Cytochrome c, oxidase

The electron leaps across the membrane from Cu near the intermanbrane space in three ET reactions to the binuclear site of heme and Cub, where dioxygen is reduced to water. Cug (Cu+/Cu +) has three histidine ligands and one OH . 

In 1992, Babcock and Wikstrom clarified the ET in CcO (Complex TV). In the Wikstrom model, as updated by H. Michel, the reduction of dioxygen to water takes place in several steps. Four electrons are obtained from Cu at arbitrary times. All intermediate steps have to be stable. Dioxygen binds to Fe at the binuclear complex. We have to ranind ourselves of the orbital structure of the oxygen molecule. There is a strong o-bond and two strong jt-bonds. However, there are another two valence electrons, and these electrons enter the antibonding jt orbital. The spins are parallel 

The second electron enters the antibonding % orbital. There is now only the o-bond between the two oxygen atoms left. The outer oxygen receives another proton whereby H2O2 is formed. Due to repulsion from the positive Fe ion of hone A, it is quite possible that the outer oxygen carries both protons. 

In the next step, the o-bond is broken by transfer of first one election and then a second electron from the iron ion to the previously unoccupied, strongly antibonding a orbital. The bond between the oxygen atoms is now fnUy canceled and one water molecule leaves the site. Another two protons are added and bind to the remaining oxygen atom. 

Of the eight protons entering the binuclear center, fonr continne to the more acid intermembrane space, while four protons are used in the 4e rednction of O2. 

3 Cytochrome c Oxidase. Cytochrome c oxidase catalyses the reduction of dioxygen to water and the redox active components comprise a dicopper centre 

Bencini and D. Gatteschi, EPR of Exchange Coupled Systems, Springer-Verlag, 

Duboc-Toia, H. Hummel, E. Bill, A.-L. Barra, G. Chouteau and K. Wieghardt, Angew. Chem. Int. Ed. Engl, 2000, 39, 2888. 

Albela, E. Bill, O. Brosch, T. Weyhermiiller and K. Wieghardt, Eur. J. Inorg. Chem., 2000,139. 

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Many key protein ET processes have become accessible to theoretical analysis recently because of high-resolution x-ray stmctural data. These proteins include the bacterial photosynthetic reaction centre [18], nitrogenase (responsible for nitrogen fixation), and cytochrome c oxidase (the tenninal ET protein in mammals) [19, 20]. Although much is understood about ET in these molecular machines, considerable debate persists about details of the molecular transfonnations. 

Tsukihara T, Aoyama H, Yamashita E, Tomizaki T, Yamaguchi H, Shinzawaitoh K, Nakashima R, Yaono R and Yoshikawa S 1995 Structures of metai sites of oxidized bovine heart cytochrome c oxidase at 2.8 angstrom Science 269 1069-74... 

Hofacker, I., Schulten, K. Oxygen and proton pathways in cytochrome-c oxidase. Proteins Str. Funct. Genet. 29 (1998) 100-107... 

Cytochrome c oxidase (from bovine heart mitochondria). [9001-16-5] Mr 100,0007haeme,... 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

Iwata, S., Ostermeier, C., Ludwig, B., Michel, H. Structure at 2.8 A resolution of cytochrome c oxidase from Paracoccus denitrificans. Nature 376 660-669, 1995. 

Tsukihara, T., et al. The whole structure of the 13-subunit oxidized cytochrome c oxidase at 2.8 A resolution. Science 272 1136-1144, 1996. 

Complex IV Cytochrome c Oxidase The Thermodynamic View of Chemiosmotic Coupling ATP Synthase... 

Cytochrome c, like UQ is a mobile electron carrier. It associates loosely with the inner mitochondrial membrane (in the intermembrane space on the cytosolic side of the inner membrane) to acquire electrons from the Fe-S-cyt C aggregate of Complex 111, and then it migrates along the membrane surface in the reduced state, carrying electrons to cytochrome c oxidase, the fourth complex of the electron transport chain. 

Complex rV is called, cytochrome c oxidase because it accepts electrons from cytochrome c and directs them to the four-electron reduction of O2 to form H2O ... 

FIGURE 21.14 All electrophoresis gel showing the complex subunit structure of bovine heart cytochrome c oxidase. The three largest subunits, I, II, and III, are coded for by mitochondrial DNA. The others are encoded by unclear DNA. (Photo kindly provided by Professor Roderick Capaldi)... 

Thus, Og and cytochrome c oxidase are the final destination for the electrons derived from the oxidation of food materials. In concert with this process, cytochrome c oxidase also drives transport of protons across the inner mitochondrial membrane. These important functions are carried out by a transmembrane protein complex consisting of more than 10 subunits (Table 21.2). 

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... 

The link with the final electron acceptor, O2, is the enzyme cytochrome c oxidase which spans the inner membrane of the mitochondrion. It consists of cytochromes a and a3 along with two, or possibly three, Cu atoms. The details of its action are not fully established but the overall reaction catalysed by the enzyme is ... 

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. 

Chemical structure and reaction mechanisms of cytochrome c oxidase. R. Lemberg, Rev. Pure Appl. Chem., 1965,15,125-136 (132). 

Cytochrome enzymes, 2, 772 Cytochrome a3 oxidase, 6, 697 Cytochrome c oxidase, 6, 683 copper complexes, 2,724,772 Cytochrome oxidases, 6, 624 bacterial, 6,696... 

Figure 8. Mechanism of cytochrome c oxidase. Explanation given in text.

Fatal infantile cytochrome c oxidase (CCO) deficiency is characterized by total absence of catalytic activity in skeletal muscle. This often occurs within the context of the Fanconi syndrome, or less commonly in association with a cardiomyopathy. Although the deficiency is global in skeletal muscle, with all fibers affected, only isolated scattered fibers show abnormal aggregations of mitochondria (ragged-red fibers). Multiple affected siblings within one family are frequently encountered and suggest autosomal recessive inheritance. The condition normally proves fatal before the age of six months and is characterized by worsening intractable lactic acidemia. 

In vivo, patients treated with AZT develop a mitochondrial myopathy with mitochondrial DNA depletion, deficiency of cytochrome c oxidase (complex IV), intracellular fat accumulation, high lactate production and marked phosphocreatine depletion (Lewis and Dalakas 1995 Dalakas 2001). Clinically, the patient presents with fatigue, myalgia, muscle weakness, wasting and elevated serum creatine kinase. Muscle biopsy shows ragged red fibers , the characteristic histopathologic changes of mitochondrial myopathy, cansed by subsarcolemmal accumulation of mitochondria (Lewis and Dalakas 1995). 

Mayer D, Ataka K, Heberle J, Offenhaeusser A. 2005. Scanning probe microscopic studies of the oriented attachment and membrane reconstitution of cytochrome c oxidase to a gold electrode. Langmuir 21 8580-8583. 

In addition to their proven capacity to catalyze a highly efficient and rapid reduction of O2 under ambient conditions (e.g., cytochrome c oxidase, the enzyme that catalyzes the reduction of >90% of O2 consumed by a mammal, captures >80% of the free energy of ORR at a turnover frequency of >50 O2 molecules per second per site), metalloporphyrins are attractive candidates for Pt-free cathodes. Probably the major impetus for a search for Pt-free cathodic catalysts for low temperature fuel cells is... 

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