An Uneven Exchange

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Science  03 Jan 2003:
Vol. 299, Issue 5603, pp. 21
DOI: 10.1126/science.299.5603.21b

The sodium pump (Na+, K+-ATPase) was one of the first active transport proteins identified almost half a century ago. Nevertheless, the structural basis for the recognition of three sodium ions in the outward half of the pumping cycle and of two potassium ions in the inward half is still largely mysterious. Ogawa and Toyoshima have built on their crystallographic analysis of Ca2+-ATPase, a related transport enzyme, by modeling the sodium pump amino acid sequence into the transmembrane domain (containing 10 helices) by homology. They find that the two sites that are known to bind Ca2+ (0.99 Å ionic radius) in the calcium pump are responsible for carrying two (I, II) of the Na+ ions (0.95 Å) and, after slight movements of the helices, both of the K+ ions (1.33 Å). In both cases, the coordinating ligands are either main chain carbonyl oxygens or side chains carboxylates and amides. The third Na+ ion (III) is nearby, and the tilting of helix M5 and the rotation of helix M6 combine to render this site inhospitable to K+. — GJC

Proc. Natl. Acad. Sci. U.S.A. 99, 15977 (2002).

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