M.A. De la Rosa

Ab initio determination of the crystal structure of cytochrome c6 and comparison with plastocyanin

BACKGROUNDnElectron transfer between cytochrome f and photosystem I (PSI) can be accomplished by the heme-containing protein cytochrome c6 or by the copper-containing protein plastocyanin. Higher plants use plastocyanin as the only electron donor to PSI, whereas most green algae and cyanobacteria can use either, with similar kinetics, depending on the copper concentration in the culture medium.nnnRESULTSnWe report here the determination of the structure of cytochrome c6 from the green alga Monoraphidium braunii. Synchrotron X-ray data with an effective resolution of 1.2 A and the presence of one iron and three sulfur atoms enabled, possibly for the first time, the determination of an unknown protein structure by ab initio methods. Anisotropic refinement was accompanied by a decrease in the free R value of over 7% the anisotropic motion is concentrated at the termini and between residues 38 and 53. The heme geometry is in very good agreement with a new set of heme distances derived from the structures of small molecules. This is probably the most precise structure of a heme protein to date.nnnCONCLUSIONSnOn the basis of this cytochrome c6 structure, we have calculated potential electron transfer pathways and made comparisons with similar analyses for plastocyanin. Electron transfer between the copper redox center of plastocyanin to PSI and from cytochrome f is believed to involve two sites on the protein. In contrast, cytochrome c6 may well use just one electron transfer site, close to the heme unit, in its corresponding reactions with the same two redox partners.

Co-evolution of cytochrome c6 and plastocyanin, mobile proteins transferring electrons from cytochrome b6f to photosystem I

Abstractu2002Cytochrome c6 and plastocyanin are soluble metalloproteins that act as mobile carriers transferring electrons between the two membrane-embedded photosynthetic complexes cytochrome b6u200af and photosystem I (PSI). First, an account of recent data on structural and functional features of these two membrane complexes is presented. Afterwards, attention is focused on the mobile heme and copper proteins – and, in particular, on the structural factors that allow recognition and confer molecular specificity and control the rates of electron transfer from and to the membrane complexes. The interesting question of why plastocyanin has been chosen over the ancient heme protein is discussed to place emphasis on the evolutionary aspects. In fact, cytochrome c6 and plastocyanin are presented herein as an excellent case study of biological evolution, which is not only convergent (two different structures but the same physiological function), but also parallel (two proteins adapting themselves to vary accordingly to each other within the same organism).

A laser flash photolysis study of the reduction of methyl viologen by conduction band electrons of TiO2 and FeTi oxide photocatalysts

The photochemical activity of undoped and iron-doped TiO2 particles was studied by laser flash photolysis. A short duration laser pulse was used to produce electron-hole pairs; methyl viologen was employed as electron scavenger. The presence of small amounts of iron ions (less than 0.5%) in TiO2 matrices is beneficial to the photoreduction of methyl viologen (MV2+); by contrast, an increase in the amount of iron doping in TiO2 samples (from 0.5% to 5%) sharply reduces the MV+ yield to a value similar to that when pure TiO2 is used. The implications of these results to charge transfer photoreactions, such as dinitrogen reduction to ammonia, are discussed.

Marine sterols with a new pattern of side-chain alkylation from the sponge Aplysina(=Verongia)Aerophoba

The principal sterols of the marine sponge Aplysina(=Verongia)aerophoba, aplysterol and 24,28-didehydroaplysterol are characterized as 26-methyl homologues of 24-methyl- and 24-methylene-cholesterol, respectively.

A laser flash photolysis study of the photochemical activity of a synthesised ZrTiO4 comparison with parent oxides, TiO2 and ZrO2

Abstract The photochemical activity of a prepared zirconium titanate, ZrTiO 4 , sample has been studied by laser flash photolysis. A short duration laser pulse was used to produce electron–hole pairs; methyl viologen was employed as electron scavenger. The same procedure has been employed to estimate the photochemical activity of two home prepared single oxides, TiO 2 (hp) and ZrO 2 (hp) and for a commercially available TiO 2 (Degussa, P-25). Our results further showed the potential of the laser flash photolysis technique to estimate the photosensitivity of semiconductor oxides and predict a poor photoactivity for the TiO 2 (hp) sample in spite of this sample exhibiting (by XRD technique) the anatase as the unique phase.

