Eukaryotic Cytochromes

Dyer R B, Einarsdottir 6, Killough P M, Lopez-Garriga J J and Woodruff W H 1989 Transient binding of photodissociated CO to of eukaryotic cytochrome oxidase at ambient temperature. Direct evidence from time-resolved infrared spectroscopy J. Am. Chem. Soc. Ill 7657-9... 

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,... 

The electron transport protein, cytochrome c, found in the mitochondria of all eukaryotic organisms, provides the best-studied example of homology. The polypeptide chain of cytochrome c from most species contains slightly more than 100 amino acids and has a molecular weight of about 12.5 kD. Amino acid sequencing of cytochrome c from more than 40 different species has revealed that there are 28 positions in the polypeptide chain where the same amino acid residues are always found (Figure 5.27). These invariant residues apparently serve roles crucial to the biological function of this protein, and thus substitutions of other amino acids at these positions cannot be tolerated. 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

FIGURE 25.14 The conversion of stearoyl-CoA to oleoyl-CoA in eukaryotes is catalyzed by stearoyl-CoA desaturase in a reaction sequence that also involves cytochrome -65 and cytochrome -65 reductase. Two electrons are passed from NADH through the chain of reactions as shown, and two electrons are also derived from the fatty acyl substrate. 

FIGURE 3.17 Cytochrome P450 systems in prokaryotes and eukaryotes. (From Neilson, A.H. and Allard, A.-S. Microbial metabolism of PAHs and heteroarenes, The Handbook of Environmental Chemistry, Vol. 3J, pp. 1-80, Springer, 1998. With permission.)... 

P450 systems (Sariaslani 1991), their widespread role in the transformation of xenobiotics (Smith and Davis 1968), and their occurrence and activities in yeasts (Kappeli 1986). The essential features of prokaryotic and eukaryotic cytochrome P450 systems are compared in Figure 3.17. 

Adamantane (A) and adamantan-4-one (B) were specifically hydroxylated at the quaternary C-1 by cytochrome P450j, to produce C and D. In contrast, the eukaryotic cytochrome P450lm2 formed in addition to the C-2 compound from adamantane, and both 5-hydroxy-adamantan-l-one (D) and the 4-a fi-hydroxyadamantan-l-one (E) from adamantan-4-one (Figure 3.18b) (White et al. 1984). 

Cytochrome P450 hydroxylation activity is well established in eukaryotic yeasts and some fungi,... 

Three distinct types of heme/Cu oxidases are currently recognized cytochrome c oxidases (CcOs or COX), quinol oxidases, and cytochrome ebbj, oxidases [Pitcher and Watmough, 2004], with CcOs being limited to eukaryotic organisms. The... 

Brautigam, D.L., Feinberg, B.A., Hoffman, B.M., Margoliash, E., Peisach, J., and Blumberg, W.E. 1977. Multiple low spin forms of the cytochrome c ferrihemochrome. EPR spectra of various eukaryotic and prokaryotic cytochromes c. The Journal of Biological Chemistry 252 574-582. 

BARNES, H.J., Maximizing expression of eukaryotic cytochromes P450 in Escherichia coli, Meth. Enzymol., 1996, 272, 3-14. 

The cytochrome he complex in eukaryotes is a homodimeric, multi-subunit entity (Figure 13.13a). Each monomer has three catalytic subunits a cytochrome b, with two b-type haems, one Rieske ISP containing a [2Fe-2S] cluster and one cytochrome c, with... 

Gafs els M, Olin M, Chowdhary BP, Raudsepp T, Andersson U. et al. 1999. Structure and chromosomal assignment of the sterol 12alpha-hydroxylase gene (CYPSBl) in human and mouse eukaryotic cytochrome P-450 gene devoid of introns. Genomics 56 184-196. 

Cytochrome bci is a multicomponent enzyme found in the inner mitochron-drial membrane of eukaryotes and in the plasma membrane of bacteria. The cytochrome bci complex functions as the middle component of the mitochondrial respiratory chain, coupling electron transfer between ubiquinone/ ubiquinol (see Figure 7.27) and cytochrome c. 

The preceding discussion of cytochromes c provides most detail on eukaryotic, mitchondrial cytochromes c, a small subset of this huge superfamily. Additionally, all the cytochromes c discussed in this section envelop one heme cofactor, although many cytochromes in nature contain more than one heme cofactor. Many other redox proteins contain a cytochrome c domain—a few of these mentioned here include the cytochrome bci complex discussed in Section 7.6, cytochrome c oxidase to be discussed in Section 7.8, and cytochrome c peroxidase, discussed briefly in Section 7.7 (see especially Section 7.7.4.2). 

Cytochrome c and ubiquinol oxidases are part of an enzyme superfamily coupling oxidation of ferrocytochrome c (in eukaryotes) and ubiquinol (in prokaryotes) to the 4 e /4 reduction of molecular oxygen to H2O. After this introduction, we will concentrate on the cytochrome c oxidase enzyme. The two enzymes, cytochrome c oxidase (CcO) and ubiquinol oxidase, are usually defined by two criteria (1) The largest protein subunit (subunit I) possesses a high degree of primary sequence similarity across many species (2) members possess a unique bimetallic center composed of a high-spin Fe(II)/(III) heme in close proximity to a copper ion. Cytochrome c oxidase (CcO) is the terminal... 

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