Na+/K(+)-ATPase with a blocked E1ATP site still allows backdoor phosphorylation of the E2ATP site.

The role of simultaneously existing ATP-binding sites in the catalytic process of Na+/K(+)-ATPase is unclear. In order to learn whether blocking the E1ATP site affects the properties of the E2ATP site, the E1ATP site was inactivated by either fluorescein 5'-isothiocyanate, the non-phosphorylating Cr(H2O)4AdoPP[CH2]P or the phosphorylating Cr(H2O)4ATP. The properties of the remaining E2ATP site were studied by measuring 'backdoor phosphorylation' in the presence of ouabain, or K(+)-activated hydrolysis of p-nitrophenyl phosphate. The involvement of the E2ATP site was further tested by the effects of Co(NH3)4ATP, a specific inactivator of this site. When the E1ATP site was inactivated by fluorescein 5'-isothiocyanate or the non-phosphorylating Cr(H2O)4AdoPP[CH2]P, backdoor phosphorylation and the activity of K(+)-activated p-nitrophenylphosphatase remained unchanged. Both processes were lost, however, when the E2ATP site was additionally inactivated by Co(NH3)4ATP. Inactivation of the E1ATP site by fluorescein 5'-isothiocyanate or Cr(H2O)4AdoPP[CH2]P decreased the affinity of the p-nitrophenylphosphatase activity of the E2ATP site for the substrate p-nitrophenyl phosphate by four times. This is consistent with a former report showing that dephosphorylation in a fluorescein 5'-isothiocyanate-inactivated Na+/K(+)-ATPase has a lowered sensitivity for ATP [Scheiner-Bobis, G., Antonipillai, J. & Farley, R. A. (1993) Biochemistry 32, 9592-9599]. Inactivation of the E1ATP site by the phosphorylating Cr(H2O)4ATP, however, led to a loss of the property of the E2ATP site to hydrolyse K(+)-dependent p-nitrophenyl phosphate and to achieve backdoor phosphorylation. Evidently, ATP sites coexist in Na+/K(+)-ATPase, and binding of ATP to one site affects the property of the other site [Scheiner-Bobis, G., Esmann, M. & Schoner, W. (1989) Eur. J. Biochem. 183, 173-178]. Although the enzyme can be phosphorylated from both ATP sites, phosphorylation of the E1ATP site excludes the phosphorylation of the E2ATP site.