铜锌超氧化物歧化酶(SOD)研究进展——从基因到功能

超氧化物歧化酶(SOD,EC 1.15.1.1),己经在多种组织中发现,它能将O2.-催化生成H2O2及O2.迄今为止,已经从哺乳动物体内分离出三种SOD:CuZnSOD(SOD1)、MnSOD(SOD2)TLEC-SOD(胞外超氧化物歧化酶,SOD3),各自具有不同的生化及分子特性.CuZnSOD(SOD1),是一类含有Cu及Zn原子的二聚体,存在于特定细胞的基质内,约占SOD总量的90%.在胞质及周质中,SOD以二聚体形式存在,而在线粒体及质外,则以四聚体形式存在.在保护脑、肺及其它组织的氧化应激中,CuZnSOD被认为起着保护作用.运动神经元肌萎缩侧索硬化症(ALS),据称也与同源二聚体CuZnSOD的错误折叠有关,己经报导,有多个CuZnSOD基因位点突变与ALS有关.本文将从基因的结构、表达、调节及蛋白的结构与功能等方面,对CuZnSOD进行简要论述.     

Mechanism and Atomic Structure of Superoxide Dismutase

The active site Cu ion in Cu,Zn superoxide dismutase is alternately oxidized and reduced during the enzymatic dismutation of superoxide to hydrogen peroxide and molecular oxygen. For oxidized Cu,Zn superoxide dismutase, an atomic structure has been determined for the human enzyme at 2.5 A resolution. The resolution of the bovine enzyme structure has been extended to 1.8 A. Atomic resolution data has been, collected for reduced and inhibitor-bound Cu,Zn superoxide dismutases. and the interpretation of the' electron density difference maps is in progress. The geometry and molecular surfaces of the active sites in these structures, together with biochemical data, suggest a specific model for the enzyme mechanism. Similarities in the active site geometry of the Mn and Fe superoxide dismutases with the Cu.Zn enzyme suggest that dismutation in these enzymes may follow a similar mechanism.     

Crystallographic structures of bovine copper-zinc superoxide dismutase reveal asymmetry in two subunits: functionally important three and five coordinate copper sites captured in the same crystal

A key feature of the generally accepted catalytic mechanism of CuZn superoxide dismutase (CuZnSOD) is the breakage of the imidazolate bridge between copper and zinc and the loss of a coordinated water molecule from copper on reduction from Cu(II) to Cu(I). Crystal structures exist for the enzyme from a number of sources in the oxidised, five coordinate copper form. For the reduced form two structures from different sources have been determined only recently but provide contradictory results. We present crystal structures of bovine CuZnSOD (BSOD) in two different space groups. The structure of the P212121 form (pBSOD), at 1.65 A resolution clearly shows one subunit with Cu in the five coordinate, oxidised form, and the other with Cu in the three coordinate form expected for the reduced state. This mixed state of pBSOD is confirmed by XANES data of these crystals. The pBSOD structure has thus captured each subunit in one of the two oxidation state conformations and thus provides direct crystallographic evidence for the superoxide dismutase mechanism involving the breakage of the imidazole bridge between Cu and Zn. A shift in the position of copper in subunit A poises the catalytic centre to undergo the first stage of catalysis via dissociation of Cu from His61 with a concomittant movement of the coordinated water molecule towards His61, which rotates by approximately 20 degrees, enabling it to form a hydrogen bond to the water molecule. The Cu-Zn separation in the reduced site is increased by approximately 0.5 A. In contrast the 2.3 A resolution structure in space group C2221 (cBSOD) shows both of the Cu atoms to be in the five coordinate, oxidised form but in this space group the whole of subunit A is significantly more disordered than subunit B. An examination of published structures of "oxidised" SODs, shows a trend towards longer Cu-Zn and Cu-His61 separations in subunit A, which together with the structures reported here indicate a potential functional asymmetry between the subunits of CuZnSODs. We also suggest that the increased separation between Cu and Zn is a precursor to breakage of His61.     

