The Specificity of Enzymatic Action

Impressed by the specificity of enzymatic action, biochemists early adopted a "lock-and-key" theory which stated that for a reaction to occur the substrate must fit into an active site precisely. Modem experiments have amply verified the idea. A vast amount of kinetic data on families of substrates and related competitive inhibitors support the idea and numerous X-ray structures of enzymes with bound inhibitors or with very slow substrates have given visual evidence of the reality of the lock-and-key concept. Directed mutation of genes of many enzymes of known three-dimensional structure has provided additional proof. 

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). 

The mechanism of chymotrypsin action is particularly well studied and, in many respects, typical. Numerous types of reaction mechanisms for enzyme action are known, and we shall discuss them in the contexts of the reactions catalyzed by the enzymes in question. To lay the groundwork, it is useful to discuss some general types of catalytic mechanisms and how they affect the specificity of enzymatic reactions. 

It is well recognized that specificity is one of the most spectacular aspects of enzymatic action. Thus, the process of alcoholic fermentation of D-glucose by a unicellular organism like yeast has been proved to consist of a sequence of elementary reactions catalyzed by sixteen individual... 

An example of the importance of enzymatic modification of hormones for the tissue specificity of hormone action is the effect of the mineral corticoid aldosterone in the presence of a large excess of the glucocorticoid cortisol. Aldosterone regulates the Na -export and K -retention in the kidney by binding on the aldosterone receptor. 

It can be concluded that the bacterial proteinases can be applied to many proplems where extensive enzymatic proteolysis is required. Because of their wide substrate specificity, they are not useful for producing one of the major sets of peptides which are required to provide overlapping sequences within large polypeptides or proteins. On the other hand, where structural studies of small peptides require extensive cleavage at a variety of bonds, the bacterial proteinases are excellent hydrolytic agents. The products of proteinase action are often quite analogous to the products... 

The approaches and logic used by Warburg, as exemplified by his monograph (1949), have markedly influenced this area of study (James, 1953). The use of inhibitors of enzymatic action, assumed to modify the rate of a reaction by specific interaction with a given metal, constitutes an important facet of this mode of investigation. Enzyme activity is the primary measurement, and the participation of the metal is inferred from the altered rate of reaction induced by the inhibitor, which is thought to combine with or remove a particular metal. 

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