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Square planar dsp2

Square planar dsp2

Octahedral d2sp3

Tetrahedral dsp3

Figure 1 Some common geometries of metal complexes.

Mg, K, Ca, and Na, are not considered 'trace' and only in rare instances are these so-called macroelements strongly bound to the surface of enzymes. Tight binding precludes loss of the metal ion by dialysis or loss to weakly dissociating agents. Metalloenzymes, however, can lose their metal cofactor and hence be rendered inactive when treated with metal chelators that have a stronger binding affinity than the enzyme and out compete the enzyme protein for the metal ion. As prosthetic groups, metals in metalloenzymes have a stoichio-metric relationship (metal ion-enzyme protein ratio) represented by a whole integer. Metalloenzymes seldom are primed to greater activity by adding its conjugate metal ion to the enzyme. Spatial geometry is also a concern. Metals in the first transition series metals (Mn, Fe, Co, Ni, Cu, Zn) must adhere to strict geometric configurations around the metal-binding site. Examples of the more common geometrical arrangements are shown in Figure 1. For metals in the first transition series one takes note of the 3d and 4s orbitals in assigning valence states and likely geometric shapes. Apart from those with Zn, enzymes with first transition series metals tend to be highly colorful; for example, the beautiful red color of hemoglobin (iron) or the blue color of ceruloplasmin (whose name means heavenly blue) associated with copper. Table 3 gives some examples of metalloenzymes and the specific metal each requires.

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