resulting in weakening or rupture of the proximal ligand-iron bond. The general question of whether NO binding generates a five-coordinate complex with proximal ligand release is important for the function of enzymes such as guanylate cyclase. This question can be addressed by studying NO binding to the myoglobin cavity mutant H93G, where the proximal histidine has been replaced by glycine. When this protein is expressed in the presence of imidazole (Im), an imidazole molecule occupies the proximal cavity and serves as a ligand to the iron [Barrick, D. (1994) Biochemistry 33, 6546-6554]. This proximal imidazole can be exchanged for a variety of exogenous ligands [DePillis, G.D., Decatur, S. M., Barrick, D., & Boxer, S.G. (1994) J. Am. Chem. Soc. 116, 6981-6982]. While CO binds to H93G(Im) to form a stable six-coordinate complex similar to that of the wild type and NO binds to wild-type myoglobin to form a six-coordinate complex, we find that the binding of NO to H93G(Im) under similar conditions results in the cleavage of the exogenous imidazole-iron bond at neutral pH, leaving a five-coordinate heme-NO complex, H93G-NO, inside the protein. When a large excess of imidazole is added to this five-coordinate NO complex, a six-coordinate complex can be formed; thus, the binding constant of a sixth ligand to the five-coordinate H93G-NO complex can be measured. This is found to be several orders of magnitude smaller than the binding constant of Im to the carbonmonoxy, deoxy, or the metcyano forms of protein. By replacement of Im with methyl-substituted imidazoles which have hindered or strained binding conformations, this binding constant can be reduced further and some of the factors responsible for favoring the five-coordinate form can be elucidated. Thus, the cavity mutant H93G provides a novel model system for studying the factors that control the coordination state of NO complexes of heme proteins and serves as a bridge between synthetic heme model complexes in simple solvents and site-directed mutants in the structured environment found in proteins.