Structure of myoglobin-ethyl isocyanide. Histidine as a swinging door for ligand entry.

The structure of myoglobin(Fe II)-ethyl isocyanide has been solved at 1.68 A resolution by X-ray crystallography. The isocyano group of the ligand is distorted from the linear conformation observed in solution and in model compounds. Local changes in the protein conformation are also seen. The side-chain of Arg-CD3 moves out into the solvent, and the side-chain of His-E7 swings up and away from the ligand. Both of these side-chains show disorder indicative of dynamic behavior. These outward movements of His-E7 and Arg-CD3 side-chains clear a path from the solvent to the heme iron, suggesting a mechanism for ligand entry.