title: Calcium ion as a probe of the monovalent cation center of sodium, potassium ATPase.
abstract: Sodium and potassium ion-transport adenosine triphosphatase from dog kidney was incubated with 0.4-2 mM Ca2+ at 23 degrees C for more than 2 min in the absence of monovalent inorganic cations, cooled to 0 degrees C, and phosphorylated from 1 mM Pi with 2.4 mM MgCl2. The resultant phosphoenzyme resembled that obtained by incubating the enzyme with K+ in place of Ca2+ in six respects. It was concluded that Ca2+ can occupy the monovalent cation-binding center for K+. The rate constant for release of Ca2+ from the dephosphoenzyme at 0 degrees C was 0.17 s-1. The rate of release from the phosphoenzyme was at least 7-fold slower. Phosphorylation stabilized the binding of Ca2+ to the enzyme in contrast to its destabilization of the corresponding K X enzyme complex. K-sensitive phosphoenzyme did not respond to free Ca2+. Thus Ca2+ was not easily accepted by nor released from the phosphoenzyme and would not be an effective substrate for transport. A selective barrier against Ca2+ between the monovalent cation binding center and the extracellular solution is proposed. Release of calcium from the dephosphoenzyme yielded a conformation that was not phosphorylated from Pi. The enzyme changed the conformation of its center for phosphorylation before or at the same time that it changed the conformation of its center for ion transport.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002414:Cations, Monovalent; D004285:Dogs; D004533:Egtazic Acid / Q000494:pharmacology; D007679:Kidney Medulla / Q000201:enzymology; D007700:Kinetics; D011188:Potassium / Q000378:metabolism; D011485:Protein Binding; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D002414:Cations, Monovalent; D004533:Egtazic Acid; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium; D002118:Calcium