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

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Abstract
Interaction of human adult methemoglobin [metHb] with p-mercuribenzoate (pMB) was examined at 21.degree. C by monitoring the hyperfine-shifted proton NMR spectra of several high- and low-spin derivatives. The NMR spectra showed that the heme methyl proton resonances from .beta. subunits in metHb were selectively affected by the binding of pMB regardless of whether the heme Fe was saturated with high-spin or low-spin ligand. Binding of pMB to metHb apparently induced a localized tertiary structural change around the .beta. heme, leaving the .alpha. heme unaffected. Structural change of the .beta. subunit was correlated with an increase in highspin character of the .beta. heme Fe. A model study of the azide-metHb complex suggested that the increase of the high-spin character of the .beta. heme Fe was due to a conformational change of the proximal histidine which weakened the interaction between the heme Fe 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 suggested that the binding of pMB to metHb induced a localized conformational change within the .beta. subunit.