Alternative pathways as mechanism for the negative effects associated with overexpression of superoxide dismutase

One of the most important antioxidant enzymes is superoxide dismutase (SOD), which catalyses the dismutation of superoxide radicals to hydrogen peroxide. The enzyme plays an important role in diseases like trisomy 21 and also in theories of the mechanisms of aging. But instead of being beneficial, intensified oxidative stress is associated with the increased expression of SOD and also studies on bacteria and transgenic animals show that high levels of SOD actually lead to increased lipid peroxidation and hypersensitivity to oxidative stress. Using mathematical models we investigate the question how overexpression of SOD can lead to increased oxidative stress, although it is an antioxidant enzyme. We consider the following possibilities that have been proposed in the literature: (i) Reaction of H(2)O(2) with CuZnSOD leading to hydroxyl radical formation. (ii) Superoxide radicals might reduce membrane damage by acting as radical chain breaker. (iii) While detoxifying superoxide radicals SOD cycles between a reduced and oxidized state. At low superoxide levels the intermediates might interact with other redox partners and increase the superoxide reductase (SOR) activity of SOD. This short-circuiting of the SOD cycle could lead to an increased hydrogen peroxide production. We find that only one of the proposed mechanisms is under certain circumstances able to explain the increased oxidative stress caused by SOD. But furthermore we identified an additional mechanism that is of more general nature and might be a common basis for the experimental findings. We call it the alternative pathway mechanism.     

Dual effects of copper-zinc superoxide dismutase

Copper-zinc superoxide dismutase (CuZnSOD) specifically catalyzes the removal of superoxide radicals to protect cellular function against the generation of superoxide-dependent hydroxyl radicals ((.)OH). However, an unexpected observation reveals that denatured CuZnSOD (dCuZnSOD) itself induces (.)OH formation. This dCuZnSOD-dependent (.)OH generation was not inhibited by active CuZnSOD, suggesting that it is a superoxide-independent process. Sodium cyanide, histidine, and N,N'-diethyldithiocarbamate abolished (.)OH generation, implying that Cu may be responsible for dCuZnSOD-induced (.)OH formation. Catalase eliminated ()OH generation, suggesting that hydrogen peroxide may be involved in the mechanism of dCuZnSOD-mediated (.)OH production. Furthermore, nitric oxide ((.)NO) completely inhibited dCuZnSOD-induced (.)OH radical generation, indicating that (.)NO is an important (.)OH radical scavenger. Our results shed new light on the effect of dysfunctional CuZnSOD and suggest that structural disorder of the enzyme may be one of the endogenous pathways of toxic (.)OH formation in biological systems.     

Structural Evidence for a Copper-Bound Carbonate Intermediate in the Peroxidase and Dismutase Activities of Superoxide Dismutase

Copper-zinc superoxide dismutase (SOD) is of fundamental importance to our understanding of oxidative damage. Its primary function is catalysing the dismutation of superoxide to O₂ and H₂O₂. SOD also reacts with H₂O₂, leading to the formation of a strong copper-bound oxidant species that can either inactivate the enzyme or oxidise other substrates. In the presence of bicarbonate (or CO₂) and H₂O₂, this peroxidase activity is enhanced and produces the carbonate radical. This freely diffusible reactive oxygen species is proposed as the agent for oxidation of large substrates that are too bulky to enter the active site. Here, we provide direct structural evidence, from a 2.15 Å resolution crystal structure, of (bi)carbonate captured at the active site of reduced SOD, consistent with the view that a bound carbonate intermediate could be formed, producing a diffusible carbonate radical upon reoxidation of copper. The bound carbonate blocks direct access of substrates to Cu(I), suggesting that an adjunct to the accepted mechanism of SOD catalysed dismutation of superoxide operates, with Cu(I) oxidation by superoxide being driven via a proton-coupled electron transfer mechanism involving the bound carbonate rather than the solvent. Carbonate is captured in a different site when SOD is oxidised, being located in the active site channel adjacent to the catalytically important Arg143. This is the probable route of diffusion from the active site following reoxidation of the copper. In this position, the carbonate is poised for re-entry into the active site and binding to the reduced copper.     

