Enzyme-catalyzed processes

Many reactions classified as dehydrogenations occur within the cells of living sys terns at 25°C H2 is not one of the products however Instead the hydrogens are lost m separate steps of an enzyme catalyzed process The enzyme indicated m the reaction... 

Most enzyme-catalyzed processes, such as the examples just discussed, are highly enantioselective, leading to products of high enantiomeric purity. Reactions with other chiral reagents exhibit a wide range of enantioselectivity. A fiequent objective of the smdy... 

FIGURE 10.3 Passive diffusion and facilitated diffusion may be distinguished graphically. The plots for facilitated diffusion are similar to plots of enzyme-catalyzed processes (Chapter 14) and they display saturation behav-... 

The rate acceleration achieved by enzymes is due to several factors. Particularly important is the ability of the enzyme to stabilize and thus lower the energy of the transition state(s). That is, it s not the ability of the enzyme to bind the substrate that matters but rather its ability to bind and thereby stabilize the transition state. Often, in fact, the enzyme binds the transition structure as much as 1012 times more tightly than it binds the substrate or products. As a result, the transition state is substantially lowered in energy. An energy diagram for an enzyme-catalyzed process might look like that in Figure 26.8. 

When 10 (R configuration about phosphorus) was treated with ribo-nuclease in aqueous methanol, nucleophilic attack by methanol in the enzyme-catalyzed process led to formation of a methyl ester, 11, which has been shown by X-ray analysis to be the isomer with the R configuration about the phosphorus (60) ... 

An intriguing influence of a cosolvent immiscible with water on the enantioselec-tivity of the enzyme-catalyzed hydrolysis was observed. It was proven that enzyme enantioselectivity is directly correlated with the cosolvent hydrophobicity. In the best example, for ethyl ether as cosolvent, the reaction proceeded with E = 55, and the target compound was obtained in 33% yield with 92.7% ee. This finding may be of great practical importance, particularly in industrial processes [24], since it will enable better optimization of enzyme-catalyzed processes. It is clear that, in future, immobilized enzymes, as heterogeneous catalysts, wiU be widely used in most industrial transformations, especially in the preparation of pharmaceuticals [25]. 

Rasor, J.P. and Voss, E. (2001) Enzyme-catalyzed processes in pharmaceutical industry. Applied Catalysis A-General, 221 (1-2), 145-158. 

An enzyme-catalyzed process also may be used to form reactive iodine species capable of iodin-ating proteins and other molecules (Marcholonis, 1969 Morrison and Bayse, 1970). The... 

The last type of CL discussed here is bioluminescence (BL). As the term suggests, BL is an enzyme-catalyzed process found in living organisms [164, 165]. In most BL reactions, luciferin is oxidized with molecular oxygen by lucifer-ase with ATP as a cofactor. In addition, the luciferase activity depends on Ca2+ or Mg2+. The analytically most often employed system is the firefly luciferase/ D-luciferin system shown in Fig. 26. Here, ATP is necessary to form the highly energetic AMP adduct required for further reaction sequence. Subsequent cleavage... 

As can be concluded from this short description of the factors influencing the overall reaction rate in liquid-solid or gas-solid reactions, the structure of the stationary phase is of significant importance. In order to minimize the transport limitations, different types of supports were developed, which will be discussed in the next section. In addition, the amount of enzyme (operative ligand on the surface of solid phase) as well as its activity determine the reaction rate of an enzyme-catalyzed process. Thus, in the following sections we shall briefly describe different types of chromatographic supports, suited to provide both the high surface area required for high enzyme capacity and the lowest possible internal and external mass transfer resistances. 

Zaks, A. and Klibanov, A.M., Enzyme-catalyzed processes in organic solvents. Proc. Natl. Acad. ScL, 1985, 42, 3192-3196. 

In an ideal kinetic resolution (common in enzyme-catalyzed processes), one enantiomer of a racemic substrate is converted tvhile the other is unreactive [70]. In such a kinetic resolution of 5-methyl-2-cyclohexenone, even with 1 equivalent of Me2Zn, the reaction should virtually stop after 50% conversion. This near perfect situation is found with ligand 18 (Fig. 7.10) [71]. Kinetic resolutions of 4-methyl-2-cyclohexenone proceed less selectively (s = 10-27), as might be expected from the lower trans selectivity in 1,4-additions to 4-substituted 2-cyclohexenones [69]. 

