Cytochrome-c complexes

FIGURE 19-15 Summary of the flow of electrons and protons through the four complexes of the respiratory chain. Electrons reach Q through Complexes I and II. QH2 serves as a mobile carrier of electrons and protons. It passes electrons to Complex III, which passes them to another mobile connecting link, cytochrome c. Complex IV... 

Electron transfer to 02 occurs stepwise, through a series of flavoproteins, cytochromes (heme-proteins), iron-sulfur proteins, and a quinone. Most of the electron carriers are collected in four large complexes, which communicate via two mobile carriers— ubiquinone (UQ) and cytochrome c. Complex I transfers electrons from NADH to UQ, and complex II transfers electrons from succinate to UQ. Both of these complexes contain flavins and numerous iron-sulfur centers. Complex III, which contains three cyto-... 

Sulfide oxidation by phototrophic bacteria is catalyzed by c-type cytochromes, flavocytochromes and even cytochrome c complexes (see 4.2). A heat-labile cytochrome c-550 of Thiocapsa roseopersicina is responsible for the oxidation of sulfide. The end product is elemental sulfur and it is assumed that this cytochrome might also catalyze the reverse reaction by reducing the intracellularly stored elemental sulfur to sulfide (4.9V... 

Subunits II, Via, and VIb are arranged on the top of subunit I and form a concave surface including a surface atom near the Cua site (Fig. 7D). The concave surface is large enough to bind two cytochrome c molecules. However, a theoretical prediction of cytochrome c binding to the enzyme based on the X-ray structures of both proteins does not seems very straightforward, particularly for the weakly bound site. Crystallization of the enzyme-cytochrome c complex is desirable. 

Gray, G., and Knaff, D. B., 1982, The role of a cytochrome C552 -cytochrome c complex in the oxidation of sulfide in Chromatium vinosum. Biochem. Biophys. Acta. 680 290n296. 

There are a number of factors which make flavocytochrome b2 an ideal model system for studying both intra- and inter-molecular electron transfer. Reasons include (i) the fact that it has been expressed at a high level in E. coli (Black et al., 1989) and is soluble and easily obtained (ii) crystal structures of the native (Xia and Mathews, 1990) and recombinant (Tegoni and Cambillau, 1994) enzymes are available (iii) a hypothetical structure for the flavocytochrome 4 2 cytochrome c complex has been proposed (Short et al., 1998) (iv) many mutant forms of the enzyme have been constructed (v) there is a wealth of data on the mechanism of action of the enzyme (Chapman et ah, 1991 Lederer, 1991). 

Guerlesquin, F., Dolla, A., and Bruschi, M., 1994, Involvement of electrostatic interactions in cytochrome c complex formations, Biochimie 76(6) 515n23. 

Evidence for more complex ET processes came from studies in which photo chemically generated reductants injected electrons into preformed Fe-cytochrome b lYt-cytochrome c complexes. In one study, the rate of c ET (1.7 X 10 s ) was reported to depend on viscosity and surface mutations. A later laser-flash photolysis study found a rate-limiting second-order reduction of Fe-cytochrome i s/Fe-cytochrome c complexes and no sign of satmation, suggesting that the intracomplex ET rate was greater than lO s-. ... 

The data given is collected from Refs. 74-79. Cytochrome aa, contains two haems and two coppers per monomer. Considerable variations in the literature are due in part to differences in protein determination, and in part to use of erroneous extinction coefficients. The ratio of Complex 1 Complex 111 Cytochrome c Complex IV Ubiquinone is about 1 4 8 8 64 in most mitochondria. The content of cytochrome c is somewhat variable, however, and is lowered by extensive washing of mitochondria is salt solutions. 

An illustrative example of combining protein modeling and electron microscopy data is the case of the apoptosome, an Apaf-1 cytochrome c complex that activates procaspase-9 [40]. The data obtained in this work helped to decipher the exact mechanism of a very important apoptosis triggering mechanism. Another interesting example is the use of computational and biochemical methods to conduct structural analyses of the seven proteins that compose the core building block of the nuclear pore complex [41]. 

Complexes I and II transfer electrons from NADH and succinate, respectively, to UQ. Complex III transfers electrons from UQH2 to cytochrome c. Complex IV transfers electrons from cytochrome c to 02. The arrows represent the flow of electrons. 

Aromatic residues have been found in proteins at positions that probably enhance the electronic coupling in systems that have been selected by evolution for efficient ET. Examples are the tryptophan mediated reduction of quinone in the photosynthetic reaction center (31), the methylamine dehydrogenase (MADH) amicyanin system, where a Trp residue is placed at the interface between the two proteins (32), as well as the [cytochrome c peroxidase-cytochrome c] complex, where a Trp seems to have a similar function (33). 

Computer-graphics models of the cytochrome 5/cytochrome c complex (left) static model produced by docking the x-ray structures of the individual proteins (right) after extension by molecular dynamics simulations." Reproduced with permission from Reference 112. 

Complex IV - Complex IV is also known as cytochrome oxidase, because it takes electrons from cytochrome c. Complex IV contains cytochromes a and a3. Cytochromes a and a3 evidently represent two identical heme A moieties, attached to the same polypeptide chain. They are within different environments in the inner membrane, however, so they have different reduction potentials. Each of the hemes is associated with a copper ion, located close to the heme iron. Electrons that pass through complex IV can be blocked by cyanide, azide, and carbon monoxide and the artificial electron carrier, ferricyanide, can accept electrons from cytochrome a in the complex (Figure 15.9). A model for the final stages in proton pumping by cytochrome oxidase is shown in Figure... 

In the cytosol, cytochrome c binds Apaf (pro-apoptotic protease activating factor). The Apaf/cytochrome c complex binds caspase 9, an initiator caspase, to form an active complex called the apoptosome. The apoptosome in turn activates execution caspases by zymogen cleavage. 

Intramolecular oxidation and reduction in cytochrome c complexes covalently modified was studied by several groups (for review see 190). Histidines (191, 192, 193) and cysteines (194) were used to attach covalently Ruthenium complexes to Fe- or Zn-substituted cytochrome c. Most of the experiments were done using laser lash photolysis. In each series of experiments, the distance was considered as constant and determined by molecular modelling. The free energies span between 0.5 to 1.4V. The L T rate constants do vary with the driving force as expected. However the reactions proceed with rate constants lower than those expected on the basis of results obtained on peptides. Results were all analyzed using Marcus theory. X and Hab were considered as adjustable parameters. Each series of experimental data was fitted separately (3 to 6 points). In all these papers, X values go from 1.15 to 1.22 eV and Hab vary from 0.1 to 0.24 cm l. Activation volumes were also measured (195). It seems that the transition state is more compact than the reactant state in both intra- and inter-molecular steps. 

X 10 M" s . This value is equal to that calculated theoretically for a diffusion controlled reaction occurring when collisions are at high frequencies. This is not the case for the cytochrome hi core - cytochrome c complex since the affinity of the latter complex is found equal to 2.5 pM while the dissociation constant of the flavocytochrome hi - cytochrome c complex is equal to 0.1 pM. 

Binding of TNS to the flavodehydrogenase - cytochrome c complex induces a conformational change around FMM (Albani, 1993). In presence of TNS, the flavin plane orientation is different from that observed in its absence. 

---