Oxidases copper-containing, electron transfer

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

Cytochrome c oxidase contains two, or possibly three, copper atoms referred to as Cua and Cub since they do not fit into the usual classification. The former (possibly a dimer) is situated outside the mitochondrial membrane, whereas the latter is associated with an iron atom within the membrane. Both have electron transfer functions but details are as yet unclear. 

Figure 1 also contains a drawing of the electronic structure of plastocy-anin (Penfield et al., 1981, 1985), oriented according to the first part of Fig. 1. The d-orbital plane appears to be normal to the peptide plane that is extended by the hydrogen-bonding pair the it orbital of the thiolate would then also be affected, poising the cluster for electron transfer and, interestingly, apparently in the direction utilized by the multi-copper protein ascorbate oxidase (see Section V,A).

Ceruloplasmin is involved in copper storage and transport as well as in iron mobilisation and oxidation. Among the blue oxidases it is unique since it contains, in addition to the usual motif of a type 1 combined with the trinuclear cluster, two other type 1 coppers. Electron transfer occurs, however, only between five of the six copper ions since one of the type 1 centres is not catalytically relevant due to its too high redox potential. The redox potentials of the centres were determined and possible electron transfer pathways among the copper sites were discussed.101... 

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. 

The phenomenon of bioelectrical catalysis with direct electron transfer from electrode to enzyme active site was primarily observed in the study of electrochemical oxygen reduction in the presence of a copper-containing oxidase - laccase, adsorbed on electrodes of different origins. This work was developed with peroxidase and hydrogenase application as the working components [2],... 

This type of active site is also known as a mixed-valence copper site. Similarly to the type 3 site, it contains a dinuclear copper core, but both copper ions have a formal oxidation state of +1.5 in the oxidized form. This site exhibits a characteristic seven-line pattern in the EPR spectra and is purple colored. Both copper ions have a tetrahedral geometry and are bridged by two sulfur atoms of two cysteinyl residues. Each copper ion is also coordinated by a nitrogen atom from a histidine residue. The function of this site is long-range electron transfer, and it can be found, for example, in cytochrome c oxidase [12-14], and nitrous oxide reductase (Figure 5.1 e). 

Galactose oxidase is an extracellular enzyme secreted by the fungus Dactylium den-droides. It is monomeric (M = 68000), contains a single copper site and catalyses the oxidation of a wide range of primary alcohols to the corresponding aldehydes. The two-electron transfer reaction RCH2OH - RCHO + 2H+ + 2e does not utilise a Cu(III)/Cu(I) couple, but a second redox site, involving a tyrosine radical which mediates the transfer of the second electron. 

Reduced cytochrome c passes its electron on to the next member of the respiratory chain, cytochrome (a -I- a3). These two cytochromes, together known as cytochrome c oxidase, comprise two iron-porphyrin prosthetic groups and two copper centres. Electrons are transferred to cytochrome a and then to a3 which contains Cu which cycles between the + 1 and + 2 oxidation states. The net function performed by this terminal member of the electron transport chain is the four-elec-... 

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). 

Ascorbate oxidase is a tetramer each subunit has 552 amino acids and contains 4 copper ions, the type-I blue copper center and the adjacent trinuclear center (arranged as a type-n center and a type-in dinuclear center) separated by /S-sheets (Figure 20) °. Ascorbate is oxidized to dehydroascorbate by dioxygen however, it is not bound directly to the metal center to be oxidized, but is proposed to bind near the type-I Cu site which may facihtate electron transfer to oxygen, presumably in the tri-Cu cluster site. Since humans cannot synthesize ascorbic acid, conservation of this important compound is highly regulated. For example, the oxidized ascorbate can be transported into red blood... 

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