D455F and D455F double mutants exhibit high apparent proton affinity for activation. Increasing proton concentration response curves of activation. The pH_50a values were 6.76 for D455F (red curve), 7.00 for D455F/H28R (blue curve), 7.03 for D455F/H28C (pink curve), and 6.99 for D455F/H32C (green curve) versus 6.68 for the WT hASIC1a (black curve). Lines are the fit to the Hill function. Each data point represents three to six independent measurements.