Electrochemical Sensors for Direct Reagentless Measurement of Superoxide Production by Human Neutrophils

Electrochemical sensors based on immobilised cytochrome c or superoxide dismutase for the measurement of superoxide radical production by stimulated neutrophils are described. Cytochrome c was immobilised covalently at a surface-modified gold electrode and by passive adsorption to novel platinised activated carbon electrodes (PACE). The reoxidation of cytochrome c at the electrode surface upon reduction by superoxide was monitored using both xanthine/xanthine oxidase and stimulated neutrophils as sources of the free radical. In addition, bovine Cu/Zn superoxide dismutase was immobilised to PACE by passive adsorption and superoxide, generated by xanthine/xanthine oxidase, detected by oxidation of hydrogen peroxide produced by the enzymic dismutation of the superoxide radical. A biopsy needle probe electrode based on cytochrome c immobilised at PACE and suitable for continuous monitoring of free radical production was constructed and characterised.     

Enzyme mimics complexing Cu(II) ion: structure-function relationships.

Five peptides containing (His-X2)-His or (His-X3)-His motifs have been designed and synthesized to coordinate Cu(II). Structural information was obtained by various spectroscopic techniques and was used as constraint to search for local conformational energy minima by molecular mechanics. Thermodynamic stability constants of the Cu(II) chelates was obtained by 19F-NMR. The synthesized Cu(II)-peptide chelates were tested as catalysts of some important red-ox processes occuring in biological systems, in particular oxidation of ascorbate and dismutation of superoxide ion. The catalytic efficiency of the five chelates was much lower than that of ascorbate oxidase. On the contrary, two of them showed kinetic constants for superoxide dismutation about one order of magnitude lower than that of the enzyme Cu,Zn superoxide dismutase. In both cases, the catalytic properties were dependent on the peptide sequence. The relationships between structure and activity are discussed to find the structural parameters crucial for catalytic activity that can be modulated by appropriate design and synthesis of the peptides.     

Inactivation of human Cu,Zn superoxide dismutase by peroxynitrite and formation of histidinyl radical

Human recombinant copper-zinc superoxide dismutase (CuZnSOD) was inactivated by peroxynitrite, the product of the reaction between nitric oxide and superoxide. The concentration of peroxynitrite that decreased the activity by 50% (IC(50)) was approximately 100 microM at 5 microM CuZnSOD and the inactivation was higher at alkaline pH. Stopped-flow determinations showed that the second-order rate constant for the direct reaction of peroxynitrite with CuZnSOD was (9.4 +/- 1.0) x 10(3) M(-1) s(-1) per monomer at pH 7.5 and 37 degrees C. Addition of peroxynitrite (1 mM) to CuZnSOD (0.5 mM) in the presence of the spin trap 2-methyl-2-nitrosopropane led to the electron paramagnetic resonance detection of an anisotropic signal typical of a protein radical adduct. Treatment with Pronase revealed a nearly isotropic signal consistent with the formation of histidinyl radical. The effects of nitrite, hydrogen peroxide, bicarbonate, and mannitol on the inactivation were assessed. Considering the mechanism accepted for the reaction of CuZnSOD with hydrogen peroxide and the fact that CuZnSOD promotes the nitration of phenolics by peroxynitrite, we herein propose that peroxynitrite reacts with CuZnSOD leading to nitrogen dioxide plus a copper-bound hydroxyl radical species that reacts with histidine residues, forming histidinyl radical.     

