Conformational change associated with membrane transport

All members of this protein family employ the same fundamental mechanism. The free energy of ATP hydrolysis drives membrane transport by effecting conformational changes associated with the addition and removal of a phosphoryl group at an analogous aspartate site in each protein. 

The above results are summarized as follows (1) the dimer form of the ADP/ATP carrier is a functional unit, (2) three loops, rather than the membrane spanning region, are important for the transport activity of the carrier, (3) pairs of each of these loops act as gates for transport, (4) a pair of the second loop M2 could constitute the primary binding site for transport substrates, and (5) changes in the locations of these loops are responsible for the interconversion of the c- and m-state conformations of the carrier, which is directly associated with its transport activity. 

The last mediator of gastric secretion in the parietal cell is an H+,K+-ATPase (proton or acid pump) which is a member of the phosphorylating class of ion transport ATPases. Hydrolysis of ATP results in ion transport. This chemical reaction induces a conformational change in the protein that allows an electroneutral exchange of cytoplasmic H+ for K+. The pump is activated when associated with a potassium chloride pathway in the canalicular membrane which allows potassium chloride efflux into the extracytoplasmic space, and thus results in secretion of hydrochloric acid at the expense of ATP breakdown. The activity of the pump is determined by the access of K+ on this surface on the pump. In the absence of K+, the cycle stops at the level of the phosphoenzyme [137]. 

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