The lipid-protein interaction in the membrane-bound Na+, K+-ATPase: a spin label study

The interrelated temperature induced structural changes in the protein and lipid fractions of the Na+, K+-ATPase preparations were studied. A spin-labelled analog of ATP (ATP) was used, where the paramagnetic fragment was attached to the 2' (3')-OH ribose group. It is shown that the rotational mobility of ATP changes with the temperature in a discontinuous way. This correlates with the behaviour of the enzymatic activity of Na+, K+-ATPase and with the state of lipids in the enzyme preparations, both being characterized by a break in the 20-23 degrees C temperature region. When the Mn2+ ions were substituted by Mn2+ a strong magnetic dipolar interaction between the spin label in ATP and the Mn2+ ion was observed, which proves that the complex E-ATP--Mn2+ is formed before the hydrolysis of ATP. The structural changes which occur near 20 degrees C were also observed in the neighbourhood of the SH-groups modified by spin label X, an analog of maleimide. The structural changes observed support the idea that the protein-lipid interactions in the Na+, K+-ATPase provide the relaxation of the system from the unstable state during hydrolysis of ATP and cation transport.