In vivo peroxidative activity of FALS-mutant human CuZnSODs expressed in yeast.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder leading to loss of motor neurons. We previously characterized the enhanced peroxidative activity of the human familial ALS (FALS) mutants of copper-zinc superoxide dismutase (CuZnSOD) A4V and G93A in vitro. Here, a similar activity is demonstrated for human FALS CuZnSOD mutants in an in vivo model system, the yeast Saccharomyces cerevisiae. Spin trap adducts of alpha-(pyridyl-4-N-oxide)-N-tert-butylnitrone (POBN) have been measured by electron paramagnetic resonance (EPR) in yeast expressing mutant (A4V, L38V, G93A, and G93C) and wild type CuZnSOD upon addition of hydrogen peroxide to the culture. The trapped radical is a hydroxyethyl adduct of POBN, identified by spectral parameters. Mutant CuZnSODs produced greater concentrations of the trapped adduct compared to the wild type enzyme. This observation provides evidence for an oxidative radical mechanism, whereby the mutants of CuZnSOD catalyze the formation of reactive oxygen species that may be related to the development or progression of FALS. This study also presents an in vivo model system to study free radical production in FALS-associated CuZnSOD mutations.     

Overexpression of manganese or copper-zinc superoxide dismutase inhibits breast cancer growth

We have studied the effects of overexpression of superoxide dismutase (SOD), a tumor suppressor protein that dismutes superoxide radical to H2O2, on breast cancer cell growth in vitro and xenograft growth in vivo. No previous work has directly compared the growth-suppressive effects of manganese SOD (MnSOD) and copper-zinc SOD (CuZnSOD). We hypothesized that either adenoviral MnSOD (AdMnSOD) or adenoviral CuZnSOD (AdCuZnSOD) gene therapy would suppress the growth of human breast cancer cells. After determining the antioxidant profiles of three human breast cell lines, MCF 10A, MDA-MB231, and MCF-7, we measured the effects of MnSOD or CuZnSOD overexpression on cell growth and survival in vitro and in vivo. Results demonstrated that infection with AdMnSOD or AdCuZnSOD increased the activity of the respective enzyme in all three cell lines. In vitro, overexpression of MnSOD or CuZnSOD decreased not only cell growth but also clonogenic survival in a dose- and transgene-dependent manner. In vivo, treatment of tumors with AdMnSOD or AdCuZnSOD decreased xenograft growth compared to controls. The first direct comparison of MnSOD to CuZnSOD overexpression indicated that CuZnSOD and MnSOD were similarly effective at suppressing cancer cell growth.     

Dietary saturated or polyunsaturated fat and copper deficiency in the rat

Dietary fat-type and copper (Cu) deficiency have been independently identified as potentially important factors in the etiology of ischemic heart disease (IHD); a disease that has been linked to inflammation and oxygen free radical (OFR) mediated damage. Group (n = 6) of male, weanling, Wistar rats were provided ad libitum with deionized water and control or low Cu diets containing (200 g/kg) either saturated or polyunsaturated fatty acids (SFA or PUFA, respectively) for 56 d. Measurement of several indices of Cu status indicated that both groups fed the low Cu diets were Cu-deficient. SFA consumption resulted in significantly increased hepatic Cu (p less than 0.001) and iron (Fe) (p less than 0.001) concentrations and xanthine oxidase activity (p less than 0.05) and significantly decreased hepatic glucose-6-phosphate dehydrogenase activity (p less than 0.001). Although Cu deficiency resulted in significantly decreased hepatic copper-zinc superoxide dismutase (CuZnSOD) activity (p less than 0.01), no significant effect on the activities of the other hepatic antioxidant enzymes, manganese superoxide dismutase, catalase, and glutathione peroxidase, or glutathione reductase, were observed. Cu deficiency also resulted in significantly decreased hepatic Cu levels (p less than 0.001) and cytochrome c oxidase activity (p less than 0.01). No significant difference in hepatic thiobarbituric acid reactive substances (TBARS), a measure of lipid peroxidation, was found between groups consuming SFA or PUFA, but both Cu-deficient groups exhibited significantly increased hepatic TBARS (p less than 0.001), compared to controls. This was probably owing to the significantly decreased hepatic CuZnSOD activity observed in the Cu-deficient, compared to control animals.     

