Figure 1.

(a) The three dominating coordination geometries and coordination numbers observed in Mn (purple sphere) containing metalloproteins are tetrahedral (Td, n = 4); trigonal bipyramidal (TBp, n = 5) and octahedral (Oh, n = 6). (b) The active site of Mn-SOD in the presence of Mn (purple sphere), with and without water molecules (W1 and W2, red spheres) in the coordinating sphere. Three His residues (His 165, 27, and 81), one Asp (161) coordinates Mn, and the involvement of 0, 1, or 2 water molecules determines the geometry of the Mn complex at the active site. Zero water produces Td, binding of W1 yields TBp, and binding of W1 and W2 produces the Oh geometry. It has been speculated that the geometry of the active sites changes during scavenging of reactive oxygen species [22].