title: Kinetic analysis of nonphotochemical quenching of chlorophyll fluorescence. 1. Isolated chloroplasts.
abstract: Nonphotochemical quenching of chlorophyll fluorescence in plants is indicative of a process that dissipates excess excitation energy from the light-harvesting antenna of photosystem II. The major fraction of quenching is obligatorily dependent upon the thylakoid DeltapH and is regulated by the de-epoxidation state of the xanthophyll cycle carotenoids associated with the light-harvesting complexes. Basic principles of enzyme kinetics have been used to investigate this process in isolated chloroplasts. The extent of quenching was titrated against the estimated thylakoid lumen pH, and a sigmoidal relationship was obtained with a Hill coefficient of 4.5 and a pK of 4.7. Upon de-epoxidation, these parameters changed to 1.6 and 5.7, respectively. Antimycin A suppressed quenching, increasing the Hill coefficient and reducing the pK. The rate of induction of quenching fitted second-order kinetics with respect to illumination time, and the rate constant was dependent upon the DeltapH, the de-epoxidation state, the presence of antimycin, and also the presence of dibucaine, a quenching enhancer. All these data are consistent with the notion that quenching is caused by a conformational transition in a chloroplast thylakoid protein; this transition shows cooperativity with respect to proton binding, and is controlled by de-epoxidation state and various exogenous reagents.
authors.affiliation: Robert Hill Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield S10 2TN, UK.
authors.forename: A V
authors.initials: AV
authors.lastname: Ruban
authors.forename: M
authors.initials: M
authors.lastname: Wentworth
authors.forename: P
authors.initials: P
authors.lastname: Horton
mesh_terms: D002734:Chlorophyll / Q000737:chemistry*; D002736:Chloroplasts / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010940:Plant Proteins / Q000737:chemistry; D011485:Protein Binding; D013050:Spectrometry, Fluorescence / Q000379:methods*; D018724:Spinacia oleracea / Q000737:chemistry; D013816:Thermodynamics; D020524:Thylakoids / Q000737:chemistry*; D013997:Time Factors
chemical_list: D010940:Plant Proteins; D002734:Chlorophyll