Simplified enzymatic mechanism

This is not a fundamental assumption, but rather one that is often made to simplify the characterization of an enzyme in vitro. However, any careful study of an enzymatic mechanism must include kinetic measurements done in the presence of all reactants and modifiers, including the products of the reaction. In such studies the reverse reactions cannot be ignored (Haldane, 1930 Alberty, 1959 Cleland,... 

The reaction under investigation is the enzymatic hydrolysis of racemic ethoxyethyl-ibuprofen ester. The (R)-ester is not active in the above reaction,1-3, thus simplifying the reaction mechanism, as shown in Figure 5.13. Because both enantiomers are converted according to fust-order kinetics, the conversion of one enantiomer is independent of the conversion of the other.4... 

The overwhelming majority of biomimetics operate in liquid. Their activity depends on the origin of solvents, reaction mixture and cell effects. Gas-phase oxidation processes are less dependent on these effects and in the first approximation can be considered as oxidation under quasi-ideal conditions. It goes without saying that enzymatic reactions do not proceed in gases. However, it is possible to simulate catalytic functions in the gas phase. This simplifies the decoding of the reaction mechanism, not complicated by factors accompanying the liquid-phase oxidation [1],... 

Despite the chemical specificity of this type of enzymatic catalysis, a rate equation can be derived in a similar manner as before. In the general case for all the steps being reversible one should use a general form of a three-step sequence, however the mechanism could be essentially simplified if the first two steps are considered to be in quasi-equlibria and the last one irreversible. 

One can probably guess that in relation to reality, the reaction examples of the illustration or of equation (3-73) are much simplified. Many enzymes of known function catalyze reactions involving more than one substrate. The mechanisms can be quite complex, however, the rate laws do generally follow the form of equation (3-73) if the composition of only one substrate is varied at one time. A good discussion of such multisubstrate enzyme-catalyzed reactions is given by Plowman [K.M. Plowman, Enzyme Kinetics, McGraw-Hill Book Co., New York, NY, (1972)]. There is a strong family resemblance between these enzymatic sequences and those encountered in the detailed collision theory of Benson and Axworthy in Chapter 1. 

The study of biomimetics can be of great benefit for the understanding of enzymatic reactions. The term biomimetic refers, in the context of this work, to a compound that mimics structural, functional and spectroscopic properties of an enzyme [67]. Often only one or two of these aspects are achieved for a model system and they usually display substantially lower activity. There are, however, advantages over the enzyme model complexes are generally more stable and robust than their enzymatic counterpart, they can be readily crystalUzed and provide easy accessible structural information on metal ion coordination. Also as these model systems are considerably less complex, kinetic and spectroscopic data interpretation is simplified and— by comparison to data derived for the enzyme— the mechanism of action and structural features can be elucidated and thus related back to the parent metalloenzyme. Also models can be obtained on a larger scale and are often less costly to synthesize, a distinct benefit for potential applications. A few structures of model complexes for dinuclear hydrolytic enzymes are shown in Fig. 1.4. The approaches for ligand and complex design are diverse. 

The enzymatic oxidation of alkanes to produce alcohols is a simplified version of the reactions that produce the adrenocortical steroid hormones. In the biosynthesis of corticosterone from progesterone (Section 4-7), two such oxidations take place successively (a, b). It is thought that the monooxygenase enzymes act as complex oxygen-atom donors in these reactions. A suggested mechanism, as applied to cyclohexane, consists of the two steps shown below the biosynthesis. 

The combination of biosensors with flow injection analysis (FIA) techniques offers the possibility to control the whole procedure, simplifying the sequence of steps and allowing an easier optimization of the reaction conditions with reducing the measurement time by a three- to fourfold. The use in such systems of enzymatic biosensors for the determination of pesticides may provide a device competitive to immunoassay kits, or single-use disposable biosensors, due to mechanization leading to more objective measurements and efunination of operations of transfer of solutions and their volumetric measurements. 

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