STRUCTURAL STUDIES OF THE ROLE OF THE ACTIVE-SITE METAL IN METALLOENZYMES

This paper describes several experimental and computational methods wh ich are currently used in the structural analysis of metal-containing macromolecules. A specific family of proteolytic enzymes which contain a zinc cation in the active site was selected to demonstrate these me thods. A range of studies using one example from this family of enzyme s is described which serves to clarify the role of the metal in the ov erall protein structure and in the local conformation of the active si te in the native enzyme, the metal-deficient enzyme, and the metal-sub stituted enzyme and in complexes of the enzyme with various chemical a nalogues. The main experimental method described is X-ray crystallogra phy, while computational methods for the examination of surface intera ctions and electrostatic potential effects are described briefly to co mplement the structural conclusions. The various experimental and comp utational results are, then assembled in order to draw general conclus ions on the structure-function relationships of metalloproteins and in particular the role of the metal in metal-containing proteolytic enzy mes. The results of these studies implicate the zinc ion in the bindin g and catalytic activation of the substrate and stabilization of the t etrahedral reaction intermediate. It appears that in this family of en zymes a divalent metal cation is important for the required catalytic arrangement of functional groups in the active site, especially the me tal ligands. However, once an appropriate metal ion is coordinated, th ere is practically no effect of the particular metal ion bound on eith er the overall three dimensional structure of the enzyme or the local detailed structure of its active site.