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β-Carotene (Car), cytochrome (Cyt) and a monomeric chlorophyll (Chl) designated as chlorophyll Z, all undergo oxidation in Photosystem (PS) II under some illumination conditions. These components are not part of the direct electron transfer that leads to water oxidation and plastoquinone reduction and are thus designated ‘side-path electron donors.’ Under the usual conditions of PS II function, the quantum yield for the oxidation of these components is low; however, under certain experimental conditions, particularly low temperatures, the dominant reactions can be those involving the side-path donors. Car is a branch point in the side-path electron donation, being oxidized by P (the kinetically competent Chl cation radical), and reduced by Cyt , which is itself reduced by electrons from the pool of plastoquinol, possibly through the Q site. This all occurs on the D2-side of the reaction center. When the Cyt is pre-oxidized, Car is reduced by Chl Z. There are two candidates for Chl Z, the more obvious candidate on the D2 side and the less straightforward candidate on D1 side of the reaction center. The side-pathway is usually rationalized as a photoprotective cycle aimed at removing long-lived P and thus limiting oxidative damage. Based on the low quantum yields, we consider this unlikely. Instead we suggest that the side-path constitutes a photoprotective cycle in which the aim is to reduce the Car cation, rather than P, returning the carotene cation to its unoxidized state, preventing adventitious reactions and allowing it to play its a role as a singlet O quencher in the heart of PS II.

This chapter briefly traces some of the early studies and key findings which have led to our current perception and understanding of Photosystem II, the water:plastoquinone oxidoreductase in oxygenic photosynthesis. Starting with the discovery of oxygen and the idea of two photosystems, the progressive identification of the unique structural and functional aspects of Photosystem II are outlined and related to the corresponding chapters in the book. The aim is to integrate the detailed descriptions in the various chapters in the context of the structure and function of Photosystem II as a whole. The chapter ends with a brief perspective for the future study and application of Photosystem II research.

The distal and extrinsic light-harvesting antennas of Photosystem II (PS II) provide the capability to match electron flow through Photosystem I thus allowing for regulated responses to environmental changes. In this chapter we provide a concise up-to-date description of these antenna complexes and discuss what is known about their function. The cyanobacteria and red algal PS II antennas are phycobiliproteins organized into complex membrane-extrinsic structures called phycobilisomes. A small group of cyanobacteria lack phycobilisomes and instead use the membrane-intrinsic prochlorophyte chlorophyll (Chl) proteins. PS II of eukaryotes is primarily served by members of the light-harvesting complex (LHC) superfamily which has become widely diversified into Chl and Chl antennas. As members of the LHC superfamily, cryptophyte algae also have novel phycobilins in the thylakoid lumen, and dinoflagellate algae have a unique peridinin-Chl protein. Atomic resolution structures of these two antennas and the major plant LHC are opening up a new era in understanding energy transfer.

The CP47 and CP43 subunits of Photosystem II bind chlorophyll a and belong to a family of (bacterio)chlorophyll-binding proteins that serve as core antenna polypeptides in both anoxygenic and oxygenic photosynthesis. Uniquely, both of these proteins possess extended hydrophilic domains that contribute to the environment of the oxygen-evolving complex. Structural studies have shown that these polypeptides are associated with both intrinsic components of the reaction center and with the extrinsic proteins that enhance O evolution under physiological conditions. The biochemical, spectroscopic and molecular techniques that have been used to define these interactions, as well as the functional domains within these proteins, are presented and encompass studies performed on photosynthetic eukaryotes and cyanobacteria.

In 1977, Chua and Gillham (J Cell Biology 74: 441–452) reported for the first time the existence of two chloroplast-encoded proteins within the thylakoid membrane of the green alga , which they termed D-l and D-2. The D1 and D2 proteins are now recognized as the Photosystem II reaction center polypeptides with a key role in binding all of the co-factors involved in photosynthetic water oxidation. In this chapter we summarize some of the biochemical and mutagenesis data that has been instrumental in shaping this view of the D1 and D2 proteins.

In this chapter the structure and function of the extrinsic proteins of Photosystem II (PS II) are examined. Higher plants and green algae contain the 33 kDa manganese-stabilizing protein and the 24 kDa and the 16 kDa extrinsic proteins while the cyanobacteria contain the same manganese-stabilizing protein, cytochrome , and the 12 kDa extrinsic protein. These proteins serve as enhancers of O evolution, optimizing PS II activity at physiological calcium and chloride concentrations. They shield the manganese cluster from exogenous reductants and reactants in the surrounding aqueous phase. A number of molecular, biochemical, and structural studies have been used to probe the structures and functions of these proteins within the photosystem. We will discuss the proposed functional roles for these components, their structure (as deduced from biochemical and X-ray crystallographic studies) and the location of their proposed binding domains within the PS II complex.

