Rl. Post et I. Klodos, INTERPRETATION OF EXTRAORDINARY KINETICS OF NA-K+-ATPASE BY A PHASE-CHANGE(), American journal of physiology. Cell physiology, 40(5), 1996, pp. 1415-1423
We interpret at a molecular level an extraordinary response in the tra
nsient kinetics of the phosphointermediate of Na+-K+-ATPase (I. Klodos
, R. L. Post, and B. Forbush III. J. Biol. Chem. 269: 1734-1743, 1994)
. The phosphointermediate comprises two principal states. The partitio
n between these states varies with salt concentration. A jump in salt
concentration changes the partition of some of the molecules more rapi
dly than they interconvert in a steady state at constant salt concentr
ation. We propose that interconversion is limited by free volume in th
e lipid of the surrounding membrane. This lipid is partitioned into ph
ases that vary with salt concentration. Free volume is larger at the i
nterface between these phases than within the phases themselves. Na+-K
+-ATPase molecules are distributed at random in the membrane. When the
phase boundary moves in response to a jump in salt concentration, it
crosses some Na+-K+-ATPase molecules, which transiently experience an
increase in free volume of the surrounding lipid. Thus their phosphoin
termediate states equilibrate more rapidly than at a constant salt con
centration. Functional and structural heterogeneity of Na+-K+-ATPase m
olecules is discussed.