Reasons for Catalytic Activity

Even though the rates of enzymatically catalyzed reactions vary, they are very high compared to the 

The specificity of substrate binding contributes substantially to the rate of an enzyme-catalyzed reaction. 

Binding to the active site of the enzyme concentrates the reaction partners in comparison with a dilute substrate solution. In addition, the reaction is now the favored one since binding places the substrate s susceptible reactive group in the proximity of the catalytically active group of the enzyme. 

Therefore the contribution of substrate binding to the reaction rate is partially due to a change in the molecularity of the reaction. The intermolecular reaction of the two substrates is replaced by an intramolecular reaction of an enzyme-substrate complex. The consequences can be clarified by using model compounds which have all the reactive groups within their molecules and, thus, are subjected to an intramolecular reaction. Their reactivity can then be compared with that of the corresponding bimolecular system and the results expressed as a ratio of the reaction rates of the intramolecular (ki) to the intermolecular (k2) reactions. Based on their dimensions, they are denoted as effective molarity . As an example, let us consider the cleavage of p-bromophenylacetate in the presence of acetate ions, yielding acetic acid anhydride  

Intramolecular hydrolysis is substantially faster than the intermolecular reaction (Table 2.6). The effective molarity sharply increases when the reactive carboxylate anion is in close proximity to the ester carbonyl group and, by its presence, retards the mobility of the carbonyl group. Thus, the effective molarity increases (Table 2.6) as the C—C bond mobility decreases. Two bonds can rotate in a glutaric acid ester, whereas only one can 

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