The conformation of H,K-ATPase determines the nucleoside triphosphate (NTP) selectivity for active proton transport.
The gastric H,K-ATPase is related to other cation transport ATPases, for example, Na,K-ATPase and Ca-ATPase, which are called E1-E2 ATPases in recognition of conformational transitions during their respective transport and catalytic cycles. Generally, these ATPases cannot utilize NTPs other than ATP for net ion transport activity. For example, under standard assay conditions, rates of NTP hydrolysis and H+ pumping by the H,K-ATPase for CTP are about 10% of those for ATP and undetectable with GTP, ITP, and UTP. However, we observed that H,K-ATPase will catalyze NTP/ADP phosphate exchange at similar rates for all of these NTPs, suggesting that a common phosphoenzyme intermediate is formed. The present study was undertaken to evaluate the specificity of nucleotides to power the H,K-ATPase and several of its partial reactions, including NTP/ADP exchange, K+-catalyzed phosphatase activity, and proton pumping. Results demonstrate that under conditions that promote the conformational change of the K+ bound form of the enzyme, K.E2, to E1, all NTPs tested support K+-stimulated NTPase activity and H+ pumping up to 30-50% of that with ATP. These conditions include (1) the presence of ADP as well as the NTP energy source and (2) reduced K+ concentration on the cytoplasmic side to approximately 0. These data conform to structural models for E1-E2 ATPases whereby adenosine binding promotes the K.E2 to E1 conformational change and K+ deocclusion.
Reenstra
William W
WW
Department of Molecular & Cell Biology, University of California, Berkeley, California 94720, USA.
Crothers
James
J
Forte
John G
JG
D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D005753:Gastric Mucosa / Q000201:enzymology*; D006160:Guanosine Triphosphate / Q000378:metabolism; D017506:H(+)-K(+)-Exchanging ATPase / Q000737:chemistry / Q000378:metabolism*; D017136:Ion Transport; D009711:Nucleotides / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D011487:Protein Conformation; D054328:Proton Pump Inhibitors; D006180:Proton-Translocating ATPases / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D013379:Substrate Specificity; D013552:Swine
D009711:Nucleotides; D054328:Proton Pump Inhibitors; D000244:Adenosine Diphosphate; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D012964:Sodium; D017506:H(+)-K(+)-Exchanging ATPase; D006180:Proton-Translocating ATPases; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium