Protein electron-mediating

The recent discovery that a class of electron mediating proteins, the ferredoxins (Sec. IVE2), characteristically contain two or more iron atoms bridged by sulfur atoms has stimulated interest in polynuclear iron-sulfur complexes. It seems appropriate therefore to review here what little is known about such species. 

The function of the electron-mediating proteins which contain a single redox active site (e.g., rubredoxin, azurins, flavodoxins, plasto-cyanins) is mainly related to the first aspect. Still, the pronounced specificity encountered in their function in biological energy conversion processes indicates that their redox center, often a transition metal ion, is embedded in an evolutionarily optimized polypeptide envelope. The... 

We have used a range of different physical and chemical approaches in the effort to better understand how the different blue copper proteins function. With the relatively simpler, electron-mediating proteins like azurin, the ultraviolet chromophores were shown to be informative in terms of copper-protein interactions. These proteins are also a useful system for detailed examination of the electron transfer pathways to and from their single copper site. 

We have discussed structure-function relations of a diverse group of electron mediating proteins with both natural (i.e., wild type) and artificial structural... 

The potential of pulse radiolysis for studying biological redox processes, particularly of macromolecules, was recognized rather early. It was initially employed for investigating radiation-induced damage and, later on, as an effective tool for resolving electron transfer processes to and within proteins. Cytochrome c, a well-characterized electron-mediating protein, was the first to be... 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

Recently, electron-mediated, scalar couplings which are active between magnetic nuclei on both sides of the hydrogen bridge have been discovered in nucleic acids [28-41], proteins [42-54] and their complexes (Tabs. 9.1-9.3) [54—56]. These couplings are closely related to similar inter- and intramolecular couplings across H-bonds in smaller chemical compounds [57-60]. It is well established [31, 58, 61-74] that such trans H-bond scalar couplings follow the same electron-mediated polarization mechanism as any covalent... 

Ferredoxins are electron-transfer proteins that can mediate between pyruvate ferredoxin oxidoreductase and hydrogenase. It appears that during the course of the evolution, different types of ferredoxin were recruited for this purpose. In Clostridia, ferredoxins of the 2[4Fe-4S] type are used (Uyeda and Rabinowitz 1971). In T. vaginalis (Chapman et al. 1986) and T. foetus (Marczak et al. 1983), [2Fe-2S] ferredoxins are used. Their axial EPR spectra at g = 1.94,2.02 (Fig. 9.2) resemble those of the ferredoxins that are involved in P450 monooxygenase systems. Similar ferredoxins, with various functions, have been isolated from... 

Willner and coworkers have extended this approach to electron relay systems where core-based materials facilitate the electron transfer from redox enzymes in the bulk solution to the electrode.56 Enzymes usually lack direct electrical communication with electrodes due to the fact that the active centers of enzymes are surrounded by a thick insulating protein shell that blocks electron transfer. Metallic NPs act as electron mediators or wires that enhance electrical communication between enzyme and electrode due to their inherent conductive properties.47 Bridging redox enzymes with electrodes by electron relay systems provides enzyme electrode hybrid systems that have bioelectronic applications, such as biosensors and biofuel cell elements.57... 

Scheme 7 Electronic transduction of photo-switchable bioelectrocatalytic functions of proteins, (A) by the tethering of photoisomerizable units to the protein (R is a diffusional electron mediator that electrically contacts the redox...

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

A relatively simple and quick procedure for the isolation of Photosystem I-enriched particles from the thermophilic cyanobacterium Phormidium laminosum, without the use of detergents for solubilization, is described. The procedure involves sonication of cells, centrifugation and DEAE-cellulose chromatography. The particles had an 02 uptake activity of up to 200 pmol 02. mg chlorophyll h 1 and appeared as vesicles of 200 100 nm diameter when observed under electron microscopy. The analysis of the chlorophyll-protein complexes by polyacrylamide gel electrophoresis showed that these particles are enriched in the complexes associated with Photosystem I and partially depleted in those associated with Photosystem II. The particles did not contain ferredoxin and were active in NADP-photoreduction only in the presence of added ferredox in. They were also able to photoreduce externally added electron mediators using ascorbate as electron donor, the reduced mediators can be coupled to hydrogenase for the production of H2 or for the activation of cyanobacterial phosphoribulokinase using a ferredoxin/thioredoxin system. 

Calcium-binding Proteins Copper Enzymes in Denitrification Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Iron Heme Proteins Electron Transport Iron-Sulfin Proteins Metal-mediated Protein Modification Metallochaperones Metal Ion Homeostasis Molybdenum MPT-containing Enzymes Nickel Enzymes Cofactors, Nitrogenase Catalysis Assembly Zinc Enzymes. 

While an inereased loading of an electron mediator on a protein enhances the effectiveness of electrical contacting, the enzyme activity suffers owing to changes in its structure. The chemical modification of redox proteins with synthetic... 

For practical photoinduced synthetic biocatalyzed transformations, it is important to integrate biocatalysts in immobilized matrices that allow the recycling of the photosystems. The fact that bipyridinium sites act as electron mediators for various redox enzymes was used to develop two paradigms for the electrical contacting and photoactivation of the biocatalyst (Figure 39). By one approach, the bipyridinium electron relays are tethered by covalent bonds to the protein backbone (Figure 39A). These electron relays act as oxidative quenchers of the excited state of the dye and, upon photoreduction of the electron acceptor units, they act as electron carriers that activate the reductive functions of the enzyme. As an example, the... 

Figure 3. Fiincvional organisation oFpholosyslem I (in protein complexes contained in the thylakoid membrane. Excitation energy is harvested by chlorophyll (Chi) and carotenoids (Car) molecules and transfered to the special pair (Chlj). Vectorial electron transfer across the membrane takes place front excited Chi to plastoqutnone (pQ) via phcopliitin (Ph) and quinone (Q) electron mediators.

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