Conformation gastric acid secretion

The final step of acid secretion is mediated by H /K -ATPase (E.C. 3.6.1.3.), the so-called gastric proton pump. Shull and Lindgrel reported that this enzyme consists of 1,033 amino acids with a combined molecular weight of 114,012 [9]. It shows about 62% homology of the amino acid sequence of NaVK -ATPase. Both enzymes are phosphorylated by ATP and potassium binding causes dephosphorylation and a conformational change in the protein. In contrast to Na /K -ATPase, which is widely distributed in all mammalian cells, HVK -ATPase is predominantly located at the apical membrane of the parietal cell, although a similar enzyme has... 

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]. 

Pepsinogen is secreted by the chief cells of the stomach. The gastric parietal cells secrete HCl. The acid in the stomach lumen alters the conformation of pepsinogen so that it can cleave itself, producing the active protease pepsin. Thus, the activation of pepsinogen is autocatalytic. 

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