Zeaxanthin Radical Cation Formation in Minor Light-harvesting Complexes of Higher Plant Antenna

Thomas J Avenson,Tae Kyu Ahn,Donatas Zigmantas,Krishna K Niyogi,Zhirong Li,Matteo Ballottari,Roberto Bassi,Graham R Fleming

doi:10.1074/jbc.m705645200

Abstract

Previous work on intact thylakoid membranes showed that transient formation of a zeaxanthin radical cation was correlated with regulation of photosynthetic light-harvesting via energy-dependent quenching. A molecular mechanism for such quenching was proposed to involve charge transfer within a chlorophyll-zeaxanthin heterodimer. Using near infrared (880-1100 nm) transient absorption spectroscopy, we demonstrate that carotenoid (mainly zeaxanthin) radical cation generation occurs solely in isolated minor light-harvesting complexes that bind zeaxanthin, consistent with the engagement of charge transfer quenching therein. We estimated that less than 0.5% of the isolated minor complexes undergo charge transfer quenching in vitro, whereas the fraction of minor complexes estimated to be engaged in charge transfer quenching in isolated thylakoids was more than 80 times higher. We conclude that minor complexes which bind zeaxanthin are sites of charge transfer quenching in vivo and that they can assume Non-quenching and Quenching conformations, the equilibrium LHC(N) <==> LHC(Q) of which is modulated by the transthylakoid pH gradient, the PsbS protein, and protein-protein interactions.

Highlights

Higher plant photosynthesis is initiated by absorption of light in pigment-binding proteins that transfer absorbed solar energy to the reaction centers of photosystems (PS)3 II and I where energy conversion begins [1]
Using near infrared (880 – 1100 nm) transient absorption spectroscopy, we demonstrate that carotenoid radical cation generation occurs solely in isolated minor light-harvesting complexes that bind zeaxanthin, consistent with the engagement of charge transfer quenching therein
Isolation of PSII light-harvesting complexes (LHCs) Containing Specific Xanthophyll Species—Our aim was to determine whether charge transfer (CT) quenching could be supported in isolated LHCs with specific xanthophyll compositions

Summary

Introduction

Higher plant photosynthesis is initiated by absorption of light in pigment-binding (antenna) proteins that transfer absorbed solar energy to the reaction centers of photosystems (PS) II and I where energy conversion begins [1]. Very little qE is exhibited in the A. thaliana mutant referred to as npq which is impaired in its ability to convert V to Z as a result of a lesion in the gene encoding the thylakoid lumen-localized enzyme violaxanthin de-epoxidase [5]. Quantum chemical calculations indicated that the lowest energy, excited singlet state of a chlorophyllzeaxanthin heterodimer ([Chl-Z]), for separations of Յϳ5Å, was a charge transfer (CT) state involving essentially complete transfer of an electron from Z to chlorophyll [9, 10] These findings led to a proposed CT quenching model for the molecular mechanism of qE in which the [Chl-Z] quenches chlorophyll singlet excited states, thereby transiently producing zeaxanthin radical cations (Z1⁄7ϩ) [9, 10]. The quenching of bulk chlorophyll by transfer of energy to a [Chl-Z] quenching complex that undergoes charge separation and subsequent recombination to the ground state provides a simple model for qE [13]

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