Characterizing protein conformational transitions of Na, K-ATPase with antibodies by fluorescence spectroscopy

Stationary and time-resolved fluorescence of FITC-Na,K-ATPase is investigated as a function of pH in the presence of different ligands, cations, and the monoclonal anti-FITC antibody 4-4-20. The binding of K+ and of the antibody leads to the same decreased fluorescence intensity level. Antibody binding is observed only under conditions where the enzyme exists in the conformational state F-1, and not in the form of the Na+ or K+ complex or when it is phosphorylated with inorganic phosphate in the presence of Mg2+. For the interpretation of the results it is shown that the fluorophore is not essentially affected by an acidity change of the bound dye, so that pK variations responsible for the observed intensity changes can be excluded in favor of a static quenching process