Electrochemical Sensors for Direct Reagentless Measurement of Superoxide Production by Human Neutrophils

Electrochemical sensors based on immobilised cytochrome c or superoxide dismutase for the measurement of superoxide radical production by stimulated neutrophils are described. Cytochrome c was immobilised covalently at a surface-modified gold electrode and by passive adsorption to novel platinised activated carbon electrodes (PACE). The reoxidation of cytochrome c at the electrode surface upon reduction by superoxide was monitored using both xanthine/xanthine oxidase and stimulated neutrophils as sources of the free radical. In addition, bovine Cu/Zn superoxide dismutase was immobilised to PACE by passive adsorption and superoxide, generated by xanthine/xanthine oxidase, detected by oxidation of hydrogen peroxide produced by the enzymic dismutation of the superoxide radical. A biopsy needle probe electrode based on cytochrome c immobilised at PACE and suitable for continuous monitoring of free radical production was constructed and characterised.     

Probing the Structural Basis for Enzyme-Substrate Recognition In Cu,Zn Superoxide Dismutase

A full understanding of enzyme-substrate interactions requires a detailed knowledge of their structural basis at atomic resolution. Crystallographic and biochemical data have been analyzed with coupled computational and computer graphic approaches to characterize the molecular basis for recognition of the superoxide anion substrate by Cu. Zn superoxide dismutase (SOD). Detailed analysis of the bovine SOD structure aligned with SOD sequences from 15 species provides new results concerning the significance and molecular basis for sequence conservation. Specific roles have been assigned for all 23 invariant residues and additional residues exhibiting functional equivalence. Sequence invariance is dominated by 15 residues that form the active site stcreochemistry. supporting a primary biological function of superoxide dismutation. Using data from crystallographic structures and site-directed mutants, we are testing the role of individual residues in the active site channel, including (in human SOD) Glu132, Glu133, Lys136, Thr137, and Arg 143. Electrostatic calculations incorporating molecular flexibility suggest that the region of positive electrostatic potential in and over the active site channel above the Cu ion sweeps through space during molecular motion to enhance the facilitated diffusion responsible for the enzyme's rapid catalytic rate.     

Copper(2+) binding to the surface residue cysteine 111 of His46Arg human copper-zinc superoxide dismutase, a familial amyotrophic lateral sclerosis mutant

Mutations in copper-zinc superoxide dismutase (CuZnSOD) cause 25% of familial amyotrophic lateral sclerosis (FALS) cases. This paper examines one such mutant, H46R, which has no superoxide dismutase activity yet presumably retains the gain-of-function activity that leads to disease. We demonstrate that Cu(2+) does not bind to the copper-specific catalytic site of H46R CuZnSOD and that Cu(2+) competes with other metals for the zinc binding site. Most importantly, Cu(2+) was found to bind strongly to a surface residue near the dimer interface of H46R CuZnSOD. Cysteine was identified as the new binding site on the basis of multiple criteria including UV-vis spectroscopy, RR spectroscopy, and chemical derivatization. Cysteine 111 was pinpointed as the position of the reactive ligand by tryptic digestion of the modified protein and by mutational analysis. This solvent-exposed residue may play a role in the toxicity of this and other FALS CuZnSOD mutations. Furthermore, we propose that the two cysteine 111 residues, found on opposing subunits of the same dimeric enzyme, may provide a docking location for initial metal insertion during biosynthesis of wild-type CuZnSOD in vivo.     

The mechanism of the enhanced antioxidant effects against superoxide anion radicals of reduced water produced by electrolysis

We reported that reduced water produced by electrolysis enhanced the antioxidant effects of proton donors such as ascorbic acid (AsA) in a previous paper. We also demonstrated that reduced water produced by electrolysis of 2 mM NaCl solutions did not show antioxidant effects by itself. We reasoned that the enhancement of antioxidant effects may be due to the increase of the ionic product of water as solvent. The ionic product of water (pKw) was estimated by measurements of pH and by a neutralization titration method. As an indicator of oxidative damage, Reactive Oxygen Species- (ROS) mediated DNA strand breaks were measured by the conversion of supercoiled phiX-174 RF I double-strand DNA to open and linear forms. Reduced water had a tendency to suppress single-strand breakage of DNA induced by reactive oxygen species produced by H2O2/Cu (II) and HQ/Cu (II) systems. The enhancement of superoxide anion radical dismutation activity can be explained by changes in the ionic product of water in the reduced water.     

