Cytochrome oxidase, also copper

Electron carriers which contain iron (and in the case of cytochrome oxidase also copper) they undergo oxidation and reduction by electron transfer alone. These are the cytochromes, in which the iron is present in a haem molecule, and nonhaem iron proteins, sometimes called iron—sulphur proteins, because the iron is bound to the protein through the sulphur of the amino acid cysteine. Figure 3.19 shows the arrangement of the iron in non-haem iron proteins and the three different types of haem that occur in cytochromes ... 

Complex IV, cytochrome oxidase. This copper-containing enzyme, which also contains cytochromes a and a3, accumulates electrons, then passes them to 02, reducing it to H20. 

Cytochrome oxidase (also called complex IV) contains two cytochromes (cytochrome a and a3). Cytochrome a is paired with a copper atom, CuA, and cytochrome a3 is paired with a different copper atom, CuB. During electron... 

Cytochrome Oxidase (also known as complex IV) is an iron and copper containing enzyme in the electron transport system. It catalyzes the final step in the electron transport process - the transfer of electrons and protons to oxygen, to form water (Figures 15.2,15.3, 15.10). Transfer of electrons through cytochrome oxidase can be blocked by cyanide, azide, and carbon monoxide. 

This enzyme [EC 1.9.3.1] (also referred to as cytochrome cytochrome oxidase) catalyzes the reaction of four ferrocytochrome c with dioxygen to produce four ferricytochrome c and two water molecules. This protein also contains copper ions as cofactors. 

Addition of NO to oxidized cytochrome oxidase produces a state in which NO binds to the copper center rather than to the heme (Brudvig et al., 1980. The Cu(Il)-NO complex is diamagnetic EPR signals can be observed at g = 6 which probably result from the ferric heme a, now uncoupled from Cu(ll). It is also possible to assign these signals to some S = f coupled state involving both iron and cooper, but this is much less likely. 

In the presence of NO and azide, cytochrome oxidase forms a complex with integral spin EPR spectra that have been assigned to a triplet state formed by coupling of S = 2 heme and copper centers (Brudvig et al., 1980). This explanation is possible, but other net integral spin possibilities could also explain the... 

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. 

The CuA center has an unusual structure.130-132 It was thought to be a single atom of copper until the three-dimensional structure revealed a dimetal center, whose structure follows. The CuB-cytochrome a3 center is also unusual. A histidine ring is covalently attached to tyrosine.133-1353 Like the tyrosine in the active site of galactose oxidase (Figs. 16-29,16-30), which carries a covalently joined cysteine, that of cytochrome oxidase may be a site of tyrosyl radical formation.135... 

Copper has an essential role in a number of enzymes, notably those involved in the catalysis of electron transfer and in the transport of dioxygen and the catalysis of its reactions. The latter topic is discussed in Section 62.1.12. Hemocyanin, the copper-containing dioxygen carrier, is considered in Section 62.1.12.3.8, while the important role of copper in oxidases is exemplified in cytochrome oxidase, the terminal member of the mitochondrial electron-transfer chain (62.1.12.4), the multicopper blue oxidases such as laccase, ascorbate oxidase and ceruloplasmin (62.1.12.6) and the non-blue oxidases (62.12.7). Copper is also involved in the Cu/Zn-superoxide dismutases (62.1.12.8.1) and a number of hydroxylases, such as tyrosinase (62.1.12.11.2) and dopamine-jS-hydroxylase (62.1.12.11.3). Tyrosinase and hemocyanin have similar binuclear copper centres. 

In the following sections, examples of all the classes of reaction given above will be discussed, with emphasis on those that have been well studied. Many of the enzymes involved in these processes are hemoproteins, but non-heme prosthetic groups are important in oxygenases and are also present in some oxidases. Copper is an important metal in this context, and is present in the oxygen transport protein hemocyanin, and in oxidases such as cytochrome oxidase and laccase. Some flavoenzymes are important too, but will not be covered in this discussion. 

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

The arrangement of the metal centers is in remarkable agreement with the structure, also reported in 1995 [49b], for another member of the superfamily of heme-copper oxidases, the cytochrome oxidase from Paracoccus denitrificans. The two structures show a strikingly similar coordination and arrangement of the five redox-active metals (the two irons and three coppers). 

Mitochondria contain ubiquinone (also known as coenzyme Q), which differs from plastoquinone A (Chapter 5, Section 5.5B) by two methoxy groups in place of the methyl groups on the ring, and 10 instead of 9 isoprene units in the side chain. A c-type cytochrome, referred to as Cyt Ci in animal mitochondria, intervenes just before Cyt c a h-type cytochrome occurring in plant mitochondria is involved with an electron transfer that bypasses cytochrome oxidase on the way to 02. The cytochrome oxidase complex contains two Cyt a plus two Cyt a3 molecules and copper on an equimolar basis with the hemes (see Fig. 5-16). Both the Fe of the heme of Cyt a3 and the Cu are involved with the reduction of O2 to H20. Cytochromes a, >, and c are in approximately equal amounts in mitochondria (the ratios vary somewhat with plant species) flavoproteins are about 4 times, ubiquinones 7 to 10 times, and pyridine nucleotides 10 to 30 times more abundant than are individual cytochromes. Likewise, in chloro-plasts the quinones and the pyridine nucleotides are much more abundant than are the cytochromes (see Table 5-3). 

Copper and Zinc in Aerobic Metabolism. Cytochrome oxidase, the terminal oxidase in the electron transport chain contains an atom of copper. On this enzyme the protons and electrons generated during oxidative metabolism combine with elemental oxygen to form water. During copper deficiency the tissue concentration of cytochrome oxidase is reduced. While the effects of lower cytochrome oxidase activity on exercise has not been described, it is likely that aerobic energy metabolism will be diminished. This effect of copper deficiency was first described in animals with myelin aplasls — the degeneration myelin (86). The oxidative process of phospholipid synthesis, a primary component of myelin, was depressed. Liver mitochondria had impaired respiratory activity (87). Cytochrome oxidase activity was also depressed in brain, heart and liver. 

Cardiovascular Disorders and Copper. Sudden cardiac failure has been associated with copper deficiency (91J. There are two attractive mechanisms. First, the coronary arteries and aorta may become weakened from an inability to synthesize elastin due to a decrease in lysyl oxidase activity. Rupture of these major blood vessels has been shown to cause sudden death in animals suffering from copper deficiency. Second, a decrease in cytochrome oxidase activity during copper deficiency Impairs aerobic metabolism of the heart and increases the risk of hypertrophy. Hypertrophy, which may lead to high output congestive heart failure, is exacerbated by hypochromic anemia also caused by copper deficiency. 

Nor is a member of the superfamily of heme-copper oxidases that also include the cytochrome oxidases (found e.g. in mitochondria ). Although no crystal structure of Nor is known at present, the number of transmembrane a-helices and the location and structure of their cofactors (heme c, heme b, heme bs and Fee) are highly similar to those of the cytochrome oxidases. 

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