Ready and Waiting

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Science  30 Jan 2009:
Vol. 323, Issue 5914, pp. 562
DOI: 10.1126/science.323.5914.562b

During the biosynthesis of chlorophyll, the central pigment in photosynthesis, the enzyme protochlorophyllide oxidoreductase (POR) catalyzes the trans addition of hydrogen across the C17=C18 double bond of the chlorophyll precursor protochlorophyllide (Pchlide). The light-activated endothermic reaction involves hydride transfer from the cofactor NADPH to C17 of Pchlide and proton transfer from a conserved tyrosine residue in POR to C18 of Pchlide; the mechanistic intricacies, however, have remained unclear.

Heyes et al. used laser excitation to trigger the addition reaction at temperatures ranging from the physiologic to the cryogenic. Kinetic measurements and density functional theory calculations showed that hydride transfer occurs by quantum-mechanical tunneling and is enabled by a fast dynamical searching for tunneling-ready configurations within the microsecond lifetime of the excited state. Subsequently, proton transfer occurs from the intermediate and involves fast motions that couple to the reaction coordinate. A breakpoint was observed in the temperature dependence of hydride transfer, but not of proton transfer, suggesting that the promoting modes are different for the two steps. — VV

J. Biol. Chem. 10.1074/jbc.M808548200 (2008).

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