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Science 11 June 2004:
Vol. 304. no. 5677, p. 1561
DOI: 10.1126/science.304.5677.1561n
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This Week in Science
Pushing an object uphill not only requires energy, but also a pawl to prevent backsliding. In the adenosine triphosphate (ATP)-driven calcium pump from sarcoplasmic reticulum, the experimental sign that the pawl has locked into place is that the pair of Ca2+ ions are trapped within the protein so that they cannot exchange with external calcium. Sørensen et al. (p. 1672) describe the crystal structures of two conformational states of the calcium pump, one with a nonhydrolyzable ATP analog, AMPPCP, and another with both adenosine diphosphate and aluminum fluoride bound. These states are analogs of the reaction intermediates prior to and just after the high-energy phosphate group has been transferred from ATP to the enzyme, respectively. In the former state, the calcium is not occluded, while in the latter, it is. The structural change induced by phosphorylation begins as an overall compaction of the cytoplasmic domain, which pulls on two of the transmembrane helices and shuts the door from the Ca2+ ion-binding site to the cytoplasm. Relaxation of the high-energy conformation then releases the ions into the lumen of the sarcoplasmic reticulum, where they are stored until needed to trigger muscle contraction once again.
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Science. ISSN 0036-8075 (print), 1095-9203 (online)
