Superoxide dismutase stoichiometry

Three Mn catalases have been purified and characterized, and all appear to have similar Mn structures (17). The Mn stoichiometry is ca. 2 Mn/subunit, suggesting a dinuclear Mn site. The optical spectrum of the as-isolated enzyme has a broad weak absorption band at ca. 450-550 nm in addition to the protein absorption at higher energies. This spectrum is similar to those observed for Mn(III) superoxide dismutase and for a variety of Mn(III) model complexes, thus implying that at least some of the Mn in Mn catalase is present as Mn(III). In particular, the absorption maximum at ca. 500 nm is similar in energy and intensity to the transitions seen for oxo-carboxylato-bridged Mn dimers, suggesting that a similar core structure may be seen for Mn catalase (18). 

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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