Electron between metalloproteins

Mitochondrial cytochrome c is perhaps the most widely studied of all metalloproteins with respect to its electrochemical properties. It is located in the inner-membrane space of mitochondria and transfers electrons between membrane-bound complex III and complex IV. The active site is an iron porphyrin with a redox potential (7) of -1-260 mV vs. NHE. The crystal structures of cytochrome c from tuna have been determined (8, 9) in both oxidation states at atomic resolution. It is found that the heme group is covalently linked to the protein via two thioether bridges, and part of its edge is exposed at the protein surface. Cytochrome c is a very basic protein, with an overall charge of -1-7/-l-8 at neutral pH. Furthermore, many of the excess basic lysine residues are clustered around the mouth of the heme crevice, giving rise to a pronounced charge asymmetry. 

The investigation of the kinetics and mechanisms of electron transfer reactions between metalloproteins and with inorganic redox reactants continues to be a rapidly growing field. The Proceedings of the 3rd International Conference on Bioinorganic Chemistry (1987) have been published in a special issue of Recueil des Travaux Chimiques des Pay-Bas, and include a section on metalloprotein electron transfer. The subject of long-distance electron transfer in metalloproteins has also been reviewed. 

MOLECULAR INTERACTIONS BETWEEN METALLOPROTEINS INVOLVED IN ELECTRON TRANSFER PROCESSES TETRAHEME CYTOCHROME Cj AND FLAVODOXIN. NMR AND MOLECULAR MODELING STUDIES... 

Molecular interactions between metalloproteins involved in electron transfer processes tetraheme cytochrome C3 and flavodoxin. NMR and molecular modeling studies... 

Electronic spectra of metalloproteins find their origins in (i) internal ligand absorption bands, such as n->n electronic transitions in porphyrins (ii) transitions associated entirely with metal orbitals (d-d transitions) (iii) charge-transfer bands between the ligand and the metal, such as the S ->Fe(II) and S ->Cu(II) charge-transfer bands seen in the optical spectra of Fe-S proteins and blue copper proteins, respectively. Figure 6.3a presents the characteristic spectrum of cytochrome c, one of the electron-transport haemoproteins of the mitochondrial... 

Redox reactions usually lead, however, to a marked change in the species, as reactions 4-6 indicate. Important reactions involve the oxidation of organic and metalloprotein substrates (reactions 5 and 6) by oxidizing complex ions. Here the substrate often has ligand properties, and the first step in the overall process appears to be complex formation between the metal and substrate species. Redox reactions will often then be phenomenologically associated with substitution. After complex formation, the redox reaction can occur in a variety of ways, of which a direct intramolecular electron transfer within the adduct is the most obvious. 

Jensen TJ, Gray HB, Winkler JR, Kuznetsov AM, Ulstrup J. Dynamic ionic strength effects in fast bimolecular electron transfer between a redox metalloprotein of high electrostatic charge and an inorganic reaction partner. J Phys Chem B 2000 104 11556-62. 

Grove TZ, Kostic NM. Metalloprotein association, self-association, and dynamics governed by hydrophobic interactions simultaneous occurrence of gated and true electron-transfer reactions between cytochrome and cytochrome c6 from Chlamydomonas reinhardii. J Am Chem Soc 2003 125 10598-607. 

The observations illustrate that inelastic and thermally activated tunnel channels may apply to metalloproteins and large transition metal complexes. The channels hold perspectives for mapping protein structure, adsorption and electronic function at metallic surfaces. One observation regarding the latter is, for example that the two electrode potentials can be varied in parallel, relative to a common reference electrode potential, at fixed bias potential. This is equivalent to taking the local redox level up or down relative to the Fermi levels (Fig. 5.6a). If both electrode potentials are shifted negatively, and the redox level is empty (oxidized), then the current at first rises. It reaches a maximum, convoluted with the bias potential between the two Fermi levels, and then drops as further potential variation takes the redox level below the Fermi level of the positively biased electrode. The relation between such current-voltage patterns and other three-level processes, such as molecular resonance Raman scattering [76], has been discussed [38]. 

Kinetic data for electron transfer between two metalloproteins are presented in Table V. The rate constants and activation parameters for the Ps(II)-Ps(III) and Az(I)-Az(II) exchange reactions were calculated from the kinetic data for the first three reactions (for which K 1, AH° 0, AS° 0 in addition, the rate constant for the Hh(II)-Ps(III) reaction is independent of ionic strength (31)). The calculated exchange data were then used to predict the kinetic parameters for the Ps(II)-Az(II) reaction. As is evident from Table V, the agreement of the observed and predicted parameters is satisfactory, particularly since the Ps( II )-Az( II) reaction has a relatively complex mechanism (57) involving conformational changes on both Ps(III) and Az(I). 

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