DNA repiication the enzyme-catalyzed process in which one molecule of DNA is converted to two molecules of DNA, each identical to the first. 

The use of enzymes to catalyze reversible reactions has proven to be an effective strategy for DCC. Enzymes work under physiological conditions (by definition), are reversible, and can also be applied to a variety of C-C and C-X bond-forming reactions. Venton and coworkers reported the first example of an enzyme-catalyzed process being used in a DCC context [32]. As their work preceded the codification of DCC in the literature, it contains little of the vocabulary that has come to define the field. It does, however, correspond perfectly with the conceptual framework of DCC, and has been widely cited as an influential early example of the DCC idea. 

CATALYSIS. Any condition promoting formation will tend to speed up the reaction rate, and catalysts are thought to accomplish rate enhancement chiefly by stabilizing the transition state. Shown in Fig. 8 is an enzyme-catalyzed process in which reactant S (more commonly called substrate in enzymology) combines with enzyme to form an enzyme-substrate complex. This complex leads to formation of the transition state complex EX which may proceed to form enzyme-product complex. The catalytic reaction cycle is then completed by the release of product P, whereupon the uncombined enzyme returns to its original state. 

The reduction in concentration of reactants, enzymes, and solute molecules can provide important information about kinetic systems. For example, one can readily differentiate a first-order process from a second-order process by testing whether the period required to reduce a reactant concentration to 50% of its initial value depends on dilution. First-order processes and intramolecular processes should not exhibit any effect on rate by diluting a reactant. In terms of enzyme-catalyzed processes, the Michaelis-Menten equation requires that the initial reaction velocity depends strictly on the concentration of active catalyst. Dilution can also be used to induce dissociation of molecular complexes or to promote depolymerization of certain polymers (such as F-actin and microtubules). 

A quantitative expression developed by Albery and Knowles to describe the effectiveness of a catalyst in accelerating a chemical reaction. The function, which depends on magnitude of the rate constants describing individual steps in the reaction, reaches a limiting value of unity when the reaction rate is controlled by diffusion. For the interconversion of dihydroxacetone phosphate and glyceraldehyde 3-phosphate, the efficiency function equals 2.5 x 10 for a simple carboxylate catalyst in a nonenzymic process and 0.6 for the enzyme-catalyzed process. Albery and Knowles suggest that evolution has produced a nearly perfect catalyst in the form of triose-phosphate isomerase. See Reaction Coordinate Diagram... 

A reaction that occurs wholly within one phase. It should be pointed out that certain homogeneous systems may still exhibit nonhomogeneous effects. For example, a reaction in a solution may be influenced by the walls of the container. If such phenomena occur, then corrections would have to be made. In general, this is not a problem for most enzyme-catalyzed processes. 

In principle, one can never exactly duplicate the transition state, because transition state theory requires that such an intermediate species would disproportionate back to E-Substrate complex as well as proceed onward to E-Product complexes. However, the scheme shown in Fig. 3 permits one to estimate the maximal affinity that should be achievable if one were to approximate closely the electronic and stereochemical configuration of the enzyme and substrate in the transition state. An accurate estimation of requires detailed knowledge that the uncatalyzed reference reaction follows the same mechanism as the enzyme-catalyzed process. See Enzyme Proficiency Reference Reaction... 

Here the problem of formulating a reaction rate expression is much more difficult because there are many atoms involved, and consequently the statistical mechanics and quantum mechanics are much more complex. We wiU consider the forms of rate expressions for surface- and enzyme-catalyzed processes in Chapter 7, but fundamental theories are usually not obtainable. 

It was necessary to establish unambiguously that the transformation from trigonal to tetrahedral geometry was an enzyme-catalyzed process, as opposed to one in which the ketone was hydrated in solution followed by binding to the enzyme. Thus, when statone analog was incubated with pepsin in 99% H2 6 for three hours, recovered ketone contained < 10% 0 at... 

In addition to deliberate enzyme-catalyzed processes, there are nonenzymatic processes that alter proteins. These include the degradative reactions described in Section 5 and also reversible reactions that may be physiologically important. For example, the N-terminal amino groups of peptides, and other amino groups of low p Ka can form carbamates with bicarbonate (Eq. 2-21 ).301-303 This provides an important mechanism of carbon dioxide transport in red blood cells (Chapter 7) and a way by which C02 pressure can control some metabolic processes. 

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