title: Kinetics of plastoquinol oxidation by the Q-cycle in leaves.
abstract: Electrochromic shift measurements confirmed that the Q-cycle operated in sunflower leaves. The slow temporarily increasing post-pulse phase was recorded, when ATP synthase was inactivated in the dark and plastoquinol (PQH(2)) oxidation was initiated by a short pulse of far-red light (FRL). During illumination by red light, the Q-cycle-supported proton arrival at the lumen and departure via ATP synthase were simultaneous, precluding extreme build-up of the membrane potential. To investigate the kinetics of the Q-cycle, less than one PQH(2) per cytochrome b(6)f (Cyt b(6)f) were reduced by illuminating the leaf with strong light pulses or single-turnover Xe flashes. The post-pulse rate of oxidation of these PQH2 molecules was recorded via the rate of reduction of plastocyanin (PC(+)) and P700(+), monitored at 810 and 950 nm. The PSII-reduced PQH(2) molecules were oxidized with multi-phase overall kinetics, τ(d)=1, τ(p)=5.6 and τ(s)=16 ms (22 °C). We conclude that τ(d) characterizes PSII processes and diffusion, τ(p) is the bifurcated oxidation of the primary quinol and τ(s) is the Q-cycle-involving reduction of the secondary quinol at the n-site, its transport to the p-site, and bifurcated oxidation there. The extraordinary slow kinetics of the Q-cycle may be related to the still unsolved mechanism of the "photosynthetic control."
authors.affiliation: Tartu Ülikooli Tehnoloogia Instituut, University of Tartu, Nooruse st. 1, Tartu 50411, Estonia. Electronic address: agu.laisk@ut.ee.
authors.forename: Agu
authors.initials: A
authors.lastname: Laisk
authors.affiliation: Tartu Ülikooli Tehnoloogia Instituut, University of Tartu, Nooruse st. 1, Tartu 50411, Estonia.
authors.forename: Vello
authors.initials: V
authors.lastname: Oja
authors.affiliation: Tartu Ülikooli Tehnoloogia Instituut, University of Tartu, Nooruse st. 1, Tartu 50411, Estonia.
authors.forename: Hillar
authors.initials: H
authors.lastname: Eichelmann
mesh_terms: D000465:Algorithms; D002734:Chlorophyll / Q000378:metabolism; D045346:Cytochrome b6f Complex / Q000378:metabolism*; D045348:Cytochromes f / Q000378:metabolism; D004579:Electron Transport; D006368:Helianthus / Q000378:metabolism / Q000528:radiation effects; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000378:metabolism; D008954:Models, Biological; D010084:Oxidation-Reduction; D010788:Photosynthesis / Q000528:radiation effects; D045331:Photosystem I Protein Complex / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism* / Q000528:radiation effects; D010940:Plant Proteins / Q000378:metabolism*; D010970:Plastocyanin / Q000378:metabolism*; D010971:Plastoquinone / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D002734:Chlorophyll; C003165:plastoquinol; C001785:chlorophyll P 700; D010970:Plastocyanin; D045346:Cytochrome b6f Complex; D045348:Cytochromes f; D010971:Plastoquinone
keywords: Electron transport; Photosynthesis; Plants; Q-cycle