NMR studies of the quaternary structure and heterogeneity of nitrosyl- and methemoglobin

NMR was used to study the quaternary structure of nitrosyl- and methemoglobin, the kinetics and equilibrium behavior of nitric oxide binding, and the oxidation of hemoglobin. The -9.6 ppm (from H2O) resonance was used as a measure of nitrosylhemoglobin molecules in the T quaternary structure. We found that stripped nitrosylhemoglobin is 70% in the T state below pH 6.4, and is in the R state above. Inositol hexaphosphate (IHP) raises this transition point to pH 7.5. For stripped aquomethemoglobin, the T marker at -10 ppm is absent. In IHP, at pH 6.5 all of the molecules are in the T state. At both higher and lower pH they shift to the R state. The intensity decreases to half of its maximum at pH 5.5 and 7.4. The relative affinity of nitric oxide binding to the alpha and beta subunits was inferred from the intensities of the resonances at -12 and -18 ppm. Under conditions in which nitrosylhemoglobin exists in the T state, NO binds to the alpha subunit 10 times more strongly than it does to the beta subunit. The kinetic experiments reveal that it binds to the two subunits at the same rate and that it dissociates at 5 x 10(-3) s-1 from the beta subunit and at 5 x 10(-4) s-1 from alpha subunit. At high pH, the two subunits are ligated at the same rate. Potassium ferricyanide oxidation, at pH 6.0 in the absence of IHP, is about 3 times more favorable for the alpha than the beta subunit. Addition of IHP raises this preferential oxidation slightly. At pH 8.44, both alpha and beta subunits were oxidized at the same rate.