title: Substrate sites for the (Na+ + K+)-dependent ATPase.
abstract: Kinetic studies on a rat brain (Na+ + K+)-dependent ATPase (EC 3.6.1.3) preparation demonstrated high-affinity sites for ATP, with a Km near 1 mum, and low affinity sites for ATP, with a Km near 0.5 mM. In addition, the dissociation constant for ATP at the low affinity sites was approached through the ability of ATP to modify the rate of photo-oxidation of the enzyme in the presence of methylene blue; a value of 0.4 mM was obtained. The temperature dependence of the Km values in these two concentration ranges also differed markedly, and the estimated entropy of binding was +27 cal/degree per mol at the high affinity sites, whereas it was -20 cal/degree per mol at the low affinity sites. Moreover, the relative affinities of various congeners of ATP as of the K+ -dependent phosphatase reaction of the enzyme indicated an interaction at the low-affinity sites for ATP: ATP, ADP, CTP, and the [beta-gamma] -imido analog of ATP all competed with Ki values near those for the ATPase reaction at the low affinity sites. Conversely, the Km for nitrophenyl phosphate as a substrate for the phosphatase reaction was near its Ki as a competitor at the low-affinity sites of the ATPase reaction. These observations are incorporated into a reaction scheme with two classes of substrate sites on a dimeric enzyme, manifesting idverse enzymatic and transport characteristics.
mesh_terms: D000227:Adenine Nucleotides / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism* / Q000528:radiation effects; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000201:enzymology; D003568:Cytidine Monophosphate / Q000378:metabolism; D007700:Kinetics; D008027:Light; D009596:Nitrophenols / Q000378:metabolism; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000494:pharmacology; D013696:Temperature; D013816:Thermodynamics
chemical_list: D000227:Adenine Nucleotides; D009596:Nitrophenols; D000255:Adenosine Triphosphate; D012964:Sodium; D000251:Adenosine Triphosphatases; D003568:Cytidine Monophosphate; D011188:Potassium