Photosynthesis Research

, Volume 98, Issue 1, pp 53–80

Photosystem II: The machinery of photosynthetic water splitting

Review

DOI: 10.1007/s11120-008-9345-7

Cite this article as:
Renger, G. & Renger, T. Photosynth Res (2008) 98: 53. doi:10.1007/s11120-008-9345-7

Abstract

This review summarizes our current state of knowledge on the structural organization and functional pattern of photosynthetic water splitting in the multimeric Photosystem II (PS II) complex, which acts as a light-driven water: plastoquinone-oxidoreductase. The overall process comprises three types of reaction sequences: (1) photon absorption and excited singlet state trapping by charge separation leading to the ion radical pair \( {\text{P}}680^{ + \bullet } {\text{Q}}_{\text{A}}^{ - \bullet } \left( { \overset{\wedge}{=}{\text{P}}_{\text{D1}}^{ + \bullet } {\text{Q}}_{\text{A}}^{ - \bullet } } \right) \) formation, (2) oxidative water splitting into four protons and molecular dioxygen at the water oxidizing complex (WOC) with \( {\text{P}}680^{ + \bullet } \) as driving force and tyrosine YZ as intermediary redox carrier, and (3) reduction of plastoquinone to plastoquinol at the special QB binding site with \( {\text{Q}}_{\text{A}}^{ - \bullet } \) acting as reductant. Based on recent progress in structure analysis and using new theoretical approaches the mechanism of reaction sequence (1) is discussed with special emphasis on the excited energy transfer pathways and the sequence of charge transfer steps: \( ^{1} \left( {\text{RC-PC}} \right)^{ *} {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}} {\text{Chl}}_{{{\text{D}}1}}^{ + \bullet } {\text{Pheo}}_{{{\text{D}}1}}^{ - \bullet } {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}}^{ + \bullet } {\text{Chl}}_{{{\text{D}}1}} {\text{Pheo}}_{{{\text{D}}1}}^{ - \bullet } {\text{Q}}_{\text{A}} \to {\text{P}}_{{{\text{D}}2}} {\text{P}}_{{{\text{D}}1}}^{ + \bullet } {\text{Chl}}_{\text{D1}} {\text{Pheo}}_{\text{D1}} {\text{Q}}_{\text{A}}^{ - \bullet } , \) where 1(RC-PC)* denotes the excited singlet state 1P680* of the reaction centre pigment complex. The structure of the catalytic Mn4OXCa cluster of the WOC and the four step reaction sequence leading to oxidative water splitting are described and problems arising for the electronic configuration, in particular for the nature of redox state S3, are discussed. The unravelling of the mode of O–O bond formation is of key relevance for understanding the mechanism of the process. This problem is not yet solved. A multistate model is proposed for S3 and the functional role of proton shifts and hydrogen bond network(s) is emphasized. Analogously, the structure of the QB site for PQ reduction to PQH2 and the energetic and kinetics of the two step redox reaction sequence are described. Furthermore, the relevance of the protein dynamics and the role of water molecules for its flexibility are briefly outlined. We end this review by presenting future perspectives on the water oxidation process.

Keywords

Photosystem IICharge separationOxidative water splittingReductive plastoquinol formationMechanisms

List of abbreviations

Ant

Antenna complexes

BChl

Bacteriochlorophyll

Bphe

Bacteriopheophytin

Chl

Chlorophyll

ChlD1, ChlD2

“accessory” Chlorophylls of the D1- and D2-branches, respectively of the RC

CP

Chlorophyll binding proteins

CP43, CP47

Core antenna subunits of PS II

DFT

Density functional theory

DGDG

Digalactosydiacylglycerol

EET

Excitation energy transfer

Em

Midpoint redox potential

EPR

Electron paramagnetic resonance

ET

Electron transfer

EXAFS

Extended X-ray absorption fine structure

FTIR

Fourier transform infrared

LHC II

Light harvesting complex II

MjLkWl

Detailed symbol for redox states Si of the WOC

MS-EPT

Multiple site electron and proton transfer

NET

Nonadiabatic electron transfer

PS II

Photosystem II

PBRC

RC of purple bacteria

PG

Phosphatidylglycerol

\( {\text{P}}680^{ + \bullet } \)

Oxidized state of the RC pigments

PD1, PD2

“special pair” Chlorophylls of the D1- and D2-branches, respectively, of the RC

PheoD1, PheoD2

Pheophytins of the D1- and D2-branches, respectively, of the RC

PQ

Plastoquinone

PT

Proton transfer

QA, QB

Plastoquinones of the RC

QENS

Quasielastic neutron scattering

RC

Reaction center

1(RC-PC)*, 1P680*

Excited singlet state of the RC pigments

SQDG

Sulfoquinovosyldiacylglycerol

WOC

Water oxidizing complex

Yz, YD

Redox active tyrosines of the D1- and D2-branches, respectively

XRDC

X-ray diffraction crystallography

Copyright information

© Springer Science+Business Media B.V. 2008

Authors and Affiliations

  1. 1.Max Volmer Laboratory for Biophysical ChemistryBerlin Institute of TechnologyBerlinGermany
  2. 2.Institute for Chemistry and BiochemistryFree University BerlinBerlinGermany