A kinetic model of non-photochemical quenching in cyanobacteria

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} Å ² at 470 nm. The transition from OCP ⁱ to OCP ^q is a dark reaction, with the first order rate constant of ~ 0.1 s ^{- 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 ~ 50%.