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Photoelectric methods to study charge transfer processes in photosynthetic organisms are reviewed along with comparative analyses of photoelectric reactions in Photosystem I, Photosystem II, and the reaction center of purple bacteria. Particular emphasis is placed on a comparison of dielectrically weighted photoelectric signal amplitudes with local structural parameters obtained from the X-ray diffraction data.

High-field frequency EPR and multifrequency quantum beat studies of the electron transfer intermediates in Photosystem I reveal new details of structure and function that could not be obtained without the enhanced resolution, both spectral and temporal, and sensitivity of these advanced spectroscopic techniques. The results of careful measurements and analyses of the resolved -tensors of the primary donor cation radical and excited triplet state show that their electronic structures differ from those of monomeric chlorophyll . Multifrequency time-resolved EPR, which includes high-field EPR, has made it possible to determine the structure of the transient charge-separated state PA and its orientation in the thylakoid membrane. High-field EPR studies are just beginning to have an impact on photosynthesis research, as the technique has only been widely accessible for the last few years. There are several important advantages of high-field EPR spectroscopy. First is the high g-tensor resolution.

In the pursuit to understand biological function in fundamental processes such as photo-induced charge separation along a chain of electron carriers as in Photosystem I (PS I), detailed knowledge of protein-cofactor interactions is desired in real time. Today, every promising time resolved molecular spectroscopic method is scrutinized, adapted, and exploited to provide that knowledge. The primary processes in photosynthetic charge separation involve the sequential generation of paramagnetic intermediates in the form of radical ion pairs. Hence, magnetic resonance techniques are a specific method of choice and promise the most detailed insight into protein-induced molecular interactions.

This chapter focuses on the kinetics of electron transfer through A in Photosystem I. The techniques used to study this step are described and their relative advantages and disadvantages are discussed. This is followed by a review of recent results that have been obtained since the publication of the 2.5 Å resolution X-ray structure. Older results are also presented where they are relevant to the new data. However, this chapter is not meant as an exhaustive review of the literature, and interested readers are directed to earlier reviews and other chapters in this volume where appropriate. The main focus of the chapter is the factors such as the energetics and structural arrangement of the cofactors that determine the kinetics of the electron transfer through phylloquinone.

Photosynthetic reaction centers are the membrane–protein complexes responsible for the capture and storage of light energy in photosynthetic organisms. These proteins contain a large number of cofactors, mostly chlorophylls, which absorb light and transfer the energy to the core of the complex, where it is used to drive a transmembrane electron transfer reaction. Since the discovery that these complexes are dimeric structures with two virtually identical branches of cofactors extending across the membrane from the primary donor, the focus of a large body of research has been directed toward understanding to what extent electron transfer occurs in the two branches.

In oxygen-evolving organisms, Photosystem I (PS I) catalyzes the light-driven reduction of ferredoxin, a small acidic soluble protein containing a low-potential [2Fe–2S] cluster. Under conditions of iron deprivation, flavodoxin, which contains a FMN cofactor, can replace ferredoxin in some algae and cyanobacteria. The reduction kinetics of ferredoxin and flavodoxin by (PS I) have been studied over the last 10 years with the unique technique of flashabsorption spectroscopy. This chapter describes and discusses the kinetic aspects of these processes, using data obtained with wild type systems as well as with mutants. A detailed summary of all the available kinetic data concerning the effects of the mutations is provided. In the case of the three stromal subunits PsaC, PsaD, and PsaE, this allows one to define structurally the region of the reaction center that is involved in ferredoxin docking, using the 2.5 Å structure of PS I from the cyanobacterium .

An overview is presented of structure/function relationships in the interactions between the small electron transfer proteins ferredoxin (Fd) and flavodoxin (Fld) and the flavoprotein enzyme ferredoxin:NADP reductase (FNR), primarily emphasizing the proteins from the cyanobacterium, , and the higher plant, spinach. Results are summarized from experiments utilizing rapid-reaction kinetic methods (stopped-flow spectrophotometry and laser flash photolysis) involving wild-type and site-specific mutants of these proteins, redox potential determinations, and X-ray crystallography, including the crystal structure of a Fd/FNR complex. These have provided detailed insights into the protein–protein recognition and electron transfer mechanisms utilized by these systems. Fd and Fld bind to FNR within a concave region of the FNR surface that contains the exposed dimethylbenzene ring of the FAD cofactor. In the Fd case, electron transfer between the iron–sulfur and flavin centers proceeds with a maximum rate constant of 5,500 sec via a direct outer-sphere mechanism. Both electrostatic and hydrophobic interactions occur between the proteins, resulting in a precise surface complementarity.

Ferredoxin, reduced by Photosystem I (PS I) in the light, serves as the electron donor for the reduction of NADP to NADPH, of sulfite to sulfide, of nitrite to ammonia and for the reductant-requiring of glutamate and 2-oxoglutarate to glutamate in all oxygenic photosynthetic organisms. Reduced ferredoxin also serves as the electron donor for the reduction of nitrate to nitrite in cyanobacteria. In addition to its role in supplying a source of electrons for the net reduction of oxidized species in reductant-requiring assimilatory pathways, reduced ferredoxin plays an important role, via the ferredoxin/thioredoxin system, in the regulation of carbon assimilation and other pathways. This chapter focuses on the interactions between ferredoxin and six enzymes that utilize reduced ferredoxin as an electron donor (NADP reductase, nitrate reductase, nitrite reductase, glutamate synthase, sulfite reductase, and thioredoxin reductase). The mechanisms of several of these enzymes will also be discussed.

Electron transfer between photosystem I (PS I) and its soluble lumenal electron donors plastocyanin (pc) or cytochrome c (cyt c) is an essential reaction in photosynthetic electron transport that is required to reduce the photooxidized primary donor P700. PS I is an integral light driven plastocyanin (cytochrome c):ferredoxin oxidoreductase that is embedded in the thylakoid membrane, which uses light energy to transport electrons from a lumenal, soluble electron carrier across the membrane to the stromal, soluble electron acceptor ferredoxin. Two types of interactions between PS I and soluble electron transfer donors can be distinguished, namely interactions that are based on electrostatic attraction and hydrophobic contact.

The photosystem I (PS I) complex of plants and algae is a large multisubunit protein complex consisting of nucleusand chloroplast-encoded subunits. Besides its redox cofactors P700, A, A, F, F, and F, the PS I complex also contains a core antenna consisting of 90 chlorophyll and 22 carotenoid molecules. An extensive forward and reverse genetics approach in and has provided important insights into the mechanisms of synthesis of the subunits and their assembly into a functional complex. The picture which emerges from these studies is that a surprisingly large number of nucleus-encoded factors are involved in several post-transcriptional steps of expression of the two large chloroplast-encoded reaction center subunits PsaA and PsaB. Thus 14 factors are required for the maturation of the mRNA through two -splicing reactions, two factors are required for the translation of the mRNA, and one is required for mRNA stability in . Several additional factors, including Ycf3 and Ycf4, are specifically required for the assembly of the PS I complex. With its three [4Fe–4S] clusters, PS I constitutes an important sink for iron. Its specific and early loss following iron deprivation is a phylogenetically conserved process and is preceded by a remodeling of the PS I antenna. Coupled genetic and biochemical approaches have revealed a new link between PS I assembly and the chlorophyll biosynthetic pathway through the identification of Crd1, a di-iron enzyme possibly involved in the aerobic oxidative cyclase reaction of chlorophyll synthesis that is essential for PS I and LHCI accumulation under copper deprivation.