Electron transfer in cytochrome

Electron Transfer in Cytochrome c and Its Redox Partners. Electron... 

Scott JR, Willie A, MacLean M, et al. Intramolecular electron transfer in cytochrome b5 labeled with ruthenium(II) polypyridine complexes rate measurements in the Marcus inverted region. J Am Chem Soc 1993 115 6820-4. 

Contzen J, Kostka S, Kraft R, Jung C. Intermolecular electron transfer in cytochrome P450cam covalently bound with tris(2,2 -bipyridyl)ruthenium(II) structural changes detected by FTIR spectroscopy. J Inorg Biochem 2002 91 607-17. 

As summarized earlier, there is consensus with regard to the sequence of electron transfer in cytochrome oxidase. The Cua center is the initial acceptor of electrons from cytochrome c (k 3 x 10 M s ). This electron transfer depends cmcially on a conserved tryptophan residne in snbnnit n ca. 5 A away from the Coa center. Then follows fast electron eqinlibration between CnA and the low-spin heme (kf 10 s Iq 5 x 10 s , kf and kr denoting the... 

Ruthenium complexes are excellent reagents for protein modification and electron-transfer studies. Ru +-aquo complexes readily react with surface His residues on proteins to form stable derivatives [20, 21]. Low-spin pseudo-octahedral Ru-complexes exhibit small structural changes upon redox cycling between the Ru + and Ru + formal oxidation states [3, 22]. Hence, the inner-sphere barriers to electron transfer (Ai) are small. With the appropriate choice of ligand, the Ru + + reduction potential can be varied from <0.0 to >1.5 V versus NHE [23]. Ru-bpy complexes bound to Lys and Cys residues have been employed to great advantage in studies of protein-protein ET reactions. The kinetics of electron transfer in cytochrome 65/cytochrome c [24], cytochrome c/cytochrome c peroxidase [12], and cytochrome c/cytochrome c oxidase [25] complexes have been measured with the aid of laser-initiated ET from a Ru-bpy label. 

Methods for Studying Electron Transfer in Cytochrome Oxidase... 

One factor that has greatly complicated elucidation of electron transfer in cytochrome oxidase is the strong redox interaction between the haems (see Refs. 12, 99). Although this phenomenon is relatively well elucidated, its functional relevance is not yet understood. 

There are three main reasons to suggest a specific function of subunit III in proton translocation. First, Casey et al. [171] showed that modification of this subunit with dicyclohexylcarbodiimide (DCCD) blocks proton translocation, but has little effect on electron transfer. Similar results have been obtained with the reconstituted oxidase from the thermophilic bacterium PS3 [164]. Prochaska et al. [160] showed that DCCD binds mainly to Glu-90 of the bovine subunit III, which is predicted to lie within the membrane domain and hence to be a site analogous to the DCCD binding site in the membranous fj, sector of the ATP-synthase (Fig. 3.8 see also Ref. 85). Since the latter is a part of a proton-conducting channel in ATP synthase, subunit III was thought to have the same function. However, there is one essential difference between the two phenomena. Modification of the membranous glutamic residue in by DCCD leads also to inhibition of ATP hydrolysis in the complex, as expected for two linked reactions. In contrast, DCCD has little or no effect on electron transfer in cytochrome oxidase under conditions where H translocation is abolished. Hence, DCCD cannot simply be judged to block a proton channel in the oxidase. More appropriately, it decouples proton translocation from electron transfer. 

Hrabakova, J., Ataka, K., Heberle, J., Hildebrandt, P., and Murgida, D.H. (2006) Long distance electron transfer in cytochrome c oxidase immobilised on electrodes. A surface enhanced resonance Raman spectroscopic smdy. Physical Chemistry Chemical Physics, 8, 759-766. 

Intramolecular electron transfer in cytochrome c has been investigated by attaching photoactive Ru complexes to the protein surface. Ru(bpy)2(C03) (bpy = 2,2 -bipyridine) has been shown to react with surface His residues to yield, after addition of excess imidazole (im), Ru(bpy)2(im)(His) +. The protein-bound Ru complexes are luminescent, but the excited states ( Ru ) are rather short lived (r 100 ns). When direct electron transfer from Ru to the heme cannot compete with excited-state decay, electron-transfer quenchers (e.g., Ru(NH3)6 + ) are added to the solution to intercept a small fraction (1-10%) of the excited molecules, yielding (with oxidative quenchers) Ru ". If, before laser excitation of the Ru site, the heme is reduced, then the Fe to Ru + reaction (ket) can be monitored by transient absorption spectroscopy. The ket values for five different modified cytochromes have been reported (Ru(His-33), 2.6(3) xlO Ru(His-39), 3.2(4) xlO Ru(His-62), 1.0(2) x 10 Ru(His-... 

Free-Energy Dependence of Electron Transfer in Cytochrome c Labeled with Ruthenium(II)-Polypyridine Complexes... 

Over the past several years, we have developed a technique that has proven extremely valuable in the study of electron transfer between redox sites in metalloproteins. We have reported kinetic studies of the reaction of cytochrome c with cytochrome c peroxidase (i-3), cytochrome oxidase (4), cytochrome bs (5, 6) plastocyanin (7), and cytochrome Ci (8). In addition, we have been able to show (9,10) that intramolecular electron transfer in cytochrome bs covalently... 

Sevrioukova, I.F., J.T. Hazzard, G. Tollin, and XL. Poulos (1999). The FMN to heme electron transfer in cytochrome P450BM-3. Effect of chemical modification of cysteines engineered at the FMN-heme domain interaction site. J. Biol. Chem. 274, 36097-36106. 

Brenner S, Hay S, Munro AW, Serutton NS (2008) Inter-flavin electron transfer in cytochrome P450 reductase—effects of solvent and pH identify hidden complexity in mechanism. FEES J 275 4540-4557... 

---