Quinol oxidase

Ambient temperature catalysis of O2 reduction at low overpotentials is a challenge in development of conventional proton exchange membrane fuel cells (pol5mer electrolyte membrane fuel cells, PEMFCs) [Ralph and Hogarth, 2002]. In this chapter, we discuss two classes of enz5mes that catalyze the complete reduction of O2 to H2O multi-copper oxidases and heme iron-containing quinol oxidases. 

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... 

CcOs from various organisms, as well as CcOs and quinol oxidases, are fairly homologous stmcturally and functionally, and both are distinct from cytochromes ebbs. Most, if not all, CcOs and quinol oxidases require two subunits for catalytic activity (subunits I and II Fig. 18.4), although some, such as mammahan CcO, may contain as many as 11 more subunits of unknown functions [Abramson et al.. 

Figure 18.4 Structures of heme/Cu oxidases at different levels of detail, (a) Position of the redox-active cofactors relative to the membrane of CcO (left, only two obligatory subunits are shown) and quinol oxidase (right), (b) Electron transfer paths in mammalian CcO. Note that the imidazoles that ligate six-coordinate heme a and the five-coordinate heme are linked by a single amino acid, which can serve as a wire for electron transfer from ferroheme a to ferriheme as. (c) The O2 reduction site of mammalian CcO the numbering of the residues corresponds to that in the crystal structure of bovine heart CcO. The subscript 3 in heme as and heme 03 signifies the heme that binds O2. The structures were generated using coordinates deposited in the Protein Data Bank, lari [Ostermeier et al., 1997] Ifft [Abramson et al., 2000] (a) and locc [Tsukihara et al., 1996] (b, c).

The reduction of ground state O2 with organic substances is fairly slow in aqueous or nonaqueous solutions [57] in spite of the high redox potential of O2. The O2 is utilized effectively, however, as the terminal oxidant in the respiratory chain in a biomembrane with redox enzymes composed of membrane proteins such as heme proteins containing cytochrome c oxidase [58-60] or quinol oxidase [61,62]. 

Suzuki T, Hashimoto T, Yabu Y, Kido Y, Sakamoto K, Nihei C, Hato M, Suzuki S, Amano Y, Nagai K, Hosokawa T, Minagawa N, Ohta N, Kita (2004) Direct evidence for cyanide-insensitive quinol oxidase (alternative oxidase) in the apicomplexan parasite Cryptosporidium parvum phylogenetic and therapeutic implications. Biochem Biophys Res Commun 313 1044-1052... 

The cytochrome aa3 terminal oxidase is produced constitutively, i.e., under all conditions. However, when cells are grown on succinate or H2 another set of enzymes is produced with the b-type cytochrome o3 as the terminal quinol oxidase (Eq. 18-2).85... 

Vos, M. H., Borisov, V. B., Liebl, U., Martin, J-L., Konstantinov, A. (2000) Femtosecond resolution of ligand-heme interaction in the high-affinity quinol oxidase bd a di-heme active site Proc. Natl. Acad. Sci. USA 97, 1554-1559. 

Multicopper blue oxidases are synthesized as a single polypeptide chain, which is composed of three BCB domains in the case of laccases (LC) and ascorbate oxidases (AO) and six such domains in ceruloplasmin (CP) and hephaestin (HP). Structurally they are arranged in a triangular manner. These enzymes, along with heme-copper oxidases (cytochrome c oxidases and quinol-oxidases) and a cyanide-resistant alternative oxidase found in mitochondria of plants and fungi, are the only known enzymes capable of catalyzing four-electron reduction of dioxygen to water. In the... 

X-ray structures of 2.8-A resolution of bovine heart cytochrome c oxidase with the metals in the fully oxidized state were reported in 1995 (Tsukihara et al., 1995). The X-ray structure of cytochrome c oxidase from Paracoccus denitrificans in the fuUy oxidized azide-bound state at 2.8-A resolution was also published in the same week (Iwata et al., 1995). The structure and location of the metal sites of the two enzymes are astonishingly similar at that resolution. Later, the resolution of the bovine enzyme structure was improved to 2.3A (Yoshikawa et al., 1998). However, resolution of the Paracoccus enzyme has been improved to 2.7-A resolution (Ostermeier et al., 1997). Recently another bacterial ba3-type oxidase at 2.3-A resolution (Soulimane et al., 2000) and Escherichia coli quinol oxidase at 3.5-A resolution were reported (Abramson et al., 2000). X-ray structures of the protein and its redox-active metal sites are discussed in terms of the bovine enzyme below. 

Cua centers exist in two redox states [Cu(II)Cu(I)] and [Cu(I)Cu(I)]. The oxidized species is a fully delocalized mixed-valence pair (formally two Cu+ 1.5 ions), as revealed by EPR spectroscopy (Kroneck et al., 1988, 1990). Despite the similar coordination geometry around copper, these systems display sharper NMR lines than do the BCP due to a shorter electron relaxation time of the paramagnetic center (wlO "s) (dementi and Luchinat, 1998). NMR studies are available for the native Cua centers from the soluble fragments of the The. thermophilus, Paracoc-cus denitrificans, Paracoccus versutus, and Bacillus subtilis oxidases (Bertini et al., 1996 Dennison et al., 1995 Luchinat et al., 1997 Salgado et al., 1998a) and Pseudomonas stutzeri N2O reductase (Holz et al., 1999), as well as for engineered Cua sites in amicyanin (Dennison et al., 1997) and Escherichia coli quinol oxidase (Kolczak et al., 1999). 

Wilmanns, M., Lappalainen, P., Kelly, M., Sauer-Eriksson, E., and Saraste, M., 1995, Crystal structure of the membrane-exposed domain from a respiratory quinol oxidase complex with an engineered dinuclear copper center, Proc. Natl. Acad. Sci. USA 92 11955nll959. 

Cua and heme 3. The Mg site is indeed lacking from the quinol oxidases, which also lack Coa, and where electrons from a bound quinol on the back of the low-spin heme are transferred via that heme gronp to the binuclear site. ... 

Membranes from T. acidophilum [ 24] oxidize ubiquinol Qio. Menaquinones are oxidized at about a 10-fold greater rate which is in keeping with the observation that a menaquinone derivative (thermoplasmaquinone) occurs in Thermoplasma membranes [127]. Quinol oxidase can be extracted using detergents and activity... 

Fig. 3.12B shows a more detailed molecular interpretation of the 6 cycle. A single ubiquinone species in a ubiquinone pocket of the Complex III monomer is shown to interact either with the quinol oxidase site (o), or with a semiquinone reductase site (i). Flip-flop of the headgroup (usually of SQ ) between the sites might take place, in principle as proposed for bulk phase ubiquinone (see Section 4.7.2). However, here it occurs within a specific proteinaceous pocket so that regulation by the enzyme is possible. 

Metal-ligand stretching modes were assigned from the resonance Raman spectrum of uu3-quinol oxidase of Acidianus ambivalens.391... 

B + 0 bo3 215 (Escherichia coli) Quinol oxidase with proton pumping activity... 

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