These analyses suggest that ATNA has the necessary elements to couple the exergonic hydrolysis of ATP with the endergonic transport of Na against its electrochemical gradient. Comparison between Na and Na K ATPases ATNA protein has 64 % identity and 72 % high similarity to AT1A1 protein from guinea pig . However, the differences are not uniformly distributed along their primary structures but grouped as clusters at the amino and carboxyl terminal ends . In addition, ATNA lacks a region of 45 amino acids present in the nucleotide binding domain of all cation K ATPases, including AT1A1 . Some features could explain the functional differences observed between the K independent, ouabain insensitive Na ATPase and the Na K ATPase . The three dimensional structure prediction using CPHmodels 3.0 shows that 45CKR is located between the phosphatase domain and the phosphorylation site in the E2P conformation. In the cation K ATPases, we have proposed that the 45CKR could prevent the approximation of the phosphatase domain to the phosphoryl aspartate, stabilizing the phosphoryl enzyme in its E2P conformation until K is bound.
Once K is bound, it should induce an additional conformational change that permits the phosphatase domain to mTOR inhibitor selleck interact with the aspartyl phosphate and the subsequent dephosphorylation of the enzyme. The absence of 45CKR in ATNA could permit direct dephosphorylation, without K binding,which would explain the K independence ofATNA. Two structural characteristic of ATNA in M1 EC1 and M5 could explain its ouabain resistance. The segments M1 and EC1 of the Na K ATPase ? subunit have been implicated in ouabain binding . Figure 6c shows the pig Na K ATPase holoenzyme bound to ouabain , which interacts closely with the M1 and EC1 segments of the enzyme . These segments show important modifications in the amino acid sequence of ATNA that could preclude ouabain binding. Moreover, the residue Thr 774, present in M5 of AT1A1 , is replaced by Ile 724 in ATNA CAVPO . It has been shown that the single substitution of Thr 774 by alanine transforms the Na K ATPase in an ouabain insensitive enzyme .
Thus, the replacement of this threonine residue by isoleucine in ATNA could result in the Idarubicin characteristic ouabain insensitivity of this enzyme. Phylogenetic analysis of ATNA Sequence alignment of 78 P type ATPases of all sub types described was performed by ClustalW 2.1 and the unrooted dendrogram was drawn with Unrooted.exe. The resulting phylogenetic tree is shown in Fig. 7a. As expected, ten clearly defined branches were identified, corresponding to the ten described sub types. These results situate ATNA in sub type IIC , which includes the four ? isoforms of the Na K ATPase , the two ? isoforms of the H K ATPase , the invertebrate ? subunit of Na K ATPase, and the Na ATPase from the alga Heterosigma akashiwo.