Complex cytochrome

Electron Transfer Within the Cytochrome fig/Cytochrome/ Complex... 

Fig. 5.8. When photosystem II is activated by absorbing photons, electrons are passed along an electron-acceptor chain and are eventually donated to photosystem I and finally to NAPD+. Photosystem II is responsible for the photolytic dissociation of water and the production of atmospheric oxygen. This pathway is sometimes referred to as the Z scheme because of its zigzag route, as depicted here, but the two arms are in fact remote in space. (Note Plastocyanin (Cu) is an alternative late replacement for an Fe cytochrome complex).

Raag, R. and Poulos, T. L. (1991) Crystal structures of cytochrome complexed with camphane, thiocamphor and adamantane factors controlling P450 substrate hydroxylation. Biochemistry 30, 2674-2684. 

Cytochrome a + a3 This cytochrome complex is the only electron carrier in which the heme iron has a free ligand that can react directly with molecular oxygen. At this site, the transported electrons, molecular oxygen, and free protons are brought together to produce water (see Figure 6.8). Cytochrome a + 83 (also called cytochrome oxidase) contains bound copper atoms that are required for this complex reaction to occur. 

Figure 23-18 Schematic view of photosynthetic reaction centers and the cytochrome //complex embedded in a thylakoid membrane. Plastocyanin (or cytochrome c6 in some algae and cyanobacteria) carries electrons to the PSI core.

Tmmpower, B. L., and Gennis, R. B. (1994). Energy Transduction by Cytochrome Complexes in Mitochondrial and Bacterial Respiration The Enzymol-... 

Coenzyme Q passes electrons through iron-sulfur complexes to cytochromes b and ch which transfer the electrons to cytochrome c. In the ferric Fe3+ state, the heme iron can accept one electron and be reduced to the ferrous state Fe2+. Since the cytochromes carry one electron at a time, two molecules on each cytochrome complex are reduced for every molecule of NADH that is oxidized. The electron transfer from coenzyme Q to cytochrome c produces energy, which pumps protons across the inner mitochondrial membrane. The proton gradient produces one ATP for every coenzyme Q-hydrogen that transfers two electrons to cytochrome c. Electrons from FADH2, produced by reactions such as the oxidation of succinate to fumarate, enter the electron transfer chain at the coenzyme Q level. 

In contrast, spinach plastocyanin binds to the soluble domain of its physiological partner cytochrome/ in a single orientation, indicating a short electron transfer path between the metal ions (Ubbink et al., 1998). A low-resolution structural model of the plastocyanin-cytochrome/complex was obtained by including paramagnetic constraints (derived from H and chemical shift differences) in molecular dynamics simulations where the structures of the two partners were kept rigid (Ubbink et al.,... 

Figure 6-5. Energetics and directionality of the coupling between electron flow and ATP formation in chloroplasts, emphasizing the role played by H+ (see also Fig. 5-19). The02 evolution from H20 and the electron flow via plastoquinones (PQ) and the cytochrome complex (Cyt b6f) lead to H+ accumulation in the lumen of a thylakoid. This H+ can moveback out through a hydrophobic channel (CF0) and another protein factor (CF, which together comprise the ATP synthetase, leading to ATP formation.

The be complexes from mitochondria, chloroplasts, and bacteria all contain three catalytic subunits harboring the four redox centers cytochrome b, the high-potential cytochrome C or /, and the Rieske iron sulfur protein. These subunits are required and sufficient to support electron transport since most bacterial bci complexes only consist of these three subunits. However, some bacterial bc complexes contain a fourth subunit with yet unknown function. Mitochondrial bc complexes contain in addition to the three catalytic subunits 7-8 subunits without redox centers two large core proteins which are peripherally located and which are members of the family of matrix proeessing peptidases (MPP), and 5-6 small subunits. In cytochrome complexes, cytochrome b is split into cytochrome b(, and subunit IV containing the C-terminal part of cytochrome b in addition, 3 small hydrophobic subimits are present [18]. 

As with all the other photosynthetic membrane complexes, the genes for the components of the cytochrome complex are distributed between the nuclear and chlo-roplast genomes of higher plants. The chloroplast genes for the Cyt /, Cyt 6-563 and 17 kDa polypeptides have been extensively characterized, but the nuclear gene(s) for the Rieske Fe-S protein have not yet been isolated. 

Figure 2.23. Shown is one half of the cytochrome complex, rotated about 40° horizontally and moved about 20° upward relative to the view in Fig. 2.22. The structure at the lower right (C) is the cytochrome / and the structure at the lower left (D) is called the Rieske protein. The upper structure (A) is the cytochrome b(. The other chains identified are a subunit (B) and 4 proteins (E-H). The area within the rectangle is enlarged in Fig. 2.24. Based on Protein Data Bank ID 1UM3 (Kurisu et al, 2003).

