Cytochrome b cyt

All known cytochrome bc complexes have three essential subunits, which contain all four redox centers in common. These subunits are cytochrome b (cyt. b), cytochrome ( (cyt. c ), and the Rieske [2Fe-2S] protein (ISP). The redox centers are heme b i and heme b in cytochrome b (L and H represent low and high potential, respectively), heme C in cytochrome ci, and Rieske [2Fe-2S] (FeS) cluster in ISP. 

Fig. 12.5. Biogenesis and assembly of cytochrome 6-c, complex in the inner mitochondrial membrane. Cytochrome fc-Cj complex contains at least five different subunits COREI (corl), COREII (corll), nonheme iron protein (Fe-S), cytochrome c, (cyt Cj), and cytochrome b (cyt b). Cytochrome f> is a mitochondrial gene product and is probably assembled into the inner membrane (IM) via vectorial translation by mitochondrial ribosomes. The other subunits are synthesized on cytoplasmic ribosomes as larger precursors. The precursors, perhaps in association with a cytoplasmic factor , are attached to receptors on the mitochondrial outer membrane (OM). The complex laterally diffuses to the junctions of the outer and inner membranes, and with the help of a hypothetical translocator the precursors are imported across the membrane. Pre-Corl, pre-Corll, and the pre-nonheme iron protein cross the two membranes, whereas cytochrome c, becomes anchored to the outer face of the inner membrane, facing the intermembrane space (IMS). Cytochrome b is assembled inside the inner membrane, and the nonheme iron protein and Corl and Corll are assembled into the matrix side of the inner membrane. The N-terminal extensions are removed by a soluble matrix protease. The N-terminal extension of cytochrome c, is removed in two steps the first is catalyzed by the matrix protease and the second probably by a protease located on the outer face of the inner membrane.

A method for species identification from pork and lard samples using polymerase chain reaction (PCR) analysis of a conserved region in the mitochondrial (mt) cytochrome b (cyt b) gene has... 

FIGURE 21.11 The structure of UQ-cyt c reductase, also known as the cytochrome hci complex. The alpha helices of cytochrome b (pale green) define the transmembrane domain of the protein. The bottom of the structure as shown extends approximately 75 A into the mitochondrial matrix, and die top of the structure as shown extends about 38 A into the intermembrane space. (Photograph kindly provided by Di Xia and Johann Deismhofer [From Xia, D., Yn, C.-A., Kim, H., Xia,J-Z., Kachnrin, A. M., Zhang, L., Yn,... 

Figure 12.2a. Photosynthetic Z-scheme for green plants. Abbreviations not included in the text are PQ, plastiquinone Cyt bse, a form of cytochrome b absorbing at 564 nm FD, ferredoxin FP a flavoprotein. Long vertical arrows indicate steps arising from photoactivation of pigment reaction centers dashed arrows indicate uncertain pathways.0185...

Scheme 3. Flavins are capable to undergo both 1 e and 2 e input/output redoxreactions as indicated. Since cytochrome b is a typical 1 e redox reagent, Hemmerich and Schmidt86) suggest a radical mechanism of the sensory transduction (assuming that the cyt b photoreduction is photo-tropically relevant). The nature and fate of the flavin-photosubstrate XH remains obsure. For the case X = cyt b the scheme represents a reversed respiration electron pathway...

Figure 18-8 Stereoscopic ribbon diagrams of the chicken bc1 complex (A) The native dimer. The molecular twofold axis runs vertically between the two monomers. Quinones, phospholipids, and detergent molecules are not shown for clarity. The presumed membrane bilayer is represented by a gray band. (B) Isolated close-up view of the two conformations of the Rieske protein (top and long helix at right) in contact with cytochrome b (below), with associated heme groups and bound inhibitors, stigmatellin, and antimycin. The isolated heme of cytochrome c, (left, above) is also shown. (C) Structure of the intermembrane (external surface) domains of the chicken bcx complex. This is viewed from within the membrane, with the transmembrane helices truncated at roughly the membrane surface. Ball-and-stick models represent the heme group of cytochrome cy the Rieske iron-sulfur cluster, and the disulfide cysteines of subunit 8. SU, subunit cyt, cytochrome. From Zhang et al.105...

Fig. 26. Z-Scheme of photosynthesis in plants. Chi is chlorophyll, cyt b, f is cytochrome b, / PC is plastocyanine, (Fe-S) is iron sulfer protein. ATP is adenosine triphosphate ADP is adenosine diphosphate Pj is the phosphate ion and NADP is the nicotinamide adenine dinucleotide phosphate ion [203]...

