In chlorophyll biosynthesis, the light-activated enzyme protochlorophyllide oxidoreductase catalyzes trans addition of hydrogen across the C-17-C-18 double bond of the chlorophyll precursor protochlorophyllide (Pchlide). This unique lightdriven reaction plays a key role in the assembly of the photosynthetic apparatus, but despite its biological importance, the mechanism of light-activated catalysis is unknown. In this study, we show that Pchlide reduction occurs by dynamically coupled nuclear

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... antum tunneling of a hydride anion followed by a proton on the microsecond time scale in the Pchlide excited and ground states, respectively. We demonstrate the need for fast dynamic searches to form degenerate "tunneling-ready" configurations within the lifetime of the Pchlide excited state from which hydride transfer occurs. Moreover, we have found a breakpoint at ؊27°C in the temperature dependence of the hydride transfer rate, which suggests that motions/vibrations that are important for promoting light-activated hydride tunneling are quenched below ؊27°C. We observed no such breakpoint for the proton-tunneling reaction, indicating a reliance on different promoting modes for this reaction in the enzyme-substrate complex. Our studies indicate that the overall photoreduction of Pchlide is endothermic and that rapid dynamic searches are required to form distinct tunneling-ready configurations within the lifetime of the photoexcited state. Consequently, we have established the first important link between photochemical and nuclear quantum tunneling reactions, linked to protein dynamics, in a biologically significant system. isotope effect; SIE, solvent isotope effect; DFT, density functional theory.