FeSOD

The mechanisms of superoxide-dismuting activity of SODs are well established. Dismutation of superoxide occurs at copper, manganese, or iron centers of SOD isoenzymes CuZnSOD, MnSOD, or FeSOD. These isoenzymes were isolated from a variety of sources, including humans, animals, microbes, etc. In the case of CuZnSOD, dismutation process consists of two stages the one-electron transfer oxidation of superoxide by cupric form (Reaction (1)) and the one-electron reduction of superoxide by cuprous form (Reaction (2)). 

Similar reactions are catalyzed by Mn and Fe centers of MnSOD and FeSOD. It is obvious that before participation in Reaction (2), superoxide must be protonized to form hydroper-oxyl radical HOO by an outer-sphere or an intra-sphere mechanisms. All stages of dismuting mechanism, including the measurement of elementary rate constants, have been thoroughly studied earlier (see, for example, Ref. [2]). 

Four distinct forms of SODs are found in nature, which fall into three families. The Cu/Zn SODs occur primarily in cytoplasm of eukaryotes and chloroplasts, hut have also heen found in a few species of bacteria, nickel-containing SODs are known in some prokaryotes, while the structurally related Mn- and FeSODs are found in... 

The intriguing question is how the seven-coordinate geometry around the metal center favors its remarkable catalytic activity, knowing that in the native MnSOD and FeSOD enzymes the active metal center has a five-coordinate geometry (3a,14f30). All SOD... 

Bacterial SODs typically contain either nonheme iron (FeSODs) or manganese (MnSODs) at their active sites, although bacterial copper/zinc and nickel SODs are also known (Imlay and Imlay 1996 Chung et al. 1999). Catalases are usually heme-containing enzymes that catalyze disproportionation of hydrogen peroxide to water and molecular oxygen (Eq. 10.2) (Zamocky and Koller 1999 Loewen et al. 2000). 

It is believed that MnSOD plays a pivotal role in many diseases. There have been many reviews of the biochemistry of MnSOD " and focusing on the structural aspects of the enzyme. Four different types of SOD are known, a Cu/Zn-containing SOD, a FeSOD, a NiSOD, and MnSOD. MnSODs, which are structurally related to the FeSODs, have a of 23,000 ( 200 amino acids) and function as a dimer or as a tetramer. MnSOD catalyzes the dismutation reaction by cycling between the - -2 and +3 oxidation states. One proton is taken up by the system in each step (Equation (2)) ... 

One area of research interest has been the metal ion specificity of the MnSOD and FeSOD molecules. The tertiary structures of these molecules are very similar and the ligands coordinated to the metal ions are identical. Many organisms contain both forms of the enzyme and each form has an absolute specificity for its metal ion, the enzyme is completely inactive if the wrong metal ion is present. Cambialistic enzymes that occur in some organisms are active with either metal ion present in the active site. Comparisons of the structures of the MnSOD, FeSOD, and the cambialistic enzymes have not revealed any single obvious structural differences that could explain this phenomenon. " ... 

Vance and Miller et al. have shown that the inactivity of enzyme is due to changes in the redox potentials of the enzyme. In order to dismute 02 the redox potential of the enzyme must lie between the E° values for the reactions shown in Equation (3). The E° value of the E. coli MnSOD enzyme is 0.290 V and that for the FeSOD is 0.220 V. The Fe-substituted form of the Mn enzyme has F ° =—0.240 V and Mn-substituted FeSOD has ii° >0.960V. These values are outside the required range and the changes in redox potentials are not due to changes in the metal ligands. Mutations of His-30 and Tyr-34, two conserved residues in the immediate vicinity of the metal binding site, do not alter the redox potential of the enzyme either " ... 

The pH dependence of the Mn(III/II) potential has not been studied for other organisms. It is interesting to note that the Fe-substituted form of MnSOD has E° = —0.24 V and that the Mn-substituted form of FeSOD has E° > 0.96 V [105]. These altered enzyme forms are ineffective in superoxide dismutation. 

The discussion here is limited to CuZnSOD for a number of reasons. First, in this system the ping-pong mechanism described in reactions (22) and (23) is operative as written whereas both MnSOD and FeSOD carry out catalysis by mechanisms that involve observable enzyme-substrate complexes under certain conditions. Secondly, this enzymatic system has been studied as a model to look at such factors as electrostatic guidance of substrate (see below). Finally, the link that was demonstrated between over 100 point mutations in CuZnSOD and the inherited version of amyotrophic lateral sclerosis (Lou Gehrig s Disease) has made underscored the importance in understanding details of enzyme function. ... 

