Superoxide dismutase metal binding

M.W. Pantoliano, P.J. McDonnell, and J.S. Valentine, Reversible loss of metal ions from the zinc binding site of copper-zinc superoxide dismutase. The low pH transition. J. Amer. Chem. Soc. 101, 6454— 6456 (1979). 

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+. 

Valentine, J. S., Pantoliano, M. W. Metal binding properties of copper-zinc superoxide dismutase. In Chemical and Biochemical Aspects of Superoxide and Superoxide Dismutase... 

Surprisingly, too, there are claims of higher oxidation states of Mn in some systems, e.g. Mnlv in photosynthetic(II) chloroplast systems and Mn111 in acid phosphatase. In the latter enzyme Tyr and Cys residues appear to form part of the metal-binding site. The metal is also involved in the phosphate binding. While superoxide dismutase (SOD) is more generally found with Cu and Zn as the active metals, an Mn-SOD form is found in certain bacteria. The Mn oscillates between different oxidation states in its catalytic activity.149... 

Some metal- (especially copper) complexes catalyse the dismutation of superoxide at rates that compare favourably with catalysis by superoxide dismutase. One could therefore argue that the presence of such complexes in vivo might be beneficial. There are, however, additional considerations (1) such metal complexes may also reduce hydrogen peroxide, which could result in the formation of hydroxyl radicals, and (2) it is extremely likely that the metal will be displaced from its ligands (even when those ligands are present in excess), and becomes bound to a biomolecule, thereby becoming less active as a superoxide dismutase mimic. As an example, copper binds well to DNA and catalyses the formation of hydroxyl radicals in the presence of hydrogen peroxide and ascorbate [30],... 

While the stoichiometries of the Mn SOD enzymes appear to vary, the properties of the Mn-binding site do not. This is borne out in the electronic spectra of these proteins, which display a great degree of similarity despite the diversity of sources from which they have been isolated (Table II). This type of spectrum is distinctive for manganese in the trivalent oxidation state (3). The native enzymes are EPR silent, as might be anticipated if they contained Mn solely as the trivalent ion (S = 2) (1, 6,12,18-20, 24). However, when the enzymes are denatured, the characteristic six-line pattern of Mn(II) (I = 5/2) appears. Magnetic susceptibility studies with the E. coli SOD were consistent with the presence of a monomeric Mn(III) complex with a zero-field splitting of 1 to 2 cm-1 (4). The enzymes are additionally metal specific (however, see Refs. 36 and 37) metal reconstitution studies with E. coli and B. stearothermophilus revealed a strict requirement for Mn for superoxide dismutase activity (2, 22, 23). 

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