The structural stability of the large cytoplasmic domain (H4 -H5loop) of mouse a1subunit of Na + /K + ATPase (L354– I777), the number and the location of its binding sites for 2¢-3¢-O-(trinitrophenyl) adenosine 5¢-triphosphate (TNP-ATP) andp-nitrophenylphosphate (pNPP) were investi-gated. C- and N-terminal shortening revealed that neither part of the phosphorylation (P)-domain are necessary for TNP-ATP binding. There is no indication of a second ATP site on the P-domain of the isolated loop, even though others reported previously of its existence by TNP-N3ADP affinity labeling of the full enzyme. .