This electron transport and proton flow is controlled by at least three other proteins on the cytosolic side of the membrane, P47, P67, and P21rac (Figure 5). Note that this is a much simpler enzyme complex than the complex III (cyt bc ) of mitochondria which drives proton export from the interior of mitochondria based on quinol oxidation. Despite its simplicity, the neutrophil enzyme may have similarities to the complex three, since it essentially carries out an oxidation of a protonated two electron flavin by a nonprotonated cytochrome b complex with two heme sites. This is the essence of the mitochondrial enzyme in that the two electron quinol is oxidized by a cytochrome b with two heme components. 

Cytochromes b are involved in the Q cycle and are not in the direct electron path from NADH to 02. UQ is reduced by cytochrome b - Ubiquinol is reoxidized by cyt b566, which then proceeds to reduce cytochrome b to complete the cycle. 

A number of other cytochromes are located in the intermembrane space and the inner surface of the outer mitochondrial membrane. Cyt bs (cytochrome b ) is a small heme protein with a hydrophihc domain of about 95 amino acids and a carboxyl terminus of about 45 hydrophobic amino acids that serve to anchor the protein to the inner surface of the outer mitochondrial membrane. Cyt bs is reduced by NADH-cyt bs reductase, also located on the inner surface of the outer membrane. Under conditions of high intermembrane ionic strength, Cc is released from the inner membrane, and can transport electrons from cyt bs on the outer membrane to CcO on the inner membrane. Cyt bs contains protoheme b with the iron atom hgated by two histidine nitrogen atoms. The heme group has a reduction potential of -1-10 mV versus NHE. Cyt bs is also found on the liver endoplasmic reticulum membrane, where it transfers electrons from NADH-cyt b ... 

The mammalian complex consists of 11 subunits, three of which contain redox centers a hydrophobic cytochrome b, which contains the high and low-potential fe-type hemes (cyt... 

SO is found in various vertebrate liver tissues and catalyzes the two-electron oxidation of sulfite to sulfate coupled to the reduction of two molecules of oxidized Cyt c. It contains molybdopterin and b-heme cofactors in separate domains connected by a polypeptide bridge, analogous to Fcyt b. The smaller N-terminal heme domain resembles cytochrome b the C-terminal domain contains Mo(VI) and catalyzes sulfite oxidation coupled to oxo group transfer. Two one-electron intramolecular ET steps, each followed by ET to Cyt c, complete the catalytic cycle. 

In purple photosynthetic bacteria, and specifically in Rps. sphaeroides and Rps. capsulata, three cytochromes of b type have been identified by means of redox titration, in the dark, of isolated chromatophores [116]. They are characterized by midpoint potentials at pH = 7.0 equal to 0.155, 0.050 and -0.090 V (in Rps. sphaeroides)-, the of the 0.050 V species is pH dependent ( — 60 mV per pH unit) [116,117]. The presence of a cytochrome cc in these organisms, interfering spectrally with cytochrome b, makes the situation unclear as far as the existence of cyt. b E j = 0.155 V) is concerned [118]. The two other cytochromes E = 0.050 and — 0.090 V) have also been resolved kinetically in studies on the photosynthetic electron transport and on the basis of their spectral characteristics (band at 561 nm and a spht bands at 558 and 556 nm, respectively these two cytochromes will be referred to as 6-561 and 6-566 in the following) [119]. 

Several observations made in whole membranes or in the isolated complexes are in line with these concepts the shifts induced by antimycin A [110,137] and myxothiazol on the absorption spectra of cytochromes b and the alterations of the ESR spectrum of the FeS protein by UHDBT or DBMIB [131]. Moreover, the oxidant-induced reduction of cytochromes b, the key observation for accepting these electron transfer schemes, has been demonstrated in all h/c, complexes isolated so far from mitochondria [134], chloroplasts [111], cyanobacteria [112] and photosynthetic bacteria [110]. In the chloroplast b /f complex this reaction has been demonstrated also in the absence of any exogenously added quinol, indicating that possibly a structurally bound quinone (quinone is always present in the isolated complexes with a stoicheiometry of about 0.5-0.7 mol/mol of cyt. c, [110,111]) is sufficient to drive the reduction of cytochromes [138]. Since a detailed treatment of the genera] mechanism, as well as of the more specific problems of the mitochondrial respiratory chain, are reported in Chapter 3 of this volume, the following discussion will deal only with the specific features of the electron transfer chains in photosynthetic membranes. 

