Proteins electron transfer rates

Beratan D N, Betts J N and Onuchic J N 1991 Protein electron transfer rates set by the bridging secondary and tertiary structure Science 252 1285-8... 

Beratan, D. N., Betts, J. N., and Onuchic, J. N., 1991, Protein Electron Transfer Rates Set by the Bridging Secondary and Tertiary Structure Science 252 1285nl288. 

To a first approximation, the interface between the Fe-protein and MoFe-protein are similar in both complexes. The relative positions of the metalloclusters observed in these complex structures indicate that electron transfer from the Fe-protein to the FeMo-cofactor proceeds through the P-clusters (Figure 2) the edge to edge distance of 14A is compatible with inter-protein electron transfer rates much more rapid (-10 sec (27, 22)) than the... 

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

The electron-transfer rate between large redox protein and electrode surface is usually prohibitively slow, which is the major barricade of the electrochemical system. The way to achieve efficient electrical communication between redox protein and electrode has been among the most challenging objects in the field of bioelectrochemistry. In summary, two ways have been proposed. One is based on the so-called electrochemical mediators, both natural enzyme substrates and products, and artificial redox mediators, mostly dye molecules and conducted polymers. The other approach is based on the direct electron transfer of protein. With its inherited simplicity in either theoretical calculations or practical applications, the latter has received far greater interest despite its limited applications at the present stage. 

The biomembrane-like films can provide a favorable microenvironment for proteins and enhance direct electron-transfer rate between proteins and electrodes, so many biopolymers such as methyl cellulose (MC) and dihexadecylphospate (DHP) have been used to immobilize Mb and make biosensors [230-232], The electrochemical catalytic reduction of oxygen by the Mb-MC/EPG was examined by CVs. Mb-MC... 

In one series of experiments the cytochrome c oxidase mutations replaced acidic residues by neutral ones, and some of them were thus expected to alter the nature of binding of the protein to cytochrome c. From the pattern of dependence of the heme c to Cua electron-transfer rate constant on these mutations it was deduced that the binding of cytochrome c to cytochrome c oxidase is mediated by electrostatic interactions between four specific acidic residues on cytochrome c oxidase and lysines on cytochrome c. In another series of experiments, tryptophan 143 of cytochrome c oxidase was mutated to Phe or Ala. These mutations had an insignificant effect on the binding of the two proteins, but they dramatically reduced the rate constant for electron transfer from heme c to Cua- It was concluded that electron transfer from... 

However, because of the mostly very slow electron transfer rate between the redox active protein and the anode, mediators have to be introduced to shuttle the electrons between the enzyme and the electrode effectively (indirect electrochemical procedure). As published in many papers, the direct electron transfer between the protein and an electrode can be accelerated by the application of promoters which are adsorbed at the electrode surface [27], However, this type of electrode modification, which is quite useful for analytical studies of the enzymes or for sensor applications is in most cases not stable and effective enough for long-term synthetic application. Therefore, soluble redox mediators such as ferrocene derivatives, quinoid compounds or other transition metal complexes are more appropriate for this purpose. 

There is currently much interest in electron transfer processes in metal complexes and biological material (1-16, 35). Experimental data for electron transfer rates over long distances in proteins are scarce, however, and the semi-metheme-rythrin disproportionation system appears to be a rare authentic example of slow electron transfer over distances of about 2.8 nm. Iron site and conformational changes may also attend this process and the tunneling distances from iron-coordinated histidine edges to similar positions in the adjacent irons may be reduced from the 3.0 nm value. The first-order rate constant is some 5-8 orders of magnitude smaller than those for electron transfer involving some heme proteins for which reaction distances of 1.5-2.0 nm appear established (35). 

Distance The affects of electron donor-acceptor distance on reaction rate arises because electron transfer, like any reaction, requires the wavefunctions of the reactants to mix (i.e. orbital overlap must occur). Unlike atom transfer, the relatively weak overlap which can occur at long distances (> 10 A) may still be sufficient to allow reaction at significant rates. On the basis of work with both proteins and models, it is now generally accepted that donor-acceptor electronic coupling, and thus electron transfer rates, decrease exponentially with distance kji Ve, exp . FCF where v i is the frequency of the mode which promotes reaction (previously estimated between 10 -10 s )FCF is a Franck Condon Factor explained below, and p is empirically estimated to range from 0.8-1.2 with a value of p 0.9 A most common for proteins. 

The bridged complexes were prepared by (a) direct interaction of the two constituents or (b) one electron reduction of the fully oxidized bridged complex (it would be the Co "-Ru " complex in one example shown above ). The speed with which the reduction must be carried out depends on the subsequent electron transfer rate. Both chemical reductants or rapidly generated reducing radicals have been used. The latter approach (b) has been an effective one for investigating electron transfer within proteins (Sec. 5.9). A special approach (c) involves... 

---