Azurins electron transfer reactions

Azoresorcinol, pyridyl-metal complexes dyes, 6, 74 Azurins, 6, 651, 652 copper(II) complexes, 2, 772 5, 721 electron transfer reactions, 6, 653 NMR, 6, 652 Raman spectra, 6, 652 spectra, 6, 652 thioether complexes, 2, 557 Azurite... 

D.R. McMillin, Purdue University In addition to the charge effects discussed by Professor Sykes, I would like to add that structural effects may help determine electron transfer reactions between biological partners. A case in point is the reaction between cytochrome C551 and azurin where, in order to explain the observed kinetics, reactive and unreactive forms of azurin have been proposed to exist in solution (JL). The two forms differ with respect to the state of protonation of histidine-35 and, it is supposed, with respect to conformation as well. In fact, the lH nmr spectra shown in the Figure provide direct evidence that the nickel(II) derivative of azurin does exist in two different conformations, which interconvert slowly on the nmr time-scale, depending on the state of protonation of the His35 residue (.2) As pointed out by Silvestrini et al., such effects could play a role in coordinating the flow of electrons and protons to the terminal acceptor in vivo. 

Parr, S. R., Barber, D., Greenwood, C., and Brunori, M. (1977). The electron-transfer reaction between azurin and the cytochrome c oxidase from Pseudomonas aeruginosa. Biochem. J. 167, 447-455. 

The mechanism of the regulation of electron transfer in metalloproteins has been investigated 61) and two relevant examples have been discussed in the first one the molecular mechanism controlling the electron transfer reactions is restricted to the immediate chemical environment of the metal center (azurin), while in the second one it involves a conformational transition of the whole quaternary structure of the enzyme. The power of the kinetic approach in detecting significant intermediates was emphasized 6t>. The Cu metal complex site of azurin has a distorted tetrahedral... 

In view of the strong photo oxidising properties of [Re(phen)(CO)3 (imidazole)]+ (14) (Re(I)VRe(O) = ca. + 1.3 V vs NHE in CH3CN), [Re(phen)(CO)3 (H20)]+ has been reacted with azurin to give [Re(phen)(CO)3(His83)]+-AzCu+, which has been used to study photoinduced electron-transfer reactions [47]. In the absence of quenchers, excitation of the rhenium(I) complex... 

Hyun, Y-L., and Davidson V. L., 1995, Electron transfer reactions between aromatic amine dehydrogenase and azurin. Biochemistry 34 12249912254. 

Quenching of excited-state [Ru(bipy)3] by reduced blue proteins involves electron transfer from the Cu with rate constants close to the diffusion limit for electron-transfer reactions in aqueous solution. It is suggested that the excited Ru complex binds close to the copper-histidine centre, and that outer-sphere electron transfer occurs from Cu through the imidazole groups to Ru. Estimated electron-transfer distances are about 3.3 A for plastocyanin and 3.8 A for azurin, suggesting that the hydrophobic bipy ligands of Ru " penetrate the residues that isolate the Cu-His unit from the solvent. ... 

The suffixes o and r refer to the oxidised and reduced protein respectively. Thus, the electron transfer reaction between the redox proteins is coupled to a transition between two forms of reduced azurin, only one of which participates directly in the redox reaction. As the mechanism involves two independent equilibria, two relaxation processes are observed following an increase in temperature. Fig. 6 shows a typical progress curve monitored at a wavelength characteristic of reduced cytochrome C55,. This transient is comprised of a rapid increase in absorbance followed by a slower decrease. The rapid relaxation reflects the initial re-equiUbration of the second-order redox reaction, an increase in temperature favouring ferrocytochrome... 

The application of direct electrochemistry of small redox proteins is not restricted to cytochrome c. For example, the hydroxylation of aromatic compounds was possible by promoted electron transfer from p-cresol methylhydroxylase (a monooxygenase from Pseudomonas putida) to a modified gold electrode [87] via the blue copper protein azurin. All these results prove that well-oriented non-covalent binding of redox proteins on appropriate electrode surfaces increases the probability of fast electron transfer, a prerequisite for unmediated biosensors. Although direct electron-transfer reactions based on small redox proteins and modified electrode surfaces are not extensively used in amperometric biosensors, the understanding of possible electron-transfer mechanisms is important for systems with proteins bearing catalytic activity. 

Azurins, 651,652 electron transfer reactions, 653 NMR, 652 Raman spectra, 652 spectra, 652 Azurite structure, 849... 

Several studies have been carried out on the electron exchange and transfer reactions of Pseudomonas aeruginosa azurin. The AzCu(I)/(II) exchange rate constant has been measured at 4 °C by fast-flow/rapid-freeze EPR methods and at 25 °C using H NMR relaxation techniques/ and shows only a small dependence on pH (4.5-9.0). The rate constants for electron transfer reactions between... 

Fig. 23. Coupling the cyclic voltammetry of horse cytochrome c (as promoted at a Au electrode in the presence of 4,4 -bipyridyl) to reduction of Oj by Pseudomonas aeruginosa cytochrome cd via a sequence of protein-protein electron-transfer reactions. Aerobic solutions contained 0.1 M NaC104, 0.02 M phosphate, pH 7.0. Scan rate 1 mVs . a) horse cytochrome c (0.44 mM) alone, b) after an addition, of cytochrome cd to 6 pM. c) after a further addition, of azurin to 0.25 pM. Redrawn from Ref. 198, with kind permission...

Hill and Walton have examined the interaction of horse heart cytochrome c with azurin and cyt C551 using an electrochemical technique and a detailed theoretical study of cytochrome c has been published. The interaction with cytochrome and redox potentials for this latter protein have also been studied. The redox-inactive [Cr(en)3] " inhibits oxidation of cyt hs with [Co(NH3)6] and other positively charged reagents by blocking the heme edge site used by cyt Electron transfer reactions of iron-sulfur proteins have been reviewed and NMR studies " have been used to pinpoint reaction sites with inorganic complexes on the two iron ferredoxin from Anabena variabilis at residues 25 and 83. 

Studies of ferredoxin [152] and a photosynthetic reaction center [151] have analyzed further the protein s dielectric response to electron transfer, and the protein s role in reducing the reorganization free energy so as to accelerate electron transfer [152], Different force fields were compared, including a polarizable and a non-polarizable force field [151]. One very recent study considered the effect of point mutations on the redox potential of the protein azurin [56]. Structural relaxation along the simulated reaction pathway was analyzed in detail. Similar to the Cyt c study above, several slow relaxation channels were found, which limited the ability to obtain very precise free energy estimates. Only semiquantitative values were... 

Earlier suggestions that the two uncoordinated and invariant residues His35 (inaccessible to solvent and covered by polypeptide) and His83 (remote and 13 A from Cu) are, from effects of [H ] on rate constants (and related pKg values), sites for electron transfer may require some re-examination. Thus, it has been demonstrated in plastocyanin studies [50] that a surface protonation can influence the reduction potential at the active site, in which case its effect is transmitted to all reaction sites. In other words, an effect of protonation on rate constants need not necessarily imply that the reaction occurs at the site of protonation. His35 is thought to be involved in pH-dependent transitions between active and inactive forms of reduced azurin [53]. The proximity of... 

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