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 (OCPo) 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 OCPo to a quenching state (OCPq) proceeds via an intermediate, non-quenching state (OCPi), and this reaction sequence can be described by a three-state kinetic model. The conversion of OCPo to OCPi is a photoinduced process with the effective absorption cross section of 4.5×10−3Å2 at 470nm. The transition from OCPi to OCPq is a dark reaction, with the first order rate constant of ~0.1s−1 at 25°C and the activation energy of 21kcal/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%.