Figure 4.

Voltage dependence of apparent affinity for K⁺ and maximum turnover rate. (A) K_0.5 as a function of voltage at 150 mM Na⁺. α1β1 (squares, n = 4), α2β1 (circles, n = 3), α2β2 (triangle, n = 6), α3β2 (stars, n = 3). (B) Maximum turnover rate (I_max/Q_tot) as a function of voltage. The I_max measured in the same experiments at +40 mV averaged 332 ±71 nA (α1β1), 251 ±24 nA (α2β1), 599 ±69 nA (α2β2), and 338 ±81 nA (α3β2). Note uncertainty of K_0.5 for α2β2 at V ≤ −60 mV due to unreliable measurements for α2β2 at very negative potentials because of small endogenous pump signals (c.f. Fig. S1). To see this figure in color, go online.