Cytochrome oxidase yeast

The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe + and Fe + during oxidation and reduction. Except for cytochrome oxidase (previously described), they are classified as dehydrogenases. In the respiratory chain, they are involved as carriers of electrons from flavoproteins on the one hand to cytochrome oxidase on the other (Figure 12-4). Several identifiable cytochromes occur in the respiratory chain, ie, cytochromes b, Cp c, a, and (cytochrome oxidase). Cytochromes are also found in other locations, eg, the endoplasmic reticulum (cytochromes P450 and h, and in plant cells, bacteria, and yeasts. 

Cytochrome c can easily be extracted from tissue particles by dilute salt solutions. It was isolated by Keilin and Hartree in 1930 and shown to contain a porphyrin ring structure. In 1933 Zeilen and Reuter established that cytochrome c was a heme (iron-porphyrin) protein. Slightly different forms of cytochrome a were distinguished in yeast and bacteria by Keilin in 1934 and the different properties of cytochrome a and a3 by Tamiya et al. in 1937. The identity of cytochrome 03, the enzyme which activates oxygen with Warburg s atmungsferment, was proposed by Keilin in 1939. Cytochrome a/a3 was renamed cytochrome oxidase by Malcolm Dixon (1939). The oxidation route then offered was ... 

Cytochrome oxidase (cytochrome aa3) represents the most important cytochrome of the a class. This is the terminal oxidase used in animals, plants, yeasts, algae and some bacteria. It contains two copper centres, giving four redox groups in total. This oxidase is discussed with other cytochromes that have a terminal oxidase function in Sections 62.1.12.4 and 62.1.12.5. These are cytochromes o, d and cd,. The oxidases fed719 and ax are not included in that discussion. The situation regarding cytochrome ax has been confused, partly due to uncertainty in the definition of this cytochrome. In some respects, the properties of cytochrome ax resemble those of mitochondrial and bacterial aa3. It functions as a terminal oxidase in some bacteria,720 but its role in E. coli is unknown. A soluble fraction from disrupted E. coli cells grown anaerobically on glycerol and fumarate contains a hemoprotein similar to cytochrome ax, which has catalase and peroxidase activity.721... 

Cytochrome oxidase also serves as a proton pump, so that the process of electron transfer is associated with the vectorial transfer of protons across the membrane, and thus contributes to the establishment of the proton gradient which is used to drive the synthesis of ATP. Cytochrome oxidase is located in the inner mitochondrial membrane of animal, plant and yeast cells (the eukaryotes) and in the cell membrane of prokaryotes. The arrangement is represented schematically in Figure 58. The complexity of cytochrome oxidase and the problems associated with its solubilization from the membrane have presented great obstacles to the elucidation of the structure and mechanism of the enzyme, but its importance has resulted in an enormous literature, which has been reviewed frequently.1296 1299... 

Bisson et al. (1980) labeled cytochrome oxidase with yeast cytochrome c to which an azido aryl group had been attached at Lys-13, and demonstrated that subunit II of the oxidase reacted specifically. In contrast, subunit III was attacked by a cytochrome c with an azido aryl group attached at Cys-102 (Moreland and Dockter, 1981). Lysine-13 and Cys-102 are on opposite sides of the cytochrome c molecule. 

Due to its central role in oxidative phosphorylation, cytochrome oxidase has a wide biological distribution. It is present in all animals and plants, in aerobic yeasts and in some bacteria. It is an integral membrane protein, being firmly associated with the inner membrane of mitochondria, the respiratory organelle of eukaryotic organisms, or, in bacteria the plasma membrane (Malstrom, 1990). 

Several additional lines of evidence indicate that presequence function is governed by conformational properties of the precursor. The target peptide of yeast mitochondrial cytochrome oxidase subunit IV (COX), fused to murine dihydrofolate reductase (DHFR, a cytoplasmic enzyme). 

