JBC, Vol. 251, Issue 2, 447-458, Jan, 1976
Binding of inositol hexaphosphate to human methemoglobin
J. S. Olson
The observed static difference spectrum produced by inositol hexaphosphate
binding to methemoglobin is the sum of a very fast and a slow spectral
transition. The more rapid absorbance change is too fast to be measured by
stopped flow techniques, whereas the slow change exhibits a half-time in
the range 1 to 6 s. From the pH dependence of the rapidly formed difference
spectrum and from a series of heme ligand binding studies, the rapid phase
is interpreted to reflect a localized tertiary conformational change which
immediately accompanies inositol hexaphosphate binding and results in a
selective increase in spin and reactivity of the beta chain heme groups. In
contrast, the slow phase appears to reflect a first order isomerization
process which involves only a small portion (less than 10%) of the
hemoglobin molecules and results primarily in a marked alteration of the
spectral properties of the alpha chains with little change in spin. While
the rapid spectral transition cannot be directly related to the overall
quaternary transition which occurs during oxygen binding to ferrous
deoxyhemoglobin, the slow spectral transition may represent the abortive
formation of a deoxyhemoglobin A-like conformation which is inhibited in
both rate and extent by the presence of water molecules bound to the heme
iron atoms.
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