Proton NMR study of methemoglobin and its isolated chains. Effect of the subunit association on the structure of the subunits.

In order to investigate the effect of the alpha beta subunit contacts on the subunit structure of human adult methemoglobin, the hyperfine shifted proton NMR spectra of several high spin complexes (water, cyanate, thiocyanate, formate, fluoride, and nitrite) and low spin complexes (imisazole, azide, and cyanide) of hemoglobin and its isolated subunits were characterized at 220 MHz and 22 degrees C. The spectra of ferric low spin derivatives of the isolated subunits were approximately superimposable on the corresponding hemoglobin spectra. On the other hand, the high spin spectra of the isolated subunits were greatly different from each other. The spectral anomaly in the ferric high spin complexes of the isolated beta subunit were interpreted to indicate other structural change than the hemichrome formation in the beta heme pocket. Difference in the subunit association effect between the high and low spin complexes of the isolated beta subunit was interpreted on the basis of a conformational change of the apoprotein dependent on the spin state of the beta heme iron.