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| - Asymmetric Distribution of Cooperativity in the Binding Cascade of NormalHuman Hemoglobin. 2. Stepwise Cooperative Free Energy
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| - Stepwise cooperative free energies and intermediate Hill coefficients are used to assess thepresence of noncooperative sequences in the database of binding free energies previously obtained forthe eight partially ligated intermediates of human hemoglobin, encompassing a variety of hemesite analogsubstitutions. This analysis is prompted by the observed noncooperative binding of two ligands tohemoglobins that are partially substituted with Zn2+-heme, an analog of deoxy Fe2+-heme (Holt et al.(2005) Biochemistry 44, XXXXX). The results show that noncooperative binding sequences are observedin all hemesite analog studied to date. The noncooperative binding observed in (α2Znβ2FeO2) and (α2FeO2β2Zn) is therefore not a Zn-specific substitution artifact. One of several binding sequences fromsingly to triply ligated hemoglobin is also observed to occur with little or no positive cooperativity. Theseresults demonstrate the variability possible among different ligation pathways in a highly cooperativemulti-subunit system such as hemoglobin. As a direct consequence of this variability, differences amongligation pathways are not always detectable using cooperativity functions based on statistical distributions,such as the Hill coefficient nH. The limitations of Hill coefficient analysis in evaluating cooperativity inintermediates of complex systems is contrasted with the utility of the stepwise binding parameters.
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