Enzyme-catalyzed reactions types

We 11 see numerous examples of both reaction types m the following sections Keep m mind that m vivo reactions (reactions m living systems) are enzyme catalyzed and occur at far greater rates than those for the same transformations carried out m vitro ( m glass ) m the absence of enzymes In spite of the rapidity with which enzyme catalyzed reactions take place the nature of these transformations is essentially the same as the fundamental processes of organic chemistry described throughout this text... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

Figure 6.44 Types of enzyme-catalyzed reactions leading to enantiopure alcohols. Stereogenic centers are included in the alcohol moiety, as indicated by an asterisk.).

In a biphasic medium, two situations are distinguished for the reaction. Biocatalysis occurs at the liquid-liquid interface [42,43] or in the bulk of the aqueous phase [25,27]. Models have been developed for both types, and interaction between mass transfer and enzyme-catalyzed reactions has been also studied. 

Inhibition Effects in Enzyme Catalyzed Reactions. Enzyme catalyzed reactions are often retarded or inhibited by the presence of species that do not participate in the reaction in question as well as by the products of the reaction. In some cases the reactants themselves can act as inhibitors. Inhibition usually results from the formation of various enzyme-inhibitor complexes, a situation that decreases the amount of enzyme available for the normal reaction sequence. The study of inhibition is important in the investigation of enzyme action. By determining what compounds behave as inhibitors and what type of kinetic patterns are followed, it may be possible to draw important conclusions about the mechanism of an enzyme s action or the nature of its active site. 

The kinetic data below were reported for an enzyme catalyzed reaction of the type E + S ES E + P. Since the data pertain to initial reaction rates, the reverse reaction may be neglected. Use a graphical method to determine the Michaelis constant and Fmax for this system at the enzyme concentration employed. 

Group specificity, in which the enzyme catalyzes reactions of only one type of functional group. 

Cyclodextrins as catalysts and enzyme models It has long been known that cyclodextrins may act as elementary models for the catalytic behaviour of enzymes (Breslow, 1971). These hosts, with the assistance of their hydroxyl functions, may exhibit guest specificity, competitive inhibition, and Michaelis-Menten-type kinetics. All these are characteristics of enzyme-catalyzed reactions. 

What about reactions of the type A + B — C This is a second-order reaction, and the second-order rate constant has units of M min-1. The enzyme-catalyzed reaction is even more complicated than the very simple one shown earlier. We obviously want to use a second-order rate constant for the comparison, but which one There are several options, and all types of comparisons are often made (or avoided). For enzyme-catalyzed reactions with two substrates, there are two Km values, one for each substrate. That means that there are two kcJKm values, one for each substrate. The kcJKA5 in this case describes the second-order rate constant for the reaction of substrate A with whatever form of the enzyme exists at a saturating level B. Cryptic enough The form of the enzyme that is present at a saturating level of B depends on whether or not B can bind to the enzyme in the absence of A.6 If B can bind to E in the absence of A, then kcJKA will describe the second-order reaction of A with the EB complex. This would be a reasonably valid comparison to show the effect of the enzyme on the reaction. But if B can t bind to the enzyme in the absence of A, kcat/KA will describe the second-order reaction of A with the enzyme (not the EB complex). This might not be quite so good a comparison. 

Despite the diverse range of documented enzyme-catalyzed reactions, there are only certain types of transformations that have thus far emerged as synthetically useful. These reactions are the hydrolysis of esters, reduction/oxidation reactions, and the formation of carbon-carbon bonds. The first part of this chapter gives a brief overview by describing some examples of various biotransformations that can easily be handled and accessed by synthetic organic chemists. These processes are now attracting more and more attention from nonspecialists of enzymes. 

Enzyme-catalyzed reactions can be described at least at two distinct levels. At the basic level, the interconversion of substrates by enzymes is governed by a set of elementary steps, including enzyme substrate binding, isomerization and dissociation steps, see Fig. 6 for a schematic depiction. Assuming the intracellular medium is an ideal solution, each elementary step is governed by mass-action kinetics, that is, the reaction rates are proportional to the probability of collision of the reactants. For a reaction of the type... 

Several types of enzymatic reactions were studied during the last decade for the biosynthesis of dextran. Reports on the application of chromatographic reactors for an enzyme catalyzed reaction were first demonstrated in the late 1980s [171, 172]. 

The protocol described in Section 7.1.2 involves isotopic competition, but with the different isotopomers held in separate containers. Equations 7.10 to 7.13 apply equally well to a type of competition experiment known in biochemistry as the perturbation method for determining KIE s of reversible enzyme catalyzed reactions. The perturbation method differs from simultaneous non-competitive measurements in several important ways. One begins by mixing equilibrium concentrations of substrate and product but with one component (substrate or product) at a different isotopic composition than the other. Thus, the mixture is in chemical, but not isotopic equilibrium. At this stage no enzyme is present and the interconversion is... 

