Superoxide dismutase protein residues

Water soluble protein with a relative molecular mass of ca. 32600, which particularly contains copper and zinc bound like chelate (ca. 4 gram atoms) and has superoxide-dismutase-activity. It is isolated from bovine liver or from hemolyzed, plasma free erythrocytes obtained from bovine blood. Purification by manyfold fractionated precipitation and solvolyse methods and definitive separation of the residual foreign proteins by denaturizing heating of the orgotein concentrate in buffer solution to ca. 65-70 C and gel filtration and/or dialysis. 

One last class of mononuclear non-haem iron enzyme that we have not yet considered, consists of the microbial superoxide dismutases with Fe(III) at their active site. The crystal structure of the E. coli enzyme shows a coordination geometry reminiscent of protocatechuate 3,4-dioxygenase, with four endogenous protein ligands, three His and one Asp residue, and one bound water molecule (Carlioz et ah, 1988). 

The chelate effect in proteins is also important, since the three-dimensional (3-D) structure of the protein can impose particular coordination geometry on the metal ion. This determines the ligands available for coordination, their stereochemistry and the local environment, through local hydrophobicity/hydrophilicity, hydrogen bonding by nearby residues with bound and non-bound residues in the metal ion s coordination sphere, etc. A good example is illustrated by the Zn2+-binding site of Cu/Zn superoxide dismutase, which has an affinity for Zn2+, such that the non-metallated protein can extract Zn2+ from solution into the site and can displace Cu2+ from the Zn2+ site when the di-Cu2+ protein is treated with excess Zn2+. 

It was clear for some time that a number of zinc enzymes required two or more metal ions for full activity, but in the absence of X-ray structural data the location of these metal centres with regard to one another was often uncertain. When the first 3-D structures began to appear, it became clear that the metals were in close proximity. A particular feature of many of these enzymes was the presence of a bridging ligand between two of the metal sites, usually an Asp residue of the protein, which is occasionally replaced by a water molecule. While some of the sites contain only Zn ions, several contain Zn in combination with Cu (in cytosolic superoxide dismutases) Fe (in purple acid phosphatases) or Mg (in alkaline phosphatase and the aminopeptidase of lens). 

Although hydroxyl radical is commonly assumed to be the most toxic of the oxygen radicals (with little direct evidence), other direct reactions are more likely to be important for understanding the cytotoxicity of peroxynitrite. A second oxidative pathway involves the heterolytic cleavage of peroxynitrite to form a nitronium-like species (N02 ), which is catalyzed hy transition metals (Beckman et al., 1992). Low molecular weight metal complexes as well as metals bound in superoxide dismutase and other proteins catalyze the nitration of a wide range of phenolics, including tyrosine residues in most proteins (Beckman et al., 1992). 

In addition to its previously mentioned role in copper transport, ceruloplasmin is an amine oxidase, a superoxide dismutase, and a ferrooxidase able to catalyze the oxidation of Fe2+ to Fe3+. Ceruloplasmin contains three consecutive homologous 350-residue sequences which may have originated from an ancestral copper oxidase gene. Like ascorbate oxidase, this blue protein contains copper of the three different types. Blood clotting factors V and VIII (Fig. 12-17), and the iron uptake protein Fet3 (Section A,l) are also closely related. 

NO to form peroxynitrite (Eq. 18-62).559b Peroxynitrite, in turn, can react with the ubitquitous C02 to give C03 and N02 radicals.559c Peroxynitrite anion also reacts with metalloenzyme centers559"4 and causes nitration and oxidation of aromatic residues in proteins.559d e However, neutrophils contain active superoxide dismutases, and most of the superoxide that is formed is converted quickly to 02 and H202. 

Polticelli F. Battistoni A. O Neill P. RotilioG. Desideri A. Identification of the residues responsible for the alkaline inhibition of Cu, Zn superoxide dismutase a site-directed mutagenesis approach. Protein Sci. 1996, 5, 248-253. 

From One Type 2 Copper Protein to Another Type 2 Copper Protein. It has been shown that a type 2 copper protein called copper-zinc superoxide dismutase (CuZnSOD) (Figure 15b) shares the same overall scaffold as its copper chaperone protein called copper chaperone for SOD (CCS). All of e zinc site ligands and three of four copper site histidine ligands in CuZnSOD are conserved in human CCS (hCCS). The zinc site in hCCS displayed the same structure as in CuZnSOD. The fourth ligand in the copper site of CuZnSOD is replaced by an aspartate residue in hCCS. Although aspartate could... 

Much of the copper in the plasma (60-95%) is bound to ceruloplasmin. The complex of copper and cenjloplasmir is assembled and secreted by the liver. A small fraction of plasma copper, under 7%, is weakly bound to albumin and to free amino acids, especially histidine, threonine, and glutamine. The copper bound to serum albumin is associated with a histidine residue near the amino terminus of the protein. The copper in red blood cells is bound to superoxide dismutase. 

Zinc usually binds to proteins via residues of cysteine and histidine. Sometimes, zinc is bound to residues of glutamate or aspartate. The zinc ion sometimes plays a catalytic role and sometimes a structural role. In the latter case, it helps maintain the three-dimensional structure or conformation of the protein. For example, carboxypeptidase A contains two atoms of zinc. One is required for catalytic activity and is boimd to cysteine and histidine. The other, which plays a structural role, is bound only to cysteine. Cytoplasmic superoxide dismutase is a dimer. It contains one atom of Cu " and one of Zn per subunit. The zinc is boimd via three residues of histidine and one residue of aspartate. It is buried deep within the enzyme and serves a structural role. The copper atom is bound via four residues of histidine. It resides close to the surface of the protein and participates in the chemistry of catalysis. 

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