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. 

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. 

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. 

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. 

B. Enzymes are highly specific in their action and this specificity of enzymatic... 

Two types of enzyme that hydrolyze acetylcholine rapidly have been identified. One is represented by an enzyme present in the serum of many animals. This esterase attacks many other esters at faster rates than acetylcholine, and has been called nonspecific or pseudocholinesterase, in distinction to the so-called specific or true cholinesterase found in erythrocytes, nervous and electrical tissue. The latter hydrolyzes acetylcholine more rapidly than other choline esters. Extensive kinetic studies have been made with both types of enzyme. Studies with highly purified true cholinesterase have been particularly revealing in explorations of the chemical nature of enzymatic action. 

Copper is required for all forms of aerobic and most forms of anaerobic life. In humans, the biological function of copper is related to the enzymatic action of specific essential copper proteins (66). Lack of these copper enzymes is considered a primary factor in cerebral degeneration, depigmentation, and arterial changes. Because of the abundance of copper in most human diets, chemically significant copper deficiency is extremely rare (67). 

Pectin degradation requires fee combined action of various enzymatic activities. However, evaluation of fee contribution of individual pectinases in Suit juice extraction and clarification is rather complicated. Most commercial pectinolytic enzyme preparations are produced by fermentation wife filamentous fungi, mostly strains belonging to fee genus Aspergillus,. plication studies with mixtures of isolat enzymes obtained by fermentation or by means of fractionation of commercial enzyme preparations can be used to assess the importance of fee various individual enzymes. Subsequently, molecular biology and fermentation technology can be used to enhance specific desirable enzymatic activities. 

Lipoxygenases catalyse the regio-specific and stereoselective oxygenation of unsaturated fatty acids. The mammalian enzymes have been detected in human platelets, lung, kidney, testes and white blood cells. The leukotrienes, derived from the enzymatic action of the enzyme on arachidonic acid, have effects on neutrophil migration and aggregation, release of lysosomal enzymes, capillary permeability, induction of pain and smooth muscle contraction (Salmon, 1986). 

---