電子線結晶学によって明かされた胃H+, K+-ATPaseのラチェット機構

概要

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Acid secretion by the stomach results in a pH of about 1. This highly acidic environment is essential for digestion and also acts as a first barrier against bacterial and viral infections. Conversely, too much acid secretion causes gastric ulcer. The mechanism by which this massive proton gradient is generated is of considerable biomedical interest. In this review, we introduce the first molecular model for this remarkable biological phenomenon. The structure of H+,K+-ATPase at 6.5 Å resolution was determined by electron crystallography of two-dimensional crystals. The structure shows the catalytic α-subunit and the non-catalytic β-subunit in a pseudo-E2P conformation. Different from Na+,K+-ATPase, the N-terminal tail of the β-subunit is in direct contact with the phosphorylation domain of the α-subunit. This interaction may hold the phosphorylation domain in place, thus stabilizing the enzyme conformation and preventing the reverse reaction of the transport cycle. Indeed, truncation of the β-subunit N-terminus allowed the reverse reaction to occur. These results suggest that the N-terminal tail of the β-subunit functions as a “ratchet”, preventing inefficient transport and reverse-flow of protons. We can thus provide a mechanistic explanation for how the H+,K+-ATPase can generate a million-fold proton gradient across the gastric parietal cell membrane, the highest cation gradient known in any mammalian tissue.