Photosystem II (PS II) has a complex arrangement of membrane spanning and soluble subunits responsible for its unique role in oxygenic photosynthesis. Advances in several areas of biology have contributed significantly to our understanding of individual subunits and the PS II complex as a whole. In recent years, the genome sequences of several plants and many cyanobacteria have been completed allowing for comparisons of their genes. This information, along with previous biochemical evidence, has strengthened the conclusion that the PS II protein components are highly conserved, from the primitive cyanobacterium PCC 7421 to the vascular plant . Advances in purification of PS II samples and protein identification have confirmed the presence of several novel proteins and provide new tools for the analysis of PS II in various mutants and under different growth conditions. Recent structural studies have also contributed tremendously to our understanding of the organization of PS II subunits and cofactors. The combination of genomic, proteomic, and structural studies will continue to be a source of new insights into the assembly, regulation and function of PS II. This chapter highlights the contributions of such studies to our understanding of the numerous low molecular weight PS II components.

This chapter reviews our current state of knowledge on the primary electron transfer in Photosystem II (PS II). Properties of chlorophyll (Chl) in solution and the basic features of pigment-pigment and pigment-protein interactions as well as the principles underlying excitation energy transfer and electron transfer are briefly outlined. Using this description as a starting point, and based on recent information available for the spatial arrangement of the cofactors, the general features of light-induced charge separation in PS II are presented. Special attention is given to the unique properties of the photoactive pigment P680, which consists of a special multimeric pigment complex of the form (Chl ) (Pheo) with x = 0, 1 or 2. The possible electronic structures of P680*, P680 and P680 as well as the underlying features that establish the extraordinarily high oxidizing power of P680 are discussed. Evidence is presented that in the first electron transfer event a ‘monomeric’ type Chl within the multichromophoric P680 transfers an electron from its excited singlet state to an associated pheophytin (Pheo) molecule which acts as the primary electron acceptor. This event is followed by rapid spin redistribution, leading to predominant localization of the electron hole on a Chl in P680 designated as P, which is part of a ‘dimeric’ structural motif termed PP and is in close proximity to the redox-active tyrosine Y. The process leading to the formation of the radical ion pair P680 Pheo comprises a cascade of radical pair states of decreasing energy through a sequence of relaxation reactions with the protein environment. The role of the protein environment in the primary charge separation process is emphasized.

The flux of reducing equivalents out of Photosystem II (PS II) occurs through the two-electron gate function catalyzed by the iron-quinone complex on the acceptor side. The mechanism of the two-electron gate has been studied more completely in bacterial reaction centers, where an understanding of function has benefited from a structural context. However, the two-electron gate was discovered in green plants, and a large body of work had suggested that the mechanism and main structural features are similar in the two systems, and this is now confirmed by structures. In PS II a number of additional properties are found, which result from the redox activity of the non-heme iron of the acceptor complex, and from the lability of its ligands. Pending structures for PS II at a higher resolution, much of the discussion on the molecular architecture had borrowed the structural context from the bacterial homologue. One theme in this chapter is the justification for this borrowing that comes from the application of spectroscopic approaches to the PS II acceptor complex. This has been especially successful in studies of the Q-site semiquinone, the magnetic interaction between the semiquinone formed at the site and the iron, and the interaction of external ligands with the iron. A second theme, reflecting the poor stability of the semiquinone of the Q-site in isolated PS II preparations, is the use of indirect approaches, including kinetic studies and structural modeling, to understand the structure-function interface. The crystallographic structures now available provide a gratifying validation of these alternative approaches.

Photosystem II (PS II) contains two redox-active tyrosines, Y and Y, located in homologous locations in the reaction center polypeptides D2 and D1, respectively. These are the best characterized of the enzymatic reactions that involve tyrosyl radical formation. The reasons for this extensive knowledge include the ability in PS II to measure tyrosine oxidation and reduction with nanosecond time resolution from liquid helium to room temperature and in preparations that have been modified by site-directed mutagenesis. The abundance and purity of PS II preparations and the recent ability to crystallize them also allows the application of a wide variety of spectroscopic techniques to the characterization of the redox-active tyrosines. This chapter describes the discovery of these redox components, the kinetics, energetics and mechanism of their oxidation and reduction and detailed information on their spectroscopic characteristics with a particular emphasis on magnetic resonance methods. The close interaction of tyrosine Y with the oxygen-evolving complex has implicated Y directly in the mechanism of water oxidation. The role of this tyrosine in this mechanism has been the source of a great deal of debate and interest in recent years, heightened by the improving resolution of the PS II crystal structure.