Predicted and experimentally determined increase in α-MSH affinity for site-directed mutants. Predicted and experimentally determined increase in affinity (i.e., computed SU and measured pK increase, log(M), respectively) for α-MSH afforded by point mutations in the MC4 receptor. Shown by red bars is the change in affinity predicted by the model utilizing physicochemical descriptions of amino acids of the receptors' transmembrane regions and facing potential ligand binding clefts to be afforded by the indicated point mutations (i.e., the predicted increase in affinity that should be gained by exchanging the indicated amino acids in the MC4 receptor with the corresponding ones in the MC1 receptor). Shown in blue bars is the experimentally determined change in affinity for these mutations vs. the wild-type MC4 receptor. Significance (nonparametric Wilcoxon Rank Sum statistical test ) denoted as follows: * p <0.05; ** p < 0.005; *** p < 0.0005.