title: Nuclear quantum tunneling in the light-activated enzyme protochlorophyllide oxidoreductase.
abstract: 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 light-driven 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 quantum 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 degrees 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 degrees 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.
authors.affiliation: Manchester Interdisciplinary Biocentre, Faculty of Life Sciences, University of Manchester, 131 Princess Street, Manchester M1 7DN, United Kingdom. derren.heyes@manchester.ac.uk
authors.forename: Derren J
authors.initials: DJ
authors.lastname: Heyes
authors.forename: Michiyo
authors.initials: M
authors.lastname: Sakuma
authors.forename: Sam P
authors.initials: SP
authors.lastname: de Visser
authors.forename: Nigel S
authors.initials: NS
authors.lastname: Scrutton
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D000458:Cyanobacteria / Q000201:enzymology*; D008027:Light; D010084:Oxidation-Reduction; D044925:Oxidoreductases Acting on CH-CH Group Donors / Q000737:chemistry* / Q000378:metabolism; D010777:Photochemistry; D011521:Protochlorophyllide / Q000737:chemistry* / Q000378:metabolism; D011522:Protons
chemical_list: D001426:Bacterial Proteins; D011522:Protons; D011521:Protochlorophyllide; D044925:Oxidoreductases Acting on CH-CH Group Donors; C023886:protochlorophyllide reductase