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. 2008 Nov;132(5):491–505. doi: 10.1085/jgp.200810094

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© 2008 Sweet and Cox

This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.jgp.org/misc/terms.shtml). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/).

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Figure 7. — The two binding sites are less than additive. The mean log ratio of NPopen at −80 mV in the presence and absence of Ca²⁺ for mutants ΔE (solid circles), ΔEΔR (solid triangles), and ΔEΔB_(D2A2) (solid squares) are plotted versus [Ca²⁺]. Various fits of log (NPopen/NPopen_min) are superimposed on the data. The fit of ΔEΔR also displayed in Fig. 2 is shown as a short dashed curve. The fit of ΔEΔB_(D2A2) also displayed in Fig. 3 is shown as a long dashed curve. We simulated the log (NPopen/NPopen_min) relation (dark solid line) predicted by the affinities determined from each of the mutants using Eq. 5. The parameters of the fit were: K_O1 = 0.88 μM, K_C1 = 3.13 μM, K_O2 = 4.88 μM, and K_C2 = 23.2 μM. Also plotted (gray curve) is a fit that incorporates cooperativity between the binding sites. The equation for the fit was log (NPopen/NPopen_min) = ((1+(K_O1+ K_O2)+K_O1K_O2b)⁴) / ((1+(K_C1+ K_C2)+K_C1K_C2a)⁴). The parameters of the fit were: K_O1 = 0.88 μM, K_C1 = 3.13 μM, K_O2 = 4.88 μM, K_C2 = 23.2 μM, a = 1, and b = 0.75. Error bars represent SEM.