title: Presence of a low-affinity nucleotide binding site on the (K+ + H+)-ATPase phosphoenzyme.
abstract: The effects of Mg2+ and nucleotides on the dephosphorylation process of the (K+ + H+)-ATPase phosphoenzyme have been studied. Phosphorylation with [gamma-32P]ATP is stopped either by addition of non-radioactive ATP or by complexing of Mg2+ with EDTA. The dephosphorylation process is slow and monoexponential when dephosphorylation is initiated with ATP. When phosphorylation is stopped by complexing of Mg2+ the dephosphorylation process is fast and biexponential. The discrepancy could be explained by a nucleotide mediated inhibition of the dephosphorylation process. The I0.5 for ATP for this inhibition is 0.1 mM and that for ADP is 0.7 mM, suggesting that a low-affinity binding site is involved. When Mg2+ is present in millimolar concentrations in addition to the nucleotides the dephosphorylation process is enhanced. Evidence has been obtained that Mg2+ acts through lowering the affinity for ATP. In contrast to K+, Mg2+ does not stimulate dephosphorylation in the absence of nucleotides. Mg2+ and nucleotides show the same interaction in the dephosphorylation process of a phosphoenzyme generated from inorganic phosphate. These findings suggest the presence of a low-affinity nucleotide binding site on the phosphoenzyme, as is found in the (Na+ + K+)-ATPase phosphoenzyme. This low-affinity binding site may function as a feed-back mechanism in proton transport.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D004492:Edetic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D017506:H(+)-K(+)-Exchanging ATPase; D008274:Magnesium / Q000494:pharmacology; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D011485:Protein Binding; D013552:Swine
chemical_list: C005390:CDTA; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D004492:Edetic Acid; D000251:Adenosine Triphosphatases; D017506:H(+)-K(+)-Exchanging ATPase; D008274:Magnesium; D011188:Potassium