Definition: the equation y(1 - y) = [S]n/Kd, where y is the fractional degree of saturation, [S] is the binding ligand concentration, n is the Hill coefficient, and Kd is the dissociation constant for the ligand. The Hill coefficient is a measure of the cooperativity of the protein: the larger the value, the higher the degree of cooperativity. This coefficient cannot be higher than the number of binding sites. For the oxygen binding curve of hemoglobin, an association constant, Ka, is used and the equation becomes y/(1 - y) = Ka[S]n. For human hemoglobin A, n = 2.5.
[Archibald V. Hill]
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