Light-Harvesting in Photosystem II

Abstract

Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some ''extra'' Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.

Light-harvesting and structural organization of Photosystem II: From individual complexes to thylakoid membrane

“…However, agreement was never reached about the assignment and interpretation of all the obtained lifetime components, (for an overview see also [36,167]). An additional complication in studies on photosynthetic membranes and chloroplasts is the fact that the thylakoid membranes are heterogeneous and contain both PSI and PSII with their spectra heavily overlapping and reaction kinetics partly occurring on similar time scales [168], making it difficult to distinguish between the various processes taking place [169].…”

“…The migration time is sometimes written as the sum of two terms, namely s mig + s del [36] to take into account the fact that the excitation energy transfer step from nearby antenna pigments to the RC can be much slower than the transfer steps within the antenna. This is due to the fact that the RC pigments are relatively far away from the antenna pigments (see Fig.…”

“…However, the distances between the pigments in these complexes and the ones in the RC (Fig. 2) are so large that it was suggested that the time for EET to the trap should give a substantial contribution to the overall trapping time [36,[63][64][65][66]. More recently, Holzwarth and coworkers [7,67] after studying the core of T. elongatus with transient absorption and fluorescence measurements with increased time resolution, concluded from the presence of several decay processes faster than 10 ps and that precede the main decay process of 40 ps, that charge separation is nevertheless trap-limited.…”

“…We will end this review with some open questions and suggestions for future research. We would also like to refer to various other reviews that have appeared in recent years discussing more/other aspects of PSII [2,3,[33][34][35][36][37][38].…”

STATE TRANSITION7-Dependent Phosphorylation Is Modulated by Changing Environmental Conditions, and Its Absence Triggers Remodeling of Photosynthetic Protein Complexes

“…Our data indicate that under high light and photoautotrophic conditions, the phosphorylation of LHCB4 is changed merely quantitatively by the absence of STT7. LHCB4 is a minor core antenna protein of PSII that is suggested to be a linker between PSII core and LHCBM proteins and likely involved in excitation energy transfer between the trimeric LHCBM proteins and the PSII core (Dainese and Bassi, 1991;Dekker and Boekema, 2005;van Amerongen and Croce, 2013) as well as in state transitions in C. reinhardtii (Tokutsu et al, 2009). Moreover, it has been suggested that the phosphorylation of LHCB4 and LHCB5 is responsible for the detachment of all LHCBM polypeptides from PSII in the transition from state 1 to state 2.…”