Metalloproteins, Metallo-enzymes and Heterogeneous Catalys

An inspection of the active site region of metallo-enzymes leads to two conclusions. (i) Individual attacking groups usually have uncommon physical properties. (ii) The active site region is cooperatively linked with a large part of the enzyme. Such an interaction is common to heterogeneous catalysts but is absent in homogeneous catalysts. It is concluded that the energetics of catalysis requires an analysis of the nature of cooperativity in microdomains of well organized material and some general remarks as to the problems which arise are described. Methods of examining large well-organized molecules (microdomains) are described and some suggestions are made as to how man-made 'enzymes' could be produced. There is a very large number of metalloproteins and metallo-enzyrnes which are now well characterized. In this article it will be assumed that the reader is familiar with much of the work before 19701. The article will be restricted to those metallo-systems which contain iron, cobalt, copper, zinc. or molybdenum. No reference will be made to either magnesium or calcium enzymes. The assumption will be made that metalloproteins, which have no enzyme activity, differ from metallo-enzymes in that. while the metal sites of metallo-proteins have relatively conventional physical properties, the sites of metallo-enzymes have more unusual properties. Elsewhere we have developed the idea that the metallo-enzymes are unusual because the metal ions in them do not have conventional ground state geometries2' 3 The geometry may be unusual by virtue of bond length, bond angle, or both properties. These energized ground states have evolved to meet the functional catalytic requirements of biological systems, i.e. so as to carry out a certain number of reactions at high speed. As will be shown below it is most illuminating to compare enzymic catalysts with heterogeneous catalysts evolved by man. As well as examining one or two new examples of enzymes which illustrate the above themes I shall also attempt to develop a better understanding of the energetics of the catalysis due to enzymes. This requires a very detailed inspection of an enzyme in solution, both in the presence and absence of the substrate. In a final section of speculative review I shall consider if it is possible to produce artificial polymers with a catalytic power equal to that of enzymes.