Superoxide dismutase, electron

The efficiency of the electron transfer from a-terthienyl to oxygen was estimated to be lower than 1% 209). The efficiency is somewhat higher, 6%, with 2,2 -bithienyl 103). The generation of superoxide was revealed experimentally by the photoreduction of cytochrome c or of nitro blue tetrazolium sensitized by a-terthienyl in aqueous solution under aerobic conditions, a reaction which is suppressed in the presence of the very specific enzyme superoxide dismutase. Electron transfer reactions can also occur directly from electronically excited a-terthienyl to cytochrome c, but they are not favored in the presence of oxygen 150). 

Nickel is found in thiolate/sulflde environment in the [NiFe]-hydrogenases and in CODH/ACS.33 In addition, either a mononuclear Ni-thiolate site or a dinuclear cysteine-S bridged structure are assumed plausible for the new class of Ni-containing superoxide dismutases, NiSOD (A).34 [NiFe]-hydrogenase catalyzes the two-electron redox chemistry of dihydrogen. Several crystal structures of [NiFe]-hydrogenases have demonstrated that the active site of the enzyme consists of a heterodinuclear Ni—Fe unit bound to thiolate sulfurs of cysteine residues with a Ni—Fe distance below 3 A (4) 35-39 This heterodinuclear active site has been the target of extensive model studies, which are summarized in Section 6.3.4.12.5. 

Carloni et al.91 applied the DFT(PZ) calculations to investigate the electronic structure of various models of oxydized and reduced Cu, Zn superoxide dismutase. The first stage of the enzymatic reaction involves the electron transfer from Cu" ion to superoxide. The theoretical investigations provided a detailed description of the electronic structure of the molecules involved in the electron transfer process. The effect of charged groups, present in the active center, on the electron transfer process were analyzed and the Argl41 residue was shown to play a crucial role. 

Carloni, P., P. E. Blochl, and M. Parrinello. 1995. Electronic Structure of the Cu, Zn Superoxide Dismutase Active Site and Its Interactions with the Substrate. J. Phys. Chem. 99, 1338. 

Superoxide Dismutase. Again, only electron-capture is important on irradiation (78). For the Cu-Zn enzyme, Cu is converted into Cu form. In the presence of oxygen, 02 is formed in competition with Cu, and on annealing reacts to re-form Cu. Thus radiolysis has proven to be a useful method for checking the mechanism of action of this dismutase. The conclusion is that the somewhat disputed mechanism [21,22] is probably correct. 

IV. Superoxide dismutase (EC 1.15.1.1) Within a cell the superoxide dismutases (SODs) constitute the first line of defense against ROS. Superoxide radical (02) is produced where an electron transport chain is present, as in mitochondria and chloroplasts, but 02 activation may occur in other subcellular locations such as glyoxysomes, peroxisomes, apoplast and the cytosol. Thus SODs are present in all these cellular locations, converting superoxide into hydrogen peroxide and water (i.e. copper/zinc SODs are typically found in the nuclei and cytosol of eukaryotic cells). 

J.M. McCord and I. Fridovich, Utility of superoxide dismutase in studying free radical reactions. II. Mechanism of the mediation of cytochrome c reduction by a variety of electron carriers. J. Biol. Chem. 245,1374-1377 (1970). 

Y. Tian, M. Shioda, S. Kasahara, T. Okajima, F. Mao, T. Hisabori, and T. Ohsaka, A facilitated electron transfer of copper-zinc superoxide dismutase (SOD) based on a cysteine-bridged SOD electrode. Biochim. Biophys. Acta. 1569, 151-158 (2002). 

X. Wu, X. Meng, Z. Wang, and Z. Zhang, Study on the direct electron transfer process of superoxide dismutase. Bioelectrochem. Bioenerg. 48, 227-231 (1999). 

T. Ohsaka, Y. Shintani, F. Matsumoto, T. Okajima, and K. Tokuda, Mediated electron transfer of polyethylene oxide-modified superoxide dismutase by methyl viologen. Bioelectrochem. Bioenerg. 37, 73-76 (1995). 

Y. Tian, T. Ariga, N. Takashima, T. Okajima, L. Mao, and T. Ohsaka, Self-assembled monolayers suitable for electron-transfer promotion of copper, zinc-superoxide dismutase. Electrochem. Commun. 6, 609-614 (2004). 

Type II copper enzymes generally have more positive reduction potentials, weaker electronic absorption signals, and larger EPR hyperfine coupling constants. They adopt trigonal, square-planar, five-coordinate, or tetragonally distorted octahedral geometries. Usually, type II copper enzymes are involved in catalytic oxidations of substrate molecules and may be found in combination with both Type I and Type III copper centers. Laccase and ascorbate oxidase are typical examples. Information on these enzymes is found in Tables 5.1, 5.2, and 5.3. Superoxide dismutase, discussed in more detail below, contains a lone Type II copper center in each of two subunits of its quaternary structure. 

Cu,Zn superoxide dismutase. Essentially, these observations support a stepwise one-electron model again. Interestingly, the oxidation state of copper does not change during the catalytic reaction, i.e. the sole kinetic role of the histidine coordinated metal center is to alter the electronic structures of the substrate and 02 in order to facilitate the electron transfer process between them. 

Carloni, P., Blochl, P. E. and Parrinello, M. Electronic structure of the Cu,Zn superoxide dismutase active site and its interactions with the substrate, J.Phys.Chem., 99 (1995), 1338-1348... 

IPHC, Intraperitoneal hyperthermic chemoperfusion/chemotherapy MMC, Mitomycin C IP, Intraperitoneal SOD, Superoxide dismutase Nd YAG, Neodymium-doped yttrium aluminium garnet Nd Y3A15012 NIR, Near infrared FITC, Fluorescein isothiocyanate PEG, Polyethylene glycol FA, Fohc acid CDDP, Cisplatin TEM, Transmission electron microscopy... 

Aerobic aerobic microorganisms utilize oxygen as a terminal electron acceptor and possess superoxide dismutase or catalase enzymes which are capable of degrading peroxide radicals. 

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