Allosteric regulation of the light-harvesting system of photosystem II

Non-photochemical quenching of chlorophyll fluorescence (NPQ) is symptomati c of the regulation of energy dissipation by the light-harvesting antenna o f photosystem II (PS II). The kinetics of NPQ in both leaves and isolated c hloroplasts are determined by the transthylakoid Delta pH and the de-epoxid ation state of the xanthophyll cycle. In order to understand the mechanism and regulation of NPQ we have adopted the approaches commonly used in the s tudy of enzyme-catalysed reactions. Steady-state measurements suggest allos teric regulation of NPQ, involving control by the xanthophyll cycle caroten oids of a protonation-dependent conformational change that transforms the P S II antenna from an unquenched to a quenched state. The features of this m odel were confirmed using isolated light-harvesting proteins. Analysis of t he rate of induction of quenching both in vitro and in vivo indicated a bim olecular second-order reaction; it is suggested that quenching arises from the reaction between two fluorescent domains, possibly within a single prot ein subunit. A universal model for this transition is presented based on si mple thermodynamic principles governing reaction kinetics.