Manganese ions binding sites

The manganese ions are bridged in both oxidation state levels by a single carboxylate (Glu70) and two nonprotein ligands, which have been modeled as p.-oxo and a water molecule in the oxidized form, and a p.-hydroxo and a water molecule in the reduced form, respectively. It appears that in the reduced form there are alternate sites approximately 2.0 A apart, for the water binding to Mn(l). In the chloride form of the reduced enzyme the chloride ion has replaced the hydroxide. 

A plausible rationalization would be that there is a competition for the transport route through the intestinal wall between cadmium and manganese on the one side, and between iron and manganese on the other. The competition, i.e., absorption in the intestinal tract, depends upon the relative concentration of these ions and kinetics of and affinity for their interaction with the binding sites in the intestinal mucosa. 

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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