Cytochrome high-potential

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Fitzsimmons, M. E., Collins, J. M., Selective biotransformation of the human immunodeficiency virus protease inhibitor saquinavir by human small-intestinal cytochrome P4503A4 potential contribution to high first-pass metabolism, Drug. Metab. Dispos. 1997, 25, 256-266. 

Metalloproteins fall into three main structure categories depending on whether the active site consists of a single coordinated metal atom, a metal-porphyrin unit, or metal atoms in a cluster arrangement. In the context of electron-transfer metalloproteins, the blue Cu proteins, cytochromes, and ferre-doxins respectively are examples of these different structure types. Attention will be confined here mainly to a discussion of the reactivity of the blue Cu protein plastocyanin. Reactions of cytochrome c are also considered, with brief mention of the [2Fe-2S] ferredoxin, and high potential Fe/S protein [HIPIP]. 

Fig. 105. Examples of small disulfide-rich or metal-rich proteins (shown on the right side) compared with their more regular counterparts in other structural categories (shown at the left), (a) Tobacco mosaic virus protein, an up-and-down helix bundle (b) cytochrome bs, a distorted up-and-down helix bundle (c) trypsin domain 1, a Greek key antiparallel /3 barrel (d) high-potential iron protein, a distorted Greek key /3 barrel (e) glutathione reductase domain 3, an open-face sandwich fi sheet (f) ferredoxin, a distorted open-face sandwich f) sheet.

The following description of the electron transfer-proton transport scheme is illustrated in Figure 7.26. First, an electron is transferred from doubly reduced dihydroplastoquinone (PQFI2) to a high potential electron transfer chain that consists of the Reiske iron-sulfur protein and the cytochrome protein containing heme f. Rappaport,Lavergne and co-workers have reported a midpoint potential at pH 7.0 of +355 mV for heme f. These two centers reside on the electropositive (lumen or p) side of the membrane, exterior to the membrane. As a result, two protons are transferred to the aqueous lumen phase. A second electron is transferred from PQH2 sequentially to heme bp. 

The proton-motive Q-cycle model, put forward by Mitchell (references 80 and 81) and by Trumpower and co-workers, is invoked in the following manner (1) One electron is transferred from ubiquinol (ubiquinol oxidized to ubisemi-quinone see Figure 7.27) to the Rieske [2Fe-2S] center at the Qo site, the site nearest the intermembrane space or p side (2) this electron can leave the bci complex via an attached cytochrome c or be transferred to cytochrome Ci (3) the reactive ubisemiquinone reduces the low-potential heme bL located closer to the membrane s intermembrane (p) side (4) reduced heme bL quickly transfers an electron to high-potential heme bn near the membrane s matrix side and (5) ubiquinone or ubisemiquinone oxidizes the reduced bn at the Qi site nearest the matrix or n side. Proton translocation results from the deprotonation of ubiquinol at the Qo site and protonation of ubisemiquinone at the Qi site. Ubiquinol generated at the Qi site is reoxidized at the Qo site (see Figure 7.27). Additional protons are transported across the membrane from the matrix (see Figure 7.26 illustrating a similar process for cytochrome b(6)f). The overall reaction can be written... 

A third, clearer explanation of the electron transfer, proton translocation cycle is given by Saratse. Each ubiquinol (QH2) molecule can donate two electrons. A hrst QH2 electron is transferred along a high-potential chain to the [2Fe-2S] center of the ISP and then to cytochrome Ci. From the cytochrome Cl site, the electron is delivered to the attached, soluble cytochrome c in the intermembrane space. A second QH2 electron is transferred to the Qi site via the cytochrome b hemes, bL and bn. This is an electrogenic step driven by the potential difference between the two b hemes. This step creates part of the proton-motive force. After two QH2 molecules are oxidized at the Qo site, two electrons have been transferred to the Qi site (where one ubiquinone (Qio) can now be reduced, requiring two protons to be translocated from the matrix space). The net effect is a translocation of two protons for each electron transferred to cytochrome c. Each explanation of the cytochrome bci Q cycle has its merits and its proponents. The reader should consult the literature for updates in this ongoing research area. 