Superoxide dismutase (SOD, EC 1.15.1.1) is a scavenger of the superoxide anion, and therefore, provides protection against oxidative stress in biological systems [259]. Most SODs are homodimeric metalloenzymes and contain redox active Fe, Ni, Mn or Cu. The superoxide dismutation by SOD is among the fastest enzyme reactions known. The rate constant for CuZnSOD is = 2x 10 s [260], FeSOD is about one order of... 

SOD-modified sensors also were demonstrated to respond to superoxide addition. Using either 3-mercaptopropionic acid [68] or cysteine [70] as a promoter on Au-electrodes superoxide sensors could be constructed where FeSOD and CuZnSOD in direct contact to the electrode acts as catalyst for the highly specific dismutation of O2 to O2 and H2O2. Either Fe or Cu of SOD are oxidized and reduced (Fe /Fe Cu°/Cu ) at the modified gold electrodes. Both, anodic and cathodic peak currents increase in the presence of O2. At a potential of 300 or- 200 mV O2 -generation could be recognized with detection limits of 5 and 6 nM, respectively [70]. 

The response of A. thaliana to Cu deficiency in roots and vegetative tissue is well established (Fig. 8.13). When Cu is limiting, cytosolic and plastid Cu/Zn SODs are downregulated and FeSOD is upregulated. The reverse is... 

The cytoplasm of Escherichia coli contains dimeric MnSOD, dimeric FeSOD,... 

FeSODs occurs also in plants (mainly in the chloroplasts) and are common in protozoa, e.g., Tetrahymena pyriformis. Entamoeba histolytica, Plasmodium falciparum, Leishmania chagasi. Trypanosoma cruzi and Trichomonas vaginalis. 

MnSODs and FeSODs from most procaryotic organisms are dimeric while MnSODs from mitochondria and some thermophilic bacteria are tetrameric [21]. However, mitochondrial MnSOD from Caenorhahditis elegans was found to be dimeric [22]. Eukaryotic MnSOD is a tetrameric protein encoded in the nucleus, synthesized in the cystosol, and imported post-translationally into the mitochondrial matrix. The 25 kDa precursor protein has a mitochondrial transit peptide that is cleaved to produce the mature 22 kDa subunit. The mature protein exists as a tetramer, each subunit containing one Mn ion. The mitochondrial MnSOD primary sequences are highly homologous to the prokaryotic Mn- and FeSOD, but has no resemblance to the CuZnSODs [23]. In MnSODs and FeSODs, the metal ions are coordinated by three His N atoms and one Asp O atom [24]. 

FeSOD acts in an analogous manner. MnSOD is slightly less efficient than FeSOD, with k at/KM values of 5.6x10 M" s" and 3x10 M" s" respectively [24]. However, while at pH 7.0 the reaction rates of CuZnSOD and MnSOD are similar, the reaction rates of MnSOD and FeSOD decrease at higher pH (so assays of tissue extracts at alkaline pH underestimate the activity of MnSOD) [13]. 

As in CuZnSOD, electrostatic surface potentials at the active site funnel steer the negatively charged substrate into the active site of MnSOD and FeSOD [24,51]. 

There are conflicting data on the redox potential of the Cu2+/Cu+ transition in CuZnSOD. It has been argued that in oxidized SOD, the presence of Zn and the tetrahedral distortion of the Cu site increase the redox potential of the Cu2+ to 0.42 V [52], which is much higher than the value of 0.17 V for an aqueous Cu2+/Cu+ pair or the 0.01 V for Cu -His/Cu -His complexes. However, a much lower value (0.12 V) has also been reported [53]. The reduction potentials of FeSOD of E. coli and human MnSOD are lower the apparent redox potential of the Fe +/Fe2+ and Mn 7Mn transition were reported to be -0.07 V and -0.29 V,respectively [53,54]. 

In E. coli FeSOD is expressed constitutively, even under anaerobic condi-... 

In pathogenic bacteria, SOD may be a virulence factor. Strains of Mycobacterium tuberculosis and Nocardia asteroides that secrete FeSOD during logarithmic phase growth are resistant to phagocytic attack, while non-secreting strains are less virulent. 

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