Enzymatic Catalysis General Principles

Several fundamental aspects of enzymatic catalysis must be considered in any discussion of the chemistry of enzymatic reactions. First, an enzyme-catalysed reaction proceeds with formation of an 

These ideas have been highly advantageous in regard to the development of chemical catalysis in aqueous solution. If the above concepts are correct, then an enzymatic reaction proceeding through an enzyme-substrate complex with the substrate bound close to appropriate functional groups is quite analogous to a chemical intramolecular reaction. Substantial effort heis therefore been expended on the study of such reactions in attempts better to comprehend enzyme catalysis (Bruice, 1970 Kirby and Fersht, 1971), 

Intramolecular nucleophilic reactions could also be facilitated over their intermolecular counterparts if the reaction centre and the nucleophile are compressed in the ground state. Part of the van der Waals repulsion energy could thereby be overcome in the ground state, resulting in a more favourable A// value. Solvation 

Activation Parameters (kcal mole ) for Nucleophilic Displacement by the Dimethylamino-Group at 25 (Bruice and Benkovic, 1963) 

As an intramolecular nucleophile is more rigidly held with respect to the reaction centre, the rate of the reaction increases as illustrated in Table 2. Bruice and Pandit (1960b) concluded that the rate increases were due to restriction of unfavourable rotamer distribution. The most energetically favourable ground state conformation would have the carboxyl group extended into the solvent, viz. 

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ENZYMATIC CATALYSIS GENERAL PRINCIPLES Intramolecular Catalysis... 

The three hydrolytic enzymes that have been discussed, a-chymo-trypsin, carboxypeptidase A, and lysozyme, cover a wide range of substrate types and mechanistic possibilities. Formulation of principles which might apply to enzymatic catalysis in general is difficult from such a small sampling, but certain features of the enzymatic and model reactions warrant some comment. 

The transition state of a reaction is difficult to study because it is so short-lived. To understand enzymatic catalysis, however, we must dissect the interaction between the enzyme and this ephemeral moment in the course of a reaction. Complementarity between an enzyme and the transition state is virtually a requirement for catalysis, because the energy hill upon which the transition state sits is what the enzyme must lower if catalysis is to occur. How can we obtain evidence for enzyme-transition state complementarity Fortunately, we have a variety of approaches, old and new, to address this problem, each providing compelling evidence in support of this general principle of enzyme action. 

In the next section, the general principles of the analytical use of reaction-rate methods are described in the subsequent sections the application of noncatalytic and catalytic techniques are treated. Several methods, instruments, and techniques for which separate entries can be found in this encyclopedia are only briefly mentioned, e.g., enzymatic catalysis, chemiluminescence, sensors, data processing. 

Although the validity of specific models for specific enzyme-catalyzed reactions may be open to question, the most general physical chemical principles are likely to be common to both enzymatic and nonenzy-matic catalysis. I would therefore like to make a plea for the consideration of the significance of such a general phenomenon to catalysis, both by metaUoenzymes and in nonbiological systems. I refer to the structure of the so-called second coordination sphere of metal complexes and its relevance to catalysis. This is an aspect of the field which has not been too widely discussed previously, either at the present symposium or elsewhere. 

The activity of an enzyme is profoundly affected by pH. Usually, enzymes display a bell-shaped activity versus pH profile (Fig. 6.1). The decrease in activity on either side of the pH optimum can be due to two general causes. First, pH may affect the stabihty of the enzyme, causing it to become irreversibly inactivated. Second, pH may affect the kinetic parameters of the enzymatic reaction It may affect the stability of the ES complex, the velocity of the rate-Mmiting step, or both. The second case is relevant to the discussion in this chapter. Interestingly, the pH dependence of enzyme-catalyzed reactions is similar to that of acid- and base-catalyzed chemical reactions. Thus, it is possible, at least in principle, to determine the pK and state of ionization of the functional groups directly involved in catalysis, and possibly their chemical nature. 

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