title: 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.
authors.affiliation: T.J. Watson Research Center, IBM, Route 134, P.O. Box 218, Yorktown Heights, NY 10598, USA.
authors.forename: Sufi
authors.initials: S
authors.lastname: Zafar
authors.forename: Arif
authors.initials: A
authors.lastname: Hussain
authors.forename: Yueyong
authors.initials: Y
authors.lastname: Liu
authors.forename: David
authors.initials: D
authors.lastname: Lewis
authors.forename: G
authors.initials: G
authors.lastname: Inesi
mesh_terms: D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000502:physiology*; D002412:Cations; D008024:Ligands; D008274:Magnesium / Q000502:physiology*; D008954:Models, Biological; D011485:Protein Binding; D011522:Protons; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology
chemical_list: D002412:Cations; D008024:Ligands; D011522:Protons; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium