These tools will no longer be maintained as of December 31, 2024. Archived website can be found here. PubMed4Hh GitHub repository can be found here. Contact NLM Customer Service if you have questions.


Pubmed for Handhelds

PUBMED FOR HANDHELDS

Search MEDLINE/PubMed

  • Title: Nuclear quantum tunneling in the light-activated enzyme protochlorophyllide oxidoreductase.
    Author: Heyes DJ, Sakuma M, de Visser SP, Scrutton NS.
    Journal: J Biol Chem; 2009 Feb 06; 284(6):3762-7. PubMed ID: 19073603.
    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.
    [Abstract] [Full Text] [Related] [New Search]