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ENZYMES IN BIOANALYTICAL CHEMISTRY
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Figure 2.4. Hydrolysis of ATP to adenosine diphosphate (ADP) and inorganic phosphate Pi .
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facilitate the cleavage of C C C and other bonds by water. Lyases cat C, O, N alyze the cleavage of these same bonds by elimination, leaving double bonds (or, in the reverse mode, catalyze the addition of groups across double bonds). Isomerases facilitate geometric or structural rearrangements or isomerizations. Finally, ligases catalyze the joining of two molecules, and often require the hydrolysis of a pyrophosphate bond in the cofactor adenosine triphosphate (ATP, Fig. 2.4) to provide the energy required for the synthetic step. The enzymes D-amino acid oxidase and lactate dehydrogenase (Eqs. 2.2 and 2.6) have the numbers E.C. 1.4.3.3 and E.C. 1.1.1.28, respectively; both are oxidoreductases, and therefore fall into the rst of the six main divisions. The enzyme cholesterol esterase catalyzes the hydrolysis of cholesterol esters into cholesterol and free fatty acids (Eq. 2.7), and has been assigned E.C. 3.1.1.13.
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2:7 In addition to the E.C. number, the source of a particular enzyme is usually given, listing the species and the organ or tissue from which it was isolated. 2.4. ENZYMES IN BIOANALYTICAL CHEMISTRY Enzymes can be employed to measure substrate concentrations as well as the concentrations of species that affect the catalytic activity of the enzyme toward its substrate, such as activators and inhibitors. The rst known enzymatic assay was reported by Osann in 1845: hydrogen peroxide (H2O2) was quantitated using the enzyme peroxidase. In 1851, Schonbein reported a detection limit of 1 part H2O2 6 in 2 10 [i.e., 500 parts per billion] using this method! Enzymatic methods
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TABLE 2.1. Substrate Selectivity of Glucose Oxidasea Substrate b-D-Glucose 2-Deoxy-D-glucose 6-Deoxy-6- uoro-D-glucose 6-Methyl-D-glucose 4,6-Dimethyl-D-glucose D-Mannose D-Xylose a-D-Glucose Relative Rate of Oxidation (%) 100 25 3 1.85 1.22 0.98 0.98 0.22
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a See Ref. 3. [Reprinted, with permission, from M. Dixon and E. C. Webb, Enzymes, 3rd ed., Academic Press, New York, 1979, p. 243. Third edition # Longman Group Ltd. 1979.]
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are popular because they are relatively simple, require little or no sample pretreatment, and do not require expensive instrumentation. The single critical advantage, however, is the lack of interferences due to the selectivity of enzymes for their natural substrates. The selectivity of glucose oxidase (E.C. 1.1.3.4) from Aspergillus niger has been studied through a comparison of the maximum rates of product formation from a variety of structurally related sugars, shown in Table 2.1.3 Glucose oxidase is most reactive toward its natural substrate, b-D-glucose (Eq. 2.8), so that this substrate has been assigned a relative oxidation rate of 100%.
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The data in Table 2.1 show that the only substrate that would represent a signi cant interference in an enzymatic assay for glucose using glucose oxidase is 2-deoxy-Dglucose. It is of particular interest that the anomeric form of the enzyme s natural substrate, a-D-glucose, cannot be oxidized at a signi cant rate, even though the two compounds differ only in the position of the hydroxyl group at the C1 position of the sugar (Eq. 2.9). It is this exquisite selectivity that is exploited in enzymatic assays, enabling their use for substrate quantitation in matrices that may be as complex as blood or fermentation broths.
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ENZYME KINETICS
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To understand how enzymes are used in bioanalytical methods, and how their concentrations are represented and determined, it is rst necessary to examine the kinetics of one- and two-substrate enzymatic reactions. 2.5. ENZYME KINETICS The dramatic increases in reaction rates that occur in enzyme-catalyzed reactions can be seen for representative systems in the data given in Table 2.2.4 The hydrolysis of the representative amide benzamide by acid or base yields second-order rate constants that are over six orders of magnitude lower than that measured for benzoyl-L-tyrosinamide in the presence of the enzyme a-chymotrypsin. An even more dramatic rate enhancement is observed for the hydrolysis of urea: The acidcatalyzed hydrolysis is nearly 13 orders of magnitude slower than hydrolysis with the enzyme urease. The disprotionation of hydrogen peroxide into water and molecular oxygen is enhanced by a factor of $1 million in the presence of catalase. Enzymes derive both their selectivities and their reaction rate enhancements by the formation of enzyme substrate complexes. This complex formation results in a transition state for the reaction, that lowers Gz , the activation energy, but does not affect the net free energy for the conversion of substrate to product. This nding is represented in Figure 2.5, for the simple one-substrate enzyme reaction shown in Eq. 2.10: E S $ E  S !E P 2:10
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where E is the enzyme, S is the substrate, E  S is the enzyme substrate complex, and P is the product of the reaction. The critical rst step in enzyme-catalyzed reactions is the formation of E  S, and this is usually represented as a simple association reaction, as in Eq. 2.10. Because it is the E  S complex that is the reactant in the substrate conversion step, its concentration determines the rate of the reaction; it follows, then, that the reaction rate
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TABLE 2.2. Examples of the Catalytic Power of Enzymesa Substrate Amide (hydrolysis) -Benzamide -Benzamide -Benzoyl-L-tyrosinamide Urea (hydrolysis) Hydrogen peroxide
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