Protein in electron transfer

Early attempts at observing electron transfer in metalloproteins utilized redox-active metal complexes as external partners. The reactions were usually second-order and approaches based on the Marcus expression allowed, for example, conjectures as to the character and accessibility of the metal site. xhe agreement of the observed and calculated rate constants for cytochrome c reactions for example is particularly good, even ignoring work terms. The observations of deviation from second-order kinetics ( saturation kinetics) allowed the dissection of the observed rate constant into the components, namely adduct stability and first-order electron transfer rate constant (see however Sec. 1.6.4). Now it was a little easier to comment on the possible site of attack on the proteins, particularly when a number of modifications of the proteins became available. 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c 

The intramolecular electron transfer kg, subsequent to the rapid reduction, must occur because the Ru(III)-Fe(II) pairing is the stable one. It is easily monitored using absorbance changes which occur with reduction at the Fe(III) heme center. Both laser-produced Ru(bpy)3 and radicals such as CO (from pulse radiolysis (Prob. 15)) are very effective one-electron reductants for this task (Sec. 3.5).In another approach, the Fe in a heme protein is replaced by Zn. The resultant Zn porphyrin (ZnP) can be electronically excited to a triplet state, ZnP which is relatively long-lived (x = 15 ms) and is a good reducing agent E° = —0.62 V). Its decay via the usual pathways (compare (1.32)) is accelerated by electron transfer to another metal (natural or artificial) site in the protein e. g.. 

The ZnPi accepts an electron from the Ru to return the system to its initial state. Although k g ATj, both can be measured. Examples of these approaches with iron and copper proteins are shown in Table 5.12. There are a number of excellent short reviews of this subject. 21-124 

Protein Redox Centers Involved Initiating Mode Kt S Distance A AE Volts Ref. 

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The important criterion thus becomes the ability of the enzyme to distort and thereby reduce barrier width, and not stabilisation of the transition state with concomitant reduction in barrier height (activation energy). We now describe theoretical approaches to enzymatic catalysis that have led to the development of dynamic barrier (width) tunneUing theories for hydrogen transfer. Indeed, enzymatic hydrogen tunnelling can be treated conceptually in a similar way to the well-established quantum theories for electron transfer in proteins. 

Callis PR, Liu T (2006) Short range photoinduced electron transfer in proteins QM-MM simulations of tryptophan and flavin fluorescence quenching in proteins. Chem Phys 326 (l) 230-239... 

J. W. Petrich, J. W. Longworth, and G. R. Fleming, Internal motion and electron transfer in proteins A picosecond fluorescence study of three homologous azurins. Biochemistry 26, 2711-2722 (1987). 

A very practical comprehensive rate expression appropriate for long-range electron transfer in proteins and other large molecules yet which retains ease of computation by anyone and on the smallest computer is obtained by assuming one high frequency harmonic mode (inner sphere reorganization) and one very... 

Acknowledgement. We thank David Beratan and Jay Winkler for helphil discussions. Our research on electron transfer in proteins is supported by grants from the National Science Foundation and the National Institutes of Health. NRSA/NIH postdoctoral fellowships were held by M. J. T., J. C, and A. L. R. and a Medical Research Council (Canada) postdoctoral fellowship was held by B. E. B. This is contribution no. 8115 from the Arthur Amos Noyes Laboratory. 

The very rapid reaetion (3.15) with a large — AG can thus be measured. We therefore have an effective method for generating very rapidly in situ a powerful reducing or oxidizing agent. One of the most impressive applications of these properties is to the study of internal electron transfer in proteins. 

Gray HB, Winkler JR (1996) Electron transfer in proteins. Annu Rev Biochem 65 537 Fedurco M (2000) Redox reactions of heme-containing metalloproteins dynamic effects of self-assembled monolayers on thermodynamics and kinetics of cytochrome c electron-transfer reactions. Coord Chem Rev 209 263... 

Amino acid side-chains may have a role in electron transfer in proteins through the well-known hopping pathway . In this process electrons could move between certain residues such as tyrosine and tryptophan, with the generation of free radical intermediates. Such free radical residues are known in certain high oxidation state species of hemoproteins. 

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),... 

Thermal and Photoinduced Long Distance Electron Transfer in Proteins and in Model Systems... 

A more sophisticated method involves the combination of metal-substituted proteins with electron acceptors covalently attached to the amino acid residue located at the protein surface. The [Ru(NH3)5]3+ complex attached to the histidine residue had been used in Zn-substituted myoglobin [71,72] or cytochrome b562 [73] as an electron acceptor. This design allows the distance between donor and acceptor to be fixed and is very useful for experimental analysis of intramolecular electron transfer in proteins. 

Gray HB, Winkler JR. Electron transfer in proteins. Annu Rev Biochem 1996 65 537-61. 

Brittain T (2008) Intra-molecular electron transfer in proteins. Protein Pept Lett 15 556-561... 

Gunner MR, Alexov E (2000) A pragmatic approach to structure based calculation of coupled proton and electron transfer in proteins. Biochim Biophys Acta 1458 63-87... 

Peluso, A., Di Donato, M., and Saracino, G.A.A.. (2000) An alternative way of thinking about electron transfer in proteins Proton assisted electron transfer between the primary and the secondary quinones in photosynthetic reaction centers, J. Chem. Phys. 113, 3212-3218. 

Although diffusion is a slow process compared to energy transfer and electron transfer at the shortest distances, it can be an exceptionally effective way to move electrons and protons over long distances. However, unlike the hard-wired cofactor chains that guide electron transfer in protein complexes, diffusion faces the problem of directing where... 

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