We have mentioned above that Transmembrane potential develops due to active transport of ions (H+) outside the cell. Since this transport takes place again& a concentration gradient it utilises energy of hydrolysis of ATP. The proton motive force generated by proton pumping provides the driving force for the transport of solutes including cations, anions, amino acids and sugars. Electrical potential and pH measurements of intact plant cells have suggested that proton pumps are localised on the plasma membrane. The plant plasma membrane ATPase is a transmembrane protein composed of a single polypeptide chain of 100 KD. The most possible coupling mechanism between ATP hydrolysis and proton transport is shown in Figure.
Figure: Simplified picture of active transport.
The enzyme exists in two conformations differing in catalytic and transport properties. In conformation I, the transport site faces the cytoplasm and has high affinity for protons. In conformation II the transport site is externally oriented and has low affinity for protons. The enzyme is forced to alternate between these two conformations and to bind and release the transported proton because neither conformation can affect the complete catalytic cycle. In conformation I, the enzyme acts as a kinase; after binding a proton it catalyses the formation of phosphorylated intermediate. In the new state (conformation II) if acts as a phosphatase and after releasing the proton it returns to its original state, conformation I.