Catálogo de publicaciones - libros

The first attempts to elucidate the molecular mechanisms that function in the bioassembly of the bound Fe/S clusters in Photosystem I (PS I) are discussed. Fe/S proteins participate in a wide variety of processes, the most important of which in photosynthetic organisms are light-mediated electron transport and stress-induced regulation of genes. One of the last steps in the biogenesis of PS I involves the assembly of the three bound [4Fe–4S] clusters F, F, and F. It has been shown that the proteins encoded by the regulon are involved in the assembly and repair of the bound Fe/S clusters in cyanobacteria. The SUF system of Fe/S cluster assembly is localized in the chloroplasts of plants; however, no homologs of the genes have been identified in nonphotosynthetic eukaryotes.

This chapter underlines the knowledge accumulated on the assembly of the reducing site of Photosystem I (PS I). A brief description of the biochemical and molecular characteristics of the reducing-site members, the peripheral, cytosol/stromal facing, PsaC, PsaD, and PsaE subunits is followed by their functional characterizations as individual subunits and as members of a cooperative entity.

Methods of obtaining the free energy and enthalpy of the steps of electron transfer through the Photosystem I reaction center are presented. The values are tabulated and discussed from the viewpoint of efficiency of the process. The bacterial system resembles Photosystem I but Photosystem II is quite different.

To understand the engineering of light induced electron transfer and energy conversion, photosystem I (PS I) has been extensively reshaped by isolation, removal of protein subunits, and redox cofactors and in some cases reconstitution of exotic redox centers. Such manipulations together with Marcus theory and its biologically focused empirical derivations show that electron tunneling dominated electron transfer kinetics are established principally by the natural selection of distance between redox centers; the driving force and reorganization energy of each electron transfer step falls within a range that assures robust function, despite the repeated impact of mutation and change during evolution. Relatively simple empirical expressions for determining electron tunneling rates are more than adequate to understand the operation of PS I, especially since kinetic and preparative heterogeneity is common. Unlike other photosystems, the typical twofold symmetry of redox centers translates into a functionally relevant, near-symmetric two-branch pattern of electron transfer that culminates in the ability of the quinone on either branch to reduce the first redox center in the terminal iron–sulfur chain. Relatively small differences apparent in the kinetics of the two branches may reflect the tolerance of evolutionary drift in the thermodynamic properties of individual redox centers. Calculation suggests that productive charge separation, while slightly favoring the B chain chlorins, initially reduces both quinones roughly equally; long-term redox equilibration and short-circuiting charge recombination, however, tend to favor electron return through the A-branch.

In this chapter, structure-based modeling of excitation energy transfer and trapping in Photosystem I (PS I) core complexes will be addressed. The prerequisite for modeling is the knowledge of the spatial arrangement of the pigments (distances between pigments, orientation of their transition dipole moments) and the understanding of the spectral properties of the core antenna and the reaction center. The former is provided by the X-ray structure of trimeric photosystem I core complexes from (formerly ) at 2.5 Å resolution (Jordan et al., 2001). The spectral properties are determined by the local transition energies of the pigments, the pigment–pigment interactions and the coupling of the electronic pigment transitions with pigment and protein vibrations. The simultaneous description of the dynamics of excitation energy transfer and of the spectral properties of PS I is a major challenge for the theory. In this chapter, recent theoretical attempts in the literature and our own work are described. The focus is put on the question of how the different approaches relate to experimental data.

In recent years significant progress has been made in gaining insight into the structure and function of the reaction centers (RCs) of plants, algae, and photosynthetic bacteria. All RCs with known structures have a porphyrin–quinone core. This core is coupled with specific donors, acceptors, and light-harvesting antenna. RCs can use both watersoluble and lipid-soluble donors and acceptors. The electron exchange of RCs with these donors and acceptors occurs via specialized processing sites, which are responsible for recognition of respective donor-acceptor, and for channeling the electrons and protons in and out of the complex. The purpose of this chapter is to describe some simple kinetic models of flash-induced electron transfer in these RCs. In our consideration we try to find the balance between solutions for the simplest models and calculations for a detailed quantitative description of electron transfer in RCs.

Cyclic electron transport around Photosystem I remains one of the last great enigmas in photosynthesis research. Although first described in 1955 by Arnon and coworkers, the molecular details of the pathway, its physiological role and even its very occurrence remain in question. Nevertheless, significant progress is starting to be made in our understanding of this process. At least two pathways of cyclic electron transport appear to operate, one involving the transfer of electrons from NADPH to plastoquinone and the other operating via the donation of electrons from ferredoxin to plastoquinone. The relative importance of these two pathways seems to vary between cyanobacteria, unicellular green algae and higher plants as do many details concerning the regulation of the pathway and its functional organization in the thylakoid membrane. Two distinct functions for cyclic electron transport can be defined — the generation of ATP and, in higher plants, the generation of ΔpH to regulate light harvesting. These two functions give rise to the need for different regulatory processes to control the ratio of cyclic and linear electron flow. We discuss recent findings that cast new light on how cyclic electron transport is regulated under a range of physiological conditions.

Photosystem I (PS I) had been believed to be stable under almost all kinds of environmental stresses, but research in the last 10 years has revealed that PS I is sensitive to photooxidative stress. When combined with low temperature stress, PS I is the main target of photoinhibition, which limits not only overall photosynthesis but also subsequent growth following the application of stress. The molecular mechanism of inhibition is being clarified, and the elucidation of the mechanism of protection from photoinhibition is in progress. Recent advances in the understanding of the photoinhibition of PS I will be discussed in connection with the protection mechanism from inhibition.

The evolutionary history of photosynthetic reaction centers has been marked by a dramatic increase in subunit and cofactor complexity among the photosystems of oxygenic photosynthetic organisms, including plants, algae, and cyanobacteria. Comparative structural and sequence analysis has made a convincing case for the common ancestry of all tetrapyrrole-based reaction centers, and the combined reaction center family is noteworthy in that multiple evolutionary developmental stages can still be found and studied in extant phototrophs. Photosystem I in particular has expanded from a two-domain homodimeric reaction center with integrated antenna and core electron transfer cofactors, to a multisubunit complex with nearly 130 total cofactors and up to 15 closely interacting subunits in some organisms. Here we discuss some of the notable events in the evolution of Photosystem I that have occurred during this dramatic increase in structural complexity, some of which are lineage-specific while others must have happened before the divergence of plastids and cyanobacteria, possibly with direct relevance to the development of oxygenic photosynthesis.

Cytochrome c and plastocyanin are an excellent case study of the convergent evolution of proteins. The two molecules differ in their primary sequence and 3D structure but function in a similar way to transfer electrons from cytochrome to Photosystem I. It seems that cytochrome c was first “discovered” by Nature when iron was much more available than copper because of the reducing character of the Earth’s atmosphere. As the atmospheric molecular oxygen concentration began to rise because of photosynthetic activity, the relative bioavailabilities of iron and copper declined and rose, respectively, and cytochrome c was replaced with plastocyanin.