Enolase. metal chelate enzyme

Metal-enzyme complexes, a subgroup of metal-protein complexes, exhibit enzymatic activity consequent to readily dissociable combination with a variety of metal ions. Many of these studies have been performed with unpurified enzymes, and, even when pure enzymes were used, the stoichiometry of the interaction of the metal and enzyme has not been measured. Enhancement of enzymatic activity as a result of the addition of metal ions and its partial loss on their removal has been the chief criterion of assessment of physiological significance. Only in a few instances, e.g., enolase, has the stability and stoichiometry been studied in relation to function (Malmstrom, 1953, 1954). The study of metal complexes and particularly metal chelates (Bjerrum, 1941 Martell and Calvin, 1952 Calvin, 1954) has provided both new experimental and new theoretical backgrounds for the study of metals in relation to the specificity of enzyme action, metal-enzyme (Calvin, 1954), metal-substrate (Najjar, 1951), and metalloenzyme interaction, as well as metal-enzyme inhibition (James, 1953). 

Such a generalization is useful in that it provides clues to the. nature of the participation of metal ions in a reaction, when the order of catalytic effect of the various metals is known. As a first example of such an approach, let us consider the case in which the order of inhibition correlates with the order of complex stability such an order is frequently observed—e.g., urease (39). It may be concluded, in such an instance, that the metals do not activate the reaction, but inhibit it only. As a second example, if metal ions activate a reaction, but they do so in inverse order from that of complex stability, it follows that inhibition competes effectively with activation. Such an order is observed with enolase (Mg+2 >Zn+2>Mn+2>Fe+2>Co+2>Ni+2) (35, 53) presumably the inhibitory effect of the more strongly binding metals is responsible for the selection of the less active Mg+2 in the natural enzyme. Probably such effects are not as important in aconitase, making it possible for a stronger chelating metal to activate that enzyme. 

Many enzymes are dependent on dissociable metal ions for their activity, and the operation of most of the important metabolic systems thus requires the presence of these cofactors. For example, the list of enzymes requiring Mg is a long one and includes the oxidases and decarboxylases for the keto acids, most of the enzymes involved in phosphate metabolism, some dehydrogenases, some peptidases, phosphoglucomutase and enolase. These enzymes may be inhibited with inhibitors forming stable complexes with Mg ions. For example, malonate and other dicarboxylic compounds are able to chelate effectively with Mg" and other metal ions, and their inhibition may result from the reduction of metal ion concentration in the medium or the removal of the metal ions from the enzyme [3] ... 

The zinc atom in carbonic anhydrases is as strongly attached to the enzyme molecule as that of enolases. It cannot be removed by electrodialysis or exchanged with the radioactive zinc of the medium. But if the enzyme is incubated at pH 5 in the presence of a strong chelating agent, the zinc is detached from the protein and the enzyme is inactivated. The enzyme is reactivated by incubating the metal-free protein with Zn or Co. However, the activity of the cobalt enzyme is only half that of the zinc enzyme. ... 

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