Efficient Light Harvesting by Photosystem II Requires an Optimized Protein Packing Density in Grana Thylakoids

Helmut Kirchhoff,Winfried Haase,Silvia Haferkamp,Herbert Van Amerongen,Andrew A Pascal

doi:10.1074/jbc.m109.077750

Abstract

A recently developed technique for dilution of the naturally high protein packing density in isolated grana membranes was applied to study the dependence of the light harvesting efficiency of photosystem (PS) II on macromolecular crowding. Slight dilution of the protein packing from 80% area fraction to the value found in intact grana thylakoids (70%) leads to an improved functionality of PSII (increased antenna size, enhanced connectivity between reaction centers). Further dilution induces a functional disconnection of light-harvesting complex (LHC) II from PSII. It is concluded that efficient light harvesting by PSII requires an optimal protein packing density in grana membranes that is close to 70%. We hypothesize that the decreased efficiency in overcrowded isolated grana thylakoids is caused by excited state quenching in LHCII, which has previously been correlated with neoxanthin distortion. Resonance Raman spectroscopy confirms this increase in neoxanthin distortion in overcrowded grana as compared with intact thylakoids. Furthermore, analysis of the changes in the antenna size in highly diluted membranes indicates a lipid-induced dissociation of up to two trimeric LHCII from PSII, leaving one trimer connected. This observation supports a hierarchy of LHCII-binding sites on PSII.

Highlights

In a previous study [30], we measured that in our BBY preparations the PSII/ PSI ratio is Ͼ30. From these data the trimeric LHCII/PSIImonomer ratio can be calculated from the equation chl/PSII ϭ R1⁄742 ϩ 63 ϩ 1/301⁄7167; R, LHCII/PSII ratio; the numbers 42, 63, and 167 are the number of chlorophylls bound to trimeric LHCII, PSII, and PSI [4, 7, 31]
Decreasing the protein packing to ϳ70% area occupation, which resembles the value in intact grana thylakoids, improves the light harvesting efficiency of PSII
We hypothesize that this improvement is due to an unbending of LHCII-bound neoxanthin, which is distorted in unfused, overcrowded BBYs where it causes energy quenching

Summary

Introduction

In higher plants and green algae LHCs are integral membrane proteins embedded in the thylakoid membrane, within chloroplast organelles [4] They are tuned to allow very efficient exciton energy transfer at high chlorophyll concentrations (ϳ0.3 mol1⁄7literϪ1) [5]. In addition to energy transfer between pigments within individual LHCs, light harvesting in thylakoid membranes requires efficient intermolecular transfer of excitations (between proteins). This is because these photosynthetic lightharvesting systems are composed of different LHCs, arranged in a modular way [4]. For example the PSII core complex in higher plants is connected to six different LHCs. Current models of PSII in stacked grana thylakoids assume a supercomplex [7] or megacomplex organization [8, 9].

Results

Discussion

Conclusion