answer 3:5 - Correct! The protein presents positive cooperativity

An experimentally determined ligand binding isotherm steeper than that calculated for a single site proten is strongly indicative of positive cooperativity. If you reached this conclusion from the analysis of the Hill plot, you may want to check the analysis of the residuals.
The Hill coefficient n=1.43 obtained for this ligand binding isoterm is compatible with the hypothesis of a homodimeric cooperative protein, whose ligand binding fucntion is described by the following equation:
Y = [X]/K1 (1 + [X]/K2) / (1 + 2[X]/K1 + [X]2/K1K2)

A non-linear least squares minimization of the above equation yields the following parameters: K1=3; K2=0.3. Since we use dissociation constants, these values indicate that the ligand affinity for the first site is lower than that for the second site. A graphical representation of the fit is depicted below.

This result is satisfactory, as shown by the distribution of the residuals, which cross the zero line at high frequency without evidence for systematic errors.

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