Proton magnetic resonance study of p-mercuribenzoate binding and structural changes in methemoglobin

Interaction of human adult methemoglobin with p-mercuribenzoate (pMB) was examined at 21 degrees C by monitoring the hyperfine-shifted proton nuclear magnetic resonance (NMR) spectra of several high- and low-spin derivatives. The NMR spectra show that the heme methyl proton resonances from the beta subunits in methemoglobin were selectively affected by the binding of pMB regardless of whether the heme iron was saturated with high-spin or low-spin ligand. This observation suggests that the binding of pMB to methemoglobin induces a localized tertiary structural change around the beta heme, leaving the alpha heme unaffected. The structural change of the beta subunit was correlated with an increase in the high-spin character of the beta heme iron. A model study of the azide-methemoglobin complex suggested that the increase of the high-spin character of the beta heme iron is due to a conformational change of the proximal histidine which weakens the interaction between the heme iron and the proximal base. A similar and more pronounced spectral change due to binding of pMB was observed for the isolated beta subunit. The NMR spectral change in the isolated beta subunit also suggests that the binding of pMB to methemoglobin induces a localized conformational change within the beta subunit.