Arginine substitution of a cysteine in transmembrane helix M8 converts Na+,K+-ATPase to an electroneutral pump similar to H+,K+-ATPase.

Na,K-ATPase and H,K-ATPase are electrogenic and nonelectrogenic ion pumps, respectively. The underlying structural basis for this difference has not been established, and it has not been revealed how the H,K-ATPase avoids binding of Na at the site corresponding to the Na-specific site of the Na,K-ATPase (site III). In this study, we addressed these questions by using site-directed mutagenesis in combination with enzymatic, transport, and electrophysiological functional measurements. Replacement of the cysteine C932 in transmembrane helix M8 of Na,K-ATPase with arginine, present in the H,K-ATPase at the corresponding position, converted the normal 3Na:2K:1ATP stoichiometry of the Na,K-ATPase to electroneutral 2Na:2K:1ATP stoichiometry similar to the electroneutral transport mode of the H,K-ATPase. The electroneutral C932R mutant of the Na,K-ATPase retained a wild-type-like enzyme turnover rate for ATP hydrolysis and rate of cellular K uptake. Only a relatively minor reduction of apparent Na affinity for activation of phosphorylation from ATP was observed for C932R, whereas replacement of C932 with leucine or phenylalanine, the latter of a size comparable to arginine, led to spectacular reductions of apparent Na affinity without changing the electrogenicity. From these results, in combination with structural considerations, it appears that the guanidine group of the M8 arginine replaces Na at the third site, thus preventing Na binding there, although allowing Na to bind at the two other sites and become transported. Hence, in the H,K-ATPase, the ability of the M8 arginine to donate an internal cation binding at the third site is decisive for the electroneutral transport mode of this pump.