The presence in chloroplasts and algae of a cytochrome b with the band at 559 nm, and therefore distinct from cyt. was described in the mid 1970s [120]. However, in spite of this early discovery and of the efforts of many laboratories, the actual role of this carrier in the photosynthetic chain is still obscure. 

Fig. 12.1. Relative sizes of mitochondrial and chloroplast chromosomes and location of protein structural genes. The figure was constructed from published data [5,15,17,22,26-28]. The structural genes are marked by wide sections. Black areas code for proteins. White areas are introns. 0x1, OxII and OxIII are subunits I, II and III of cytochrome c oxidase. Cyt b, cytochrome b. Fo and Fo, are subunits 6 and 9 of the proton ATPase complex. In the chloroplast chromosome the arrows indicate the transcription direction and the size of the transcripts. CF,a, CFj/8, CFjc and CFoIII are subunits a, /S, t and III of the chloroplast proton ATPase complex [30]. PSII5], PSII44, and PSII34 are subunits of photosystem II reaction center with the corresponding molecular weights of 51000, 44000 and 34000. PSI70 is subunit I of photosystem I reaction center. Cyt /is cytochrome/ cyt is cytochrome b and b -flV is subunit IV of cytochrome b(,-f complex.

A) An intBr e for the light-controlled bitxling of Cyt c.The cytochrome electrochem-tstry i promoted by the pyridine component, but it is repelled from I lb. (B) Cyclic vottammagrams of cytochrome c, Cyt c, I x 1monolayer electrode. (b)The pyrldlne-protonated-nitromerocyanine (I lb -mb(ed monolayer electrode. Data recorded at a scan rate of 50 mV sec . (Adapted from reference 89, Figure 1. Reproduced by permission ofThe Royal Society of Chemistry.)... 

Abbreviations FAD, flavin adenine dinucleotide Fe-S, iron-sulfur proteins that can he identified in separate clusters by electron paramagnetic resonance analysis (the s-1, s-2 subscripts identify these iron-sulfur proteins as part of the succinate dehydrogenase complex) His, the histidine linkage between FAD and the large (70,000 daltons) protein moiety of the enzyme FMN, flavin mononucleotide N-la, N-2 subscripts identify these iron-sulfur proteins as part of the NADH-dehydro-genase complex UQ, ubiquinone Cyt bf and Cyt b, cytochrome b-566 and b-563, respectively. 

As indicated, above, the two Z -hemes of the Cyt b f complex provide a pair of reacting sites spanning the thylakoid membrane, one near the stromal side and the other near the lumenal side of the thylakoid membrane. The plastohydroquinone is first oxidized by the Rieske FeS to a semiquinone, which is then oxidized by cytochrome/, which then releases the electron to the copper protein plastocyanin. After loss of one electron by the plastohydroquinone, the resulting semiquinone loses an electron to the two fc-hemes in series. The Z -hemes operate in the so-called Q-cycle, similar to that in the mitochondrial or bacterial cytochrome bc complex, and provide a translocation of additional protons across the membrane into the lumenal space. Discussion of the cytochrome b(,f complex and the Q-cycle will be presented in Chapter 35. 

Cytochrome b /also serves as an intermediate in a non-linear, or so-called cyclic, electron-transport pathway around PS I, as formulated in Fig. 1 (B). A third function of Cyt b /is translocation of protons across the thylakoid membrane during electron transfer from plastoquinol to plastocyanin [Fig. 1 (C)]. The unique effects resulting from electron transport and proton translocation in the cytochrome b(f complex are the production of an electrochemical potential and a pH gradient across the thylakoid membrane to provide energy in a form needed for ATP synthesis (see the following chapter). 

Fig. 1. The location and relationship of cytochrome b/complex with respect to photosystems I and II and the three major functions of Cyt b/[ A), (B) and (C)] in oxygenic photosynthesis. The stroma and lumen sides are also represented by n (negative) and p (positive) to indicate that protons are pumped from the n- to the p-side, or the sign of ph+ generated during translocation.

Fig. 2. (A) A model for b/in chloroplast thylakoid membrane (B) Topological arrangement of the four subunits of the purified Cyt b/complex (C) A densitometric scan of an SDS-PAGE gel for b/ (B) from Hauska, Schiitz and Biittner (1996) The cytochrome b/ complex - composition, structure and function. In DR Ort and CF Yocum (eds) Oxygenic Photosynthesis - The Light Reactions, p 384. Kluwer (C) from Black, Widger and Cramer (1987) Large-scale purification of active cytochrome b f complex from spinach chloroplasts. Arch Biochem Biophys 252 657.

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