Role of Metal Ions in Enzyme Systems

ODERN BIOCHEMISTRY has made great strides in the identification of the components of enzyme systems and in the purification of enzyme pro-teins; few advances have been made, however, in understanding the nature of catalysis by enzyme proteins in chemical and physical terms. Those enzymes de-pending for their activity on metallic functional groups appear to offer an approach to these more fundamental problems since in many cases the catalytic activity of metalloenzymes is already present in a primitive form in simple inorganic compounds of the metallic moiety. The role of the protein moiety in enhancing these inherent properties of the metal is not yet known although recent advances in the field of the heme enzymes, for instance, offer considerable encouragement for the view that the role of the protein may eventually become comprehensible in terms of modem theories of molecular structure. In this article the role of metal ions in the action of various types of metalloenzymes is reviewed in the light of some general considera-tions on the physical properties of metallic ions as they relate to metal ion specifici-ties among different metalloenzymes, and the nature of the structural linkages be-