Molecular insight into intrinsic heme distortion in ligand binding in hemoprotein.

A pair of myoglobins containing inherently distorted alpha-ethyl-2,4-dimethyldeuteroheme or undistorted 2,4-dimethyldeuteroheme were prepared, and the functional consequence of intrinsic heme deformation was investigated. The visible absorption peaks of the myoglobin bearing the distorted heme exhibited a bathochromic shift, indicating that the heme was deformed in the protein pocket. Ligand affinities for the ferric myoglobin with the distorted heme were found to be higher than those of the myoglobin bearing the undistorted heme. The observation suggested that the iron atom was more displaced toward the proximal histidine to weaken the coordination of the water molecule. In the paramagnetic proton NMR spectrum of ferrous deoxy protein, the deformed heme caused a 3.2 ppm lower-field shift of the proximal histidine signal, supporting an enhanced iron-histidine interaction. The deformed heme in ferrous myoglobin lowered the oxygen and carbon monoxide affinities by 25- and 480-fold, respectively, and caused the cleavage of the iron-histidine bond in a fractional population of the nitric oxide derivative. These results demonstrate a distinctive controlling mechanism for ligand binding by the deformed heme. Upon the heme distortion, the iron atom is more attracted by the proximal histidine to reduce the affinity of exogenous ligands for the ferrous heme.