Redox proteins, electron transfer

Chi Q, Farver O, Ulstrup J (2005) Long-range protein electron transfer observed at the singlemolecule level in situ mapping of redox-gated tunneling resonance. Proc Natl Acad Sci USA 102 16203-16208... 

In order to probe these effects, a number of studies on the kinetics of electron transfer between small molecule redox reagents and proteins, as well as protein-protein electron transfer reactions, have been carried out (38-41). The studies on reactions of small molecules with electron transfer proteins have pointed to some specificity in the electron transfer process as a function of the nature of the ligands around the small molecule redox reagents, especially the hydrophobicity of these... 

As shown in Table V, a number of Fe S-containing proteins perform reactions other than redox or electron transfer. That is, the function of the cluster does not include a change in oxidation state, even as a transient step in catalysis. This role is best illustrated by aconitase, one of the most extensively studied Fe S proteins, regardless of function. The elegant recent work on this enzyme is largely under the guiding hand of H. Beinert and is summarized in the Krebs Memorial Lecture (Beinert and Kennedy, 1989). 

Many proteins are exclusively involved in intra-protein electron transfer and typically function in ordered structures such as mitochondria. Under these circumstances, the redox-active centers are generally accessible on the outer surface of the protein. In contrast, the redox reactions catalyzed by oxidoreductases involve small molecules with the reaction involving two redox couples, i.e. the substrate and the co-factor or co-substrate. Because the catalytic center of the enzyme is often located... 

The physical biochemistry of these two classes of 4Fe-4S proteins has been extensively studied and reviewed, especially with regard to redox and electron transfer chemistry (2-22), whereas the magnetic properties of the cluster have been probed by electron paramagnetic resonance (EPR) (23-34), nuclear magnetic resonance (NMR) (24, 25, 35-54), and susceptibility measurements (56-61). Mossbauer spec-... 

Peroxidases are heme-iron proteins involved in oxidative stress control. The mechanisms of this class of enzymes involve rather complex redox and electron transfer processes. These include alterations of spin state and ligands to the active-site heme as well as a novel role for Ca ions in the process as discussed by Moura and colleagues (Chapter 6). The topology and mechanism of the electron transfer process can be studied in detail using dynamic NMR data and molecular modeUng tools as illustrated by Pettigrew and co-workers (Chapter 7). The action of these types of peroxidases can be considered of crucial importance for the redox state regulation of the cell. 

There is little doubt that MgATP both causes a conformational change in the Fe protein to lower its redox potential and is hydrolyzed during inter-protein electron transfer. However, its possible roles in substrate reduction remain obscure. There is also uncertainty about the site of inhibition by MgADP. Some evidence suggests that it may be associated with the MoFe... 

Finally, inverted regions have also been observed in electron transfers between redox centres embedded in proteins. This is both the case of modified intra-protein electron transfers briefly mentioned before [2,34], and of the reaction centres in photosynthetic bacteria [35]. 

In the first stages of photosynthesis carried out in proteins such as bacterial reaction centers (RCs) the energy of an absorbed photon is used to initiate a series of intra-protein electron transfers. These reactions have been well characterized (see reference I for a review). One important consequence of the resulting charge separation is proton uptake from the interior of the cell. The stoichiometry of proton uptake on formation of different redox states as a function of pH has been described in detail. More recent studies have monitored the influence of specific amino acids on proton uptake stoichiometry and kinetics. " It is of interest to understand how the RCs three dimensional structure yields the observed coupling of electron and proton transfers. 

Cytochromes c (Cyt c) can be defined as electron- transfer proteins having one or several haem c groups, bound to the protein by one or, more commonly two, thioether bonds. Cyt c possesses a wide range of properties and function in a large number of different redox processes. 

The properties of electron transfer proteins that are discussed here specifically affect the electron transfer reaction and not the association or binding of the reactants. A brief overview of these properties is given here more detailed discussions may be found elsewhere (e.g.. Ref. 1). The process of electron transfer is a very simple chemical reaction, i.e., the transfer of an electron from the donor redox site to the acceptor redox site. 

---