The preparation of apo-Cu,Zn superoxide dismutase by ion-exchange chromatography on iminodiacetic acid-sepharose

The superoxide dismutases (EC 1.15.1.1) are a family of enzymes that catalyze the dismutation of superoxide radical anion to dioxygen and hydrogen peroxide. The active site contains a critical metal ion such as manganese, iron, or copper. The copper-containing protein also has one zinc ion bound per subunit. The standard method used to remove the metal ions from Cu,Zn superoxide dismutase has been to exhaustively dialyze the protein against chelating agents at low pH. We have developed a new method where the protein is bound to ion-exchange medium based on iminodiacetic acid immobilized on Sepharose. The bound protein is treated with a buffer containing edta at pH 3.5 to remove metal ions; the buffer is then exchanged for acetate buffer to remove edta, after which the protein is eluted by a salt gradient. An advantage of this method is that a single chromatography step is sufficient to produce apo protein. Results are shown for both human and bovine dimeric Cu,Zn superoxide dismutase and the monomeric Escherichia coli Cu,Zn superoxide dismutase. In every case, the metals were removed efficiently.     

Functional aspects of the superoxide dismutative action of Cu-penicillamine.

The superoxide dismutative action of Cu-penicillamine was examined by pulse radiolysis. The second order rate constand of the reaction wpith superoxide was 0.4 +/- o.2.10(9) M-1.s-1, comparable to the action of Fe and Mn-superoxide dismutases. No marked pH-dependence was seen. Neither ethylene diamine tetraacetic acid nor cyanide affected the catalytic action of Cu-penicillamine. The cyanide resistant reactivity as well as further X-ray photoelectron spectrometric measurements supported the suggestion of a Cu(I) stabilized sulphur radical being the active species involved in the catalysis of superoxide dismutation.     

Role of the electrostatic loop of Cu,Zn superoxide dismutase in the copper uptake process.

Cu,Zn superoxide dismutases are characterized by the presence of four highly conserved charged residues (Lys120, Glu/Asp130, Glu131 and Lys134), which are placed at the edge of the active site channel and have been shown to be individually involved in the electrostatic attraction of the substrate toward the catalytically active copper ion. By genetic engineering we mutated these four residues into neutrally charged ones (Leu120, Gln130, Gln131, Thr134). The effects of these mutations on the rate of superoxide dismutation were not dramatic. In fact, at two different pH and ionic strength values, the mutant enzyme had a catalytic constant even higher with respect to the wild-type protein, showing that electrostatic interaction at these surface sites is not essential for high catalytic efficiency of the enzyme. The mutant and the wild-type enzyme showed the same degree of inhibition by CN(-), and both were not affected by I(-), showing that mutations did not alter the sensitivity of the enzyme to anions. On the other hand, reconstitution of active enzyme from either the wild-type or mutant copper-free enzymes with a copper(I)-glutathione [Cu(I)-GSH] complex showed that metal uptake by the mutant was much slower than by the wild-type enzyme. The demonstration that the 'electrostatic loop' is apparently conserved to assure optimal copper uptake by the enzyme, rather than fast dismutation, may provide further support to the idea that Cu,Zn superoxide dismutase is a bifunctional protein, acting in cellular defense against oxidative stress both as a copper buffer and as a superoxide radical scavenger.     

Function of periplasmic copper-zinc superoxide dismutase in Caulobacter crescentus.

Caulobacter crescentus is one of a small number of bacterial species that contain a periplasmic copper-zinc superoxide dismutase (CuZnSOD). A C. crescentus mutant, with the CuZnSOD gene interrupted by a promoterless cat gene, was constructed and characterized to analyze CuZnSOD function. Periplasmic SOD does not protect against oxyradical damage in the cytosol or play a major role in maintaining the integrity of the cell envelope. Studies of the effect of sodium citrate on plating efficiency suggest that CuZnSOD protects a periplasmic or membrane function(s) requiring magnesium or calcium.