Leghemoglobin. Kinetic, nuclear magnetic resonance, and optical studies of pH dependence of oxygen and carbon monoxide binding.

The rate of dissociation of oxygen from soybean oxyleghemoglobin a increases about 5-fold between pH 4 and pH 7, with apparent pK = 5.46 and n = 1. The rate of dissociation of carbon monoxide from carbon monoxyleghemoglobin a and the rates of combination of oxygen and carbon monoxide with ferrous leghemoglobin a are all invariant in this range of pH. The optical spectrum of oxyleghemoglobin in the visible region and the resonances of the four heme meso protons (protons of the bridge carbon atoms) in the NMR spectrum of oxyleghemoglobin are also dependent on pH with pK near 5.5. We suggest that protonation of the imidazole of the distal histidine residue (His 61) leads to formation of a hydrogen bond to the bound oxygen molecule which affects the electronic configuration or conformation of the heme and decreases the rate of oxygen dissociation.