Pushing H from K to L

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Science  16 May 2003:
Vol. 300, Issue 5622, pp. 1055
DOI: 10.1126/science.300.5622.1055c

The active transport of ions across cell membranes establishes electrochemical gradients that are then used to take in nutrients. Recently, atomic structures of membrane transporters have become available, and Lanyi and Schobert provide a close-up look at one of the early intermediates in the reaction cycle of bacteriorhodopsin. This protein catalyzes the conversion of light (a single photon) into the uphill transport of one proton from the inside to the outside of the cell. Absorption of the photon isomerizes the C13=C14 bond from trans to cis (the K state), which has the detrimental effect of rotating the proton (attached to the nitrogen of the Schiff base) away from nearby water molecule 402. Relaxation leads to the L state, in which the N-H rotates back toward water 402, which serves as a stepping-stone for the proton on its way to aspartate 85 and thence to the extracellular space. The angular and translational movements are small indeed, but they suffice to push a proton around.—GJC

J. Mol. Biol. 328, 439 (2003).

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