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Publication
Archives of Biochemistry and Biophysics
Paper
Specificity of ligand binding to transport sites: Ca2+ binding to the Ca2+ transport ATPase and its dependence on H+ and Mg2+
Abstract
Ligand binding to transport sites constitutes the initial step in the catalytic cycle of transport ATPases. Here, we consider the well characterized Ca2+ ATPase of sarcoplasmic reticulum (SERCA) and describe a series of Ca2+ binding isotherms obtained by equilibrium measurements in the presence of various H+ and Mg2+ concentrations. We subject the isotherms to statistical mechanics analysis, using a model based on a minimal number of mechanistic steps. The analysis allows satisfactory fits and yields information on occupancy of the specific Ca2+ sites under various conditions. It also provides a fundamental method for analysis of binding specificity to transport sites under equilibrium conditions that lead to tightly coupled catalytic activation. © 2008 Elsevier Inc. All rights reserved.