However, in both the yeast and the E. coli systems, the stoichiometries for NADPH S and SOj -rS - are 3 1 and 1 1, respectively. These results and the inability to detect 2-electron- and 4-electron-reduced intermediates in these systems have suggested that such intermediates, if present at all, must be firmly held on the surface of the enzyme. It has further been suggested that the presence of multiple flavins and hemes in the enzyme might be a device for achieving a rapid six-electron reduction of sulfite without the release of intermediates (414)- This situation is analogous to the four-electron reduction of 0, to 2H,0 by cytochrome oxidase and the six-electron reduction of nitrite to ammonia by various assimilatory nitrite reductases. However, unlike cytochrome oxidase,... 

The true physiological role of cytochrome c peroxidase in yeast is yet to be established. It may serve as a part of the systems which prevent intracellular accumulation of harmful hydrogen peroxide. It would be of interest to know if cytochrome c peroxidase is synthesized concurrently with or in competition with the production of other peroxide-decomposing systems such as catalase. Although cytochrome c peroxidase is present in mitochondria of aerobically grown yeast in a concentration comparable to that of cytochrome oxidase (19) and possesses an extremely high molecular activity (fcj = 10 sec ) toward yeast ferrocytochrome c (17), it has not been unequivocally shown that ferrocytochrome c is a true substrate of this enzyme. 

The structure of cytochrome c is known from X-ray studies to < 2 A resolution [112], It binds to subunit II, and also to cytochrome c, of Complex 111 [113,114], with a lysine-rich domain near the solvent-accessible haem edge. This domain mainly involves lysines 13, 72, 86, possibly 79, and 27 to a lesser extent [115-118], When bound to the dimeric oxidase cytochrome c may lie in a cleft between the monomers so that the side opposite to the lysine-rich region interacts weakly with subunit III of the second monomer [119], The binding site on subunit II has recently been mapped more accurately. Four acidic residues have been suggested to be involved, i.e Asp-158 [120], Asp-112, Glu-198 [121,122] and Glu-114 [123] (numbering refers to the bovine subunit see also Fig. 3.4 below). Of these, Glu-114 is not conserved in subunits II from human, yeast and maize cytochrome oxidase (see Ref. 92). 

Although there is a 5-fold difference between the sizes of the mitochondrial genomes of yeast (84 kb) and mammals (16 kb), the number of proteins synthesized within mitochondria is similar. Proteins produced by mammalian mitochondria are those involved in electron-transport and oxidative-phosphorylation systems. These include cytochrome b, three subunits of cytochrome oxidase, one subunit of ATPase, and six subunits of NADH dehydrogenase. Apart from these differences, protein synthesis in mitochondria follows the same steps and mechanisms as those in the cytoplasm. 

Fi>2 is a member of a family of homologous flavoproteins that catalyze the oxidation of a-hydroxy acids. It is located in the intermembrane space of yeast mitochondria and provides pyruvate for the Krebs cycle as well as participates in a short electron-transfer chain involving cytochrome c and cytochrome oxidase, making it an important respiratory enzyme. Unlike many other flavoproteins that oxidize a-hydroxy acids, F 2 has very poor reactivity toward oxygen. Fi>2 is a homotetramer with each monomer containing both an FMN and a heme The catalytic cycle has five redox steps (Scheme 7). In the first step, lactate is oxidized to... 

Many type b cytochromes associated with the classic cytochrome oxidase (a and 03) show their a peaks at 561-563 nm, and they are usually designated as cytochrome b. The name cytochrome 61 was originally used for the pigments with a peak at 557-560 nm, but it is now generally applied to the cytochrome in the nitrate reductase system. Cytochrome 62 (a, 557 nm) is the entity of yeast lactate dehydrogenase which contains FMN and protoheme. Cytochromes 63 (a, 559 nm) and 65 (a, 556 nm) participate in the microsomal electron transport system in plants and animals, respectively. Both the primary and ternary structures of cytochrome 65 are known. Cytochromes bg (a, 563 nm) and 6-559 are the components of the photosynthetic electron transport system in plant. 

The behavior of zeolites in biological systems is a bit mysterious. For example, treatment of yeast mitochondria with zeolite A or X in the sodium or potassium cation forms disaggregates the mitochondria and solubilizes cytochrome oxidase in an aqueous system. However, the same zeolites in calcium or magnesium form showed little activity of this type (67). Similarly, zeolite A can disrupt bacterial cells (83). 

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