Reaction center proteins, modeling electron transfer from

Figure 8 Model and proposed reaction cycle for arsenite oxidase from A.faecalis. Reaction steps are (1) binding of arsenite, AsOjOH, to the enzyme, (2) two-electron transfer to Mo, oxidizing As(III) to As(V) and reducing Mo(Vl) to Mo(lV), (3) release of arsenate oxyanion, (4) two-electron transfer from Mo(Vl) to [3Fe-4S] center, regenerating Mo(IV) reaction center, (5) two-electron transfer from [3Fe-4S] center in large subunit to [2Fe-2S] Rieske center of small subunit, and (6) electron transfer from the [2Fe-2S] center of arsenite oxidase to the associated small copper protein azurin. (Based on Refs. 63 and 64.)...

Most of the interest in mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer in solution from chlorophyll a to p-benzoquinone (21, 22) which has been shown to occur via the excited triplet state of chlorophyll a. However, these solution studies are not very good models of the in vivo reaction center because the in vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other in the reaction-center protein. 

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

In this model, the FeMoco centers of the MoFe protein are reduced independently by a series of eight one-electron transfers from the Fe protein with concommitant hydrolysis of MgATP. Following each electron transfer, dissociation of the protein-protein complex occurs in a reaction that is rate limiting when dithionite is used as reductant. Following this dissociation, the oxidized Fe protein, with MgADP... 

Modeling the First Electron Transfer from Qa to Qb in Reaction Center Proteins from Rb, sphaeroides... 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

We begin with a summary of the standard single-electron rigid-bridge model for electron transport [1,2], and then describe effects that arise from bridge dynamics. We next examine issues in multistep multi-center electron transfer. The closely related problem of two-electron transfer is then discussed. Multi-center and multielectron processes are of great relevance for ET in DNA, proteins, and catalytic reactions. 

Fig. 6. Photochemical cycles showing coupling of electron transfer to proton transfer, cytochrome oxidation and quinone exchange in (A) native reaction centers where two Cyt c are oxidized in the cycie, (B) reaction centers where uptake of the first proton is inhibited, and (C) reaction centers where uptake ofthe second proton is inhibited (shading indicates the quinone pool). Figure source (A) Paddock, Rongey, McPherson, Juth, Feher and Okamura (1994) Pathway of proton transfer in bacterial reaction centers role of aspartate-L21Z in proton transfers associated with reduction of quinone to dihydroquinone. Biochemistry 33 734 (B) Okamura and Feher (1992) Proton transfer in reaction centers from photosynthetic bacteria. Annu Rev Biochemistry. 61 868 (C) Feher, Paddock, Rongey and Okamura (1992) Proton transfer pathways in photosynthetic reaction centers studied by site-directed mutagenesis. In A Pullman, J Jortner and B Pullman (eds) Membrane Proteins Structures, Interactions and Models, p 485. Kluwer.

The nonheme diiron centers in proteins and model complexes with 0,N-donors can reach a number of oxidation states spanning from FenFen to FeIVFeIV The diiron(II) state is reactive with dioxygen yielding different products depending on the nature of ligands and reaction conditions (Figure 4.21). Outer-sphere electron transfer may occur for coordinatively saturated and sterically impeded complexes with sufficiently low redox potential.17... 

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