Nitric oxide myoglobin: crystal structure and analysis of ligand geometry

The structure of the ferrous nitric oxide form of native sperm whale myoglobin has been determined by X-ray crystallography to 1.7 angstroms resolution. The nitric oxide ligand is bent with respect to the heme plane: the Fe-N-O angle is 112 degrees. This angle is smaller than those observed in model compounds and in lupin leghemoglobin. The exact angle appears to be influenced by the strength of the proximal bond and hydrogen bonding interactions between the distal histidine and the bound ligand. Specifically, the N(epsilon) atom of histidine64 is located 2.8 angstroms away from the nitrogen atom of the bound ligand, implying electrostatic stabilization of the FeNO complex. This interpretation is supported by mutagenesis studies. When histidine64 is replaced with apolar amino acids, the rate of nitric oxide dissociation from myoglobin increases tenfold.