A dimensionless quantity used to assess the extent of inhibition by a particular compound at a specific concentration on the initial rate of an enzyme-catalyzed reaction. The degree of inhibition, symbolized by ei, is equal to (vo Vi)/Vo where Vo in the reaction rate in the absence of the inhibitor and Vi is the rate in the presence of the inhibitor. Whenever ej values are reported, the inhibitor concentration and initial rate conditions have to be provided as well. The degree of inhibition is a useful parameter in the early stages of an investigation. However, it does not address the type of inhibition nor provide much information on the dissociation constant for the inhibitor. See Inhibition (as well as specific type of inhibition)... 

A sequential enzyme-catalyzed reaction mechanism in which two substrates react to form two products and in which there is a preferred order in the binding of substrates and release of products. Several enzymes have been reported to have this type of binding mechanism, including alcohol dehydrogenase , carbamate kinase , lactate dehydrogenase , and ribitol dehydrogenase. ... 

Symbol for the temperature coefficient, a quotient equal to Vt+wIvt, where Vr+io and Vj are the rates of a process (e.g., an enzyme-catalyzed reaction) at two temperatures differing by 10°C. This parameter is usually evaluated at saturating concentrations of substrate(s), so that temperature-dependent changes in Michaelis constant(s) are inconsequential. The <2io value is a characteristic property of a particular enzyme from a specific organism and cell type. For example, one cannot use the Qio value for one hexokinase from yeast to infer the temperature dependence of another hexokinase, say from rat brain. Likewise, the Qio value need not remain the same for a mutant form and a wild-type enzyme. 

We wdl encounter similar rate expressions of this type when we consider surface or enzyme-catalyzed reactions in Chapter 7. These rate expressions are called Langmuir-Hinshelwood and Michaelis-Menten kinetics, respectively. 

The monolithic stirrer reactor (MSR, Figure 2), in which monoliths are used as stirrer blades, is a new reactor type for heterogeneously catalyzed liquid and gas-liquid reactions (6). This reactor is thought to be especially useful in the production of fine chemicals and in biochemistry and biotechnology. In this work, we use cordierite monoliths as stirrer blades for enzyme-catalyzed reactions. Conventional enzyme carriers, including chitosan, polyethylenimine and different are used to functionalize the monoliths. Lipase was... 

The enzymes catalyzing this type of reaction are called sulfotransferases. 

Similarly, for enzyme-catalyzed reactions of the Michaelis-Menten type, we can derive Equation 7.3 from Equation 3.31. 

The answer to these questions has two distinct but interwoven parts. The first lies in the rearrangements of covalent bonds during an enzyme-catalyzed reaction. Chemical reactions of many types take place between substrates and enzymes functional groups (specific... 

The general principles outlined above can be illustrated by a variety of recognized catalytic mechanisms. These mechanisms are not mutually exclusive, and a given enzyme might incorporate several types in its overall mechanism of action. For most enzymes, it is difficult to quantify the contribution of any one catalytic mechanism to the rate and/or specificity of a particular enzyme-catalyzed reaction. 

Any substance that can diminish the velocity of an enzyme-catalyzed reaction is called an inhibitor. Reversible inhibitors bind to enzymes through noncovalent bonds. Dilution of the enzyme-inhibitor complex results in dissociation of the reversibly bound inhibitor, and recovery of enzyme activity. Irreversible inhibition occurs when an inhibited enzyme does not regain activity on dilution of the enzyme-inhibitor complex. The two most commonly encountered types of inhibition are competitive and noncompetitive. 

Apparent Temperature Optimum. A rise in temperature has a dual effect upon an enzyme-catalyzed reaction it increases the rate of the reaction, but it also increases the rate of thermal inactivation of the enzyme itself. Like the pH optimum, the temperature optimum may in certain instances be altered by environmental conditions, e.g., pH, type and strength of buffer, etc. The term temperature optimum, therefore, is useless unless the incubation time and other conditions are specified. A more enlightening term is apparent temperature optimum, which indicates that the optimum has been obtained under a... 

The 500-residue subunits of pyruvate kinase consist of four domains,891 the largest of which contains an 8-stranded barrel similar to that present in triose phosphate isomerase (Fig. 2-28). Although these two enzymes catalyze different types of reactions, a common feature is an enolic intermediate. One could imagine that pyruvate kinase protonates its substrate phosphoenolpyruvate (PEP) synchronously with the phospho group transfer (Eq. 12-42). However, the enzyme catalyzes the rapid conversion of the enolic form of pyruvate to the oxo form (Eq. 12-43) adding the proton sterospecifically to the si face. This and other evidence favors the enol as a true intermediate... 

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