Sigmoid shape of the oxygen equilibrium curve and the P50 of human hemoglobin.

The fractional saturation of hemoglobin with oxygen was plotted against P/P50 and the slope of the abscissa at 1 was calculated for 38 OEC data sets of human Hb A. There was a linear correlation between the slope and the Hill coefficient (nmax), and the slope was about one-fourth that of nmax. This implies that the slope of the abscissa at 1 of Y vs P/P50 plot can provide information about the magnitude of cooperativity in hemoglobin oxygen binding.