Light-driven hydrogen peroxide production as a way to solar energy conversion

Abstract The existence of a number of biochemical pathways through which molecular oxygen is reduced to hydrogen peroxide, either directly in a two-electron transfer process or indirectly via superoxide, is well known. The Mehler reaction, which is the photoreduction of oxygen by isolated chloroplasts in light with the concomitant formation of hydrogen peroxide, is a worthwhile example of such biochemical pathways. Taking advantage of these facts, the possibility of designing a photobiochemical system for solar energy conversion aimed to generate a valuable product such as hydrogen peroxide — which is both a powerful source of energy (it releases more than 100 kJ per mol upon decomposition into water and oxygen) and a powerful source of oxygen (it is a strong bleaching agent widely used in industry) — from oxygen reduced by illuminated chloroplasts has been considered. Likewise, the construction of several artificial photosystems for the production of hydrogen peroxide was also studied. Such model systems, which try to simulate the natural process with a higher efficiency and simplicity, are based on the use of a visible light-absorbing pigment (flavins, for example) as a photosensitizer.

Coupling between redox and acid-base energy by cytochrome b-564 in baker's yeast mitochondria

Bakers yeast mitochondrial cytochrome b-564 is characterized by exhibiting both a labile pH-independent high-potential form (Eo, pH 7 = + 190 mV) and a stable pH-dependent (pKa = 6.8) low-potential form (Eo, pH 7 = + 70 mV). The different behavior of these two forms of cytochrome b-564 versus pH seems to be a decisive factor for transduction of redox energy into acid-base energy in oxidative phosphorylation site 2. Deenergizing treatments, such as ADP plus Pi, result in the conversion of all the mitochondrial cytochrome b-564 into its low-potential form, whereas energization with ATP specifically transforms the cytochrome into its high-potential form, the ATP effect being neutralized by the ATPase inhibitor oligomycin and by the uncoupler FCCP. Accordingly, a minimal model for coupling between redox energy and acid-base energy through an electronically energized and protonated ferricytochrome b-564 intermediate is proposed. The energy-transducing properties of mitochondrial cytochrome b-564 seems to be shared by chloroplast cytochrome b-559.

Energy transduction by bioelectrochemical systems

Summary Electronic energy seems to be the obligatory link between the different forms of energy (light, redox, acid-base, phosphate-bond) transduced by the bioelectrochemical systems. These energy-transducing systems can operate, according to their nature and depending on their energization state, either at two alternate midpoint redox potentials ( U 0 ′ and U 0 ′* ), or at two p K a ′ s (p K a and p K a * ), or at two phosphate transfer potentials ( PTP and PTP * ). The key point in energy coupling between any two of these biochemical systems lies apparently in the fact that both of them share a common intermediate, which cyclically participates in the overall process by alternating between its electronically energized state and its unenergized basal state. Electronic energization of the coupling intermediate may proceed in one or two steps and can oscillate between approximately 0.2 and 1 eV molecule −1 , i.e., between 20 and 100 kJ mole −1 .

Kinetic Mechanisms of Psi Reduction by Plastocyanin and Cytochrome c 6 in the Ancient Cyanobacteria Pseudanabaena sp. Pcc 6903 and Prochlorothrix hollandica

Cytochrome c6 (Cyt) and plastocyanin (Pc) act as mobile electron carriers between the cytochrome b d f and PSI complexes. Whereas some cyanobacteria and alga can synthesize both electron donors and other cyanobacteria just produce Cyt, in higher plants Pc is only found. Pc and Cyt are acidic in eukaryotic organisms, but can be either acidic, basic or neutral in cyanobacteria. Both molecules are thus an excellent example of convergent evolution of proteins with different structures but playing the same function [1].

Sequential transduction of light into redox and acid—base energy in photosynthesis

Abstract Photosynthesis consists essentially in the conversion, by the plant kingdom, of sunlight energy into chemical energy (cell material and molecular oxygen). From a physicochemical point of view, photosynthesis is intrinsically a light-driven oxidation—reduction/acid—base process. In the course of photosynthesis, water is oxidized to molecular oxygen, whereas carbon dioxide, nitrate or dinitrogen, and sulfate are respectively reduced to carbohydrate, ammonia and sulfide; water is moreover ionized, in similar amounts, into protons and hydroxide anions. Phosphorus does not change its oxidation state, but orthophosphate becomes energized to metaphosphate, at the expense of the ionization products of water, in a peculiar acid—base process. Metaphosphate is mostly used as an energy shuttle in many cell processes, among them in the reduction and assimilation of the primordial bioelements, as well as in the polymerization of the resulting monomers — sugars, lipid components, amino acids, nucleotides — and deenergizes itself back to orthophosphate, again in acid—base reactions. The role of the oxide anion, that is, an oxygen atom with two additional electrons, O 2− , in these bioenergetic processes is especially discussed.