Fluorescence measurements of nucleotide association with the Na+/K+-ATPase

The Na+/K+-ATPase, a membrane-associated ion pump, uses energy from the hydrolysis of ATP to pump 3 Na+ ions out of and 2 K+ into cells. The dependence of ATP hydrolysis on ATP concentration was measured using a fluorescence coupled-enzyme assay. The dependence on concentration of nucleotide association with the ATPase was examined using ADP and ATP-induced quenching of the fluorescence of ATPase labeled with Cy3-maleimide (Cy3-ATPase) or Alexa Fluor 546 carboxylic acid, succinimidyl ester (AF-ATPase). The kinetics of ATP hydrolysis in the presence of Na+ and K+ exhibited negative cooperativity with a Hill coefficient (nH) of 0.66 and a half-maximal concentration (K0.5) of 61 μM; in the absence of K+, nH was 0.58 and K0.5 was 13 μM. Nucleotide-induced fluorescence quenching exhibited negative cooperativity with an nH of 0.3–0.5. These results suggest that negative cooperativity observed in ATP hydrolysis is attributable to negative cooperativity in nucleotide association to the ATPase. Interaction between AF-ATPase and ATP labeled with Alexa Fluor 647 (AF-ATP) showed significant Förster resonance energy transfer (FRET). These results indicate that the ATPase exists as oligoprotomeric complexes in this preparation, and that this aggregation has significant effects on enzyme function.