Because the cytochromes can only carry one electron at a time, two molecules in each cytochrome complex must be reduced for every molecule of NADH that is oxidized. 

The Cyt f complex lying between PS II and PS I in the electron-transport system resembles the Cyt be complex of mitochondria and photosynthetic bacteria. These cytochrome complexes possess one Rieske iron-sulfur protein R-FeS (a [2Fe-2S] protein discovered by John Rieske) and a so-called subunit IV. The two fc-hemes of Cyt b(, and the subunit IV span the thylakoid membrane, while the R-FeS and Cyt/ are located near the lumen side. As previously noted, the placement of the i>-hemes across the thylakoid membrane helps form a redox chain across the membrane. The function of the Cyt complex in green-plant thylakoids is to oxidize the plastohydroquinone formed by PS II and to transfer these electrons to plastocyanin. Accordingly, the Cyt ig/ complex has therefore also been called the plastohydroquinone-plastocyanin-oxidoreductase. ... 

Fig. 7. A frame of reference for the Rp. viridis RC-associated cytochrome complex (A) and a more detailed view of the cytochrome subunit with the four hemes shown (B). See text for the various nomenclatures used. P represents the [BChllj (the primary donor). The table also includes the redox-potential values of the hemes, and the wavelength of the a-band of the hemes both at room and cryogenic temperatures. Figure (A) the same as Fig. 7 in Chapter 2. (B) is taken from CRD Lancaster, U Ermler and H Michel (1995) The structure of photosynthetic reaction centers from purple bacteria as revealed by X-ray crystallography. In RE Blankenship, MT Madigan and CE Bauer (eds) Anoxygenic Photosysnthetic Bacteria, p 511. Kluwer.

Fig. 2 (C) shows a model representing the thylakoid membrane of a cyanobacterium or a red alga, consisting of photosystems I and II interconnected by the cytochrome- /complex, and the ATP synthase, CFo CFi. The phycobilisomes are seen as attached to the stromal surface at the PS-II reaction-center core complex.

Fig. 9. Room-temperature absorption spectrum of spinach Cytbe in the presence of dithionite (A) reduced-minus-oxidized difference spectra as indicated for spinach Cyt bg compiex measured at room temperature in (A) inset and at low temperature (77 K) in (B) (C) 77 K difference spectra of the isolated (spinach) Cyt bef complex titrated to the potentials indicated. See text for details. Figure source (A) and (B) Hurt and Hauska (1981) A cytochrome flb complex of five polypeptides with plastoquinol-plastocyanin-oxidoreductase activity from spinach chloroplasts. Eur J Biochem 117 594 (C) Hurt and Hauska (1983) Cytochrome /) from isolated cytochrome complexes. Evidence for two spectral forms with different midpoint potentials. FEBS Lett 153 415.

The Q-cycle hypothesis and other alternative versions of it were attempts to explain the two important reactions occurring during electron transport and proton translocation in the mitochondrial cytochrome be complex and also in the chloroplast cytochrome complex by the so-called oxidant-induced reduction of Cyt b and the interheme electron transport in the Cyts b. Abundant experimental work to obtain evidence for these two reactions as well as other aspects relating to the structure and function of the be complexes has been performed. In addition to what has been mentioned above, we will present several selected examples to elucidate the oxidant-induced reduction of Cvt b and the need for two quinone-binding sites, using the chloroplast CyX-b(f complex or the Cyi-bcx complex from photosynthetic bacteria as examples, all monitored by absorbance changes of cytochromes induced by either steady or flash illumination. 

Figure 30 summarizes the main proposals for cytochrome c involvement in Chromatium photosynthesis (377-380). Cytochromes C662 and Cess are the components of the cytochrome complex listed in Table XX. There are obvious analogies with green plant and purple nonsulfur bacterial photosynthesis (Figs. 27 and 29), but with less known about intermediate substances. It has been proposed that the high potential Ciu participates in cyclic photosynthesis and the lower potential cs52 is involved mainly in substrate-fed noncyclic photosynthesis, but this distinction is muddied because there is some communication between the two cytochromes... 

Hope AB, Liggins J and Matthews DB (1988) The kinetics of reactions in and near the cytochrome complex of chloroplast... 

The last cytochrome complex is cytochrome oxidase, which passes electrons from cytochrome c to O2 (see Fig. 21.5). It contains cytochromes a and a and the oxygen binding site. A whole oxygen molecule, O2, must accept four electrons to be reduced to 2 EI2O. Bound copper (Cu ) ions in the cytochrome oxidase complex facilitate the collection of the four electrons and the reduction of O2. 

Our Dl-D2-cytochrome complexes were stable at 406.7 and 441.6 nm, but photodegraded slowly at 436 nm, the Soret absorption maximum of chlorophyll a. Therefore, the data collection time at 436 nm was adjusted so that no degradation products were observed. 

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