Configuration of subunits within crystals of Na, K-ATPase maintained in the frozen-hydrated state.

Two-dimensional crystalline sheets of Na, K-ATPase were studied in the vitrified, frozen-hydrated state by electron microscopy and image processing. The technique of correlation averaging was used to determine the projected structure. The projection map shows asymmetry between the pair of "alpha beta" protomers comprising a dimer of Na, K-ATPase molecules. The two protomers differ in overall density as well as in shape. One protomer has an oblong shape, whereas the other with higher density has a head and a hook region. Such an asymmetry has not been reported by other laboratories. This asymmetry may either be due to the coexistence of two different conformations of the enzyme in the dimeric form or due to the simultaneous existence of two molecular species of Na, K-ATPase.