Fe atoms. It had been anticipated that the c-type cytochrome center would have His/Met coordination, but His/His is observed. The former is the more usual coordination, especially at the high potential end E° > +200 mV) ofthe typical bacterial electron transfer chain to which the nitrite reductase is connected (Fig. 2) (7). The second curious feature is that the di heme iron is also six-coordinate thus, the enzyme does not offer a substrate-binding site at either heme. In addition to an expected axial histidine ligand there was an axial tyrosine (residue 25) ligand to the d heme (Fig. 4a). Each monomer is organized into two domains. 

Fig. 7. The structure of the oxidized state of cytochrome c peroxidase from P. aeruginosa (only a monomer is shown here). The small sphere between the low-potential heme (top) and the high-potential heme (bottom) domains shows the position of the Ca ion.

As it inhibits microsomal cytochrome P450 cimetidine has a high potential for drug interactions not shared by the other H2 receptor antagonists. The oxidative metabolism of agents such as anticoagulants, most antiepileptics, some beta-blockers, warfarin, theophylline and many hypnotics, neuroleptics and antidepressants may be reduced, leading to increased effects. 

In contrast to the above reductases which contain only a single flavin moiety, NADPH-cytochrome P450 reductase contains both FMN and FAD One flavin may be presumed to accept two reducing equivalents from NADPH while the other serves as a one-electron reductant for the heme iron of cytochrome 450- Flavin resolution and reconstitution studies have shown that the FAD moiety is the low-potential flavin that accepts reducing equivalents from NADPH while the FMN moiety is the high potential flavin that serves as the one-electron reductant for cytochrome 1 450 ... 

Complex III (ubiquinol-cytochrome c oxido-reductase or cytochrome bct complex). Mitochondrial complex III is a dimeric complex, each subunit of which contains 11 different subunits with a total molecular mass of 240 kDa per monomer.104-107 However, in many bacteria the complex consists of only three subunits, cytochrome b, cytochrome c , and the high potential ( 0.3 V) Rieske iron-sulfur protein, which is discussed in Chapter 16, Section A,7. These three proteins are present in all bc1 complexes. 

Finally, as usual, bacteria provide an exception to the general rule. In this case the di-haem cytochrome c peroxidase from Pseudomonas aeruginosa removes its electron, not from a porphyrin, nor an amino acid, but instead from a separate high-potential haem, which is converted from Fe11 to Fe111 [22]. This proves that, at least for peroxidases, nature can manage very well without protein-bound free radicals if necessary. 

The cytochromes are the electron carrier heme proteins occurring in the mitochondrial respiratory chain.449 There are five cytochromes linking coenzymes Q (ubiquinone) and 02 in this electron transport chain (Scheme 7). Cytochromes are also involved in energy transfer in photosynthesis. The iron atom in cytochromes cycles between the Fe11 and Fe111 states, i.e. they are one-electron carriers, in contrast to CoQ and the NADH flavins they act upon which are two-electron carriers. Thus, one molecule of reduced CoQ transfer its two high potential electrons to two molecules of cytochrome b, the next member of the electron transport chain. 

Relaxation phenomena are equally evident at the donor side of the reaction center. The well studied, fast (< 1 //s) electron transfer reactions from bound c-type cytochromes of RCs from such species as Chromatium and Rps. viridis f requentlv show a progressive shutoff of electron transfer from the high potential heme, which is closest to P (Gao et al., 1990). It is suggested that this is due to a large (> 100 mV) increase in the midpoint potential of the heme, associated with the freezing out of solvent or hydration-related relaxation processes (Kaminskaya et al., 1990). This is consistent with a deuterium solvent isotope effect for this reaction, as reported by Kihara and McCray (1973). 

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. 

Three alternative mechanisms were proposed based only on the thermodynamic data (403). All of these assumed distinct functions for each flavin and interaction between the flavins. They also assumed that electrons would be transferred to cytochrome P-450 one at a time this has been shown to be the case with cytochromes P-450 that receive electoons from iron-sulfur proteins rather than from the flavoprotein directly (or through the indirect mediation of lipid) (405, 406). One of these mechanisms (403) is shown below. It seems to fit best with the kinetic data determined for the lipase-solubilized reductase (345, 398). In this scheme, SH is a hydroxylatable substrate and SOH its hydroxylated product, and Fli and FU are the high potential and low potential flavins, respectively. 

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]. 

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