title: A kinetic model of non-photochemical quenching in cyanobacteria.
abstract: High light poses a threat to oxygenic photosynthetic organisms. Similar to eukaryotes, cyanobacteria evolved a photoprotective mechanism, non-photochemical quenching (NPQ), which dissipates excess absorbed energy as heat. An orange carotenoid protein (OCP) has been implicated as a blue-green light sensor that induces NPQ in cyanobacteria. Discovered in vitro, this process involves a light-induced transformation of the OCP from its dark, orange form (OCP(o)) to a red, active form, however, the mechanisms of NPQ in vivo remain largely unknown. Here we show that the formation of the quenching state in vivo is a multistep process that involves both photoinduced and dark reactions. Our kinetic analysis of the NPQ process reveals that the light induced conversion of OCP(o) to a quenching state (OCP(q)) proceeds via an intermediate, non-quenching state (OCP(i)), and this reaction sequence can be described by a three-state kinetic model. The conversion of OCP(o) to OCP(i) is a photoinduced process with the effective absorption cross section of 4.5 × 10(-3)Å² at 470 nm. The transition from OCP(i) to OCP(q) is a dark reaction, with the first order rate constant of approximately 0.1s(-1) at 25°C and the activation energy of 21 kcal/mol. These characteristics suggest that the reaction rate may be limited by cis-trans proline isomerization of Gln224-Pro225 or Pro225-Pro226, located at a loop near the carotenoid. NPQ decreases the functional absorption cross-section of Photosystem II, suggesting that formation of the quenched centers reduces the flux of absorbed energy from phycobilisomes to the reaction centers by approximately 50%.
authors: Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers, the State University of New Jersey, New Brunswick, NJ 08901, USA. gorbunov@marine.rutgers.edu
authors: Maxim Y
authors: MY
authors: Gorbunov
authors: Fedor I
authors: FI
authors: Kuzminov
authors: Victor V
authors: VV
authors: Fadeev
authors: John Dongun
authors: JD
authors: Kim
authors: Paul G
authors: PG
authors: Falkowski
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D000458:Cyanobacteria / Q000737:chemistry / Q000378:metabolism*; D008027:Light; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D010788:Photosynthesis / Q000502:physiology*; D011487:Protein Conformation
chemical_list: D001426:Bacterial Proteins; C109473:orange carotenoid protein, Synechocystis