Enzymes fumarase reaction

Optically inactive starting materials can give optically active products only if they are treated with an optically active reagent or if the reaction is catalyzed by an optically active substance The best examples are found m biochemical processes Most bio chemical reactions are catalyzed by enzymes Enzymes are chiral and enantiomerically homogeneous they provide an asymmetric environment m which chemical reaction can take place Ordinarily enzyme catalyzed reactions occur with such a high level of stereo selectivity that one enantiomer of a substance is formed exclusively even when the sub strate is achiral The enzyme fumarase for example catalyzes hydration of the double bond of fumaric acid to malic acid m apples and other fruits Only the S enantiomer of malic acid is formed m this reaction... 

The enzyme fumarase catalyzes the stereospecific addition of water to fumarate to form L-malate. A standard solution of fumarase, with a concentration of 0.150 tM, gave a rate of reaction of 2.00 tM mim under conditions in which the concentration of the substrate was significantly greater than K. The rate of reaction for a sample, under identical conditions, was found to be 1.15 tM mimh What is the concentration of fumarase in the sample ... 

The activity of many enzymes is pH-dependent because the enzyme may ionize in solution and the biological activity of unionized and ionized forms may be different. In this case, the rate of an enzyme-mediated reaction can be expected to depend on the acidity of the solution. If the enzyme can lose more than one proton as the pH increases (Figure 8.12), the rate of reaction as a function of pH may display a maximum if the forms of the enzyme in strongly acidic or strongly basic solution are inactive, but the intermediate, monoanion, is active. An example of this behavior is provided by fumarase (Figure 8.13). 

The enzyme fumarase catalyses the stereospecific iram -addition of water to fumaric acid giving (5)-malic acid, and the reverse reaction, the rrans-elimination of water from (S )-malic acid ... 

Example 5.4A Detailed model of the fumarase reaction The basic mechanism of enzyme reactions is... 

The reaction is remarkable for a number of reasons. It is readily reversible and is catalyzed by an enzyme (fumarase) at nearly neutral conditions (pH s 7). Without the enzyme, no hydration occurs under these conditions. Also, the enzymatic hydration is a completely stereospecific antarafacial addition and creates L-malic acid. The enzyme operates on fumaric acid in such a way that the proton adds on one side and the hydroxyl group adds on the other side of the double bond of fumaric acid. This is shown schematically in Figure 10-9. 

Hydration and dehydration reactions are common in biological pathways. The enzyme fumarase catalyzes the reversible addition of waterto the double bond of fumaratetoform malate. In contrast to the harsh conditions used in the chemical reaction, the enzymatic reaction takes place at neutral pH and at 37 °C. 

Generally speaking, these distinctions have not been observed by biochemists. Stereoselective has been little used, and stereospecific has been used to cover almost all aspects of the impact of stereochemical influences on reactions in living tissues or enzyme systems. Consider, for instance, the enzymatic hydration of fumarate by the enzyme, fumarase. Since there is a relationship between the structure of the substrate and product, the process could be described as stereospecific. Yet the definition of stereospecific requires that it be shown that the isomer of fumaric acid gives rise to a product which is stereochemically different from L-malate. Since the enzyme, however, does not catalyze any reaction with the (Z)-isomer (maleic acid) it is not clear whether stereospecific actually applies. 

Biological organic chemistry takes place in the aqueous medium inside cells rather than in organic solvents, and it involves complex catalysts called enzymes. Nevertheless, the kinds of biological roactions that occur are remarkably nimiUr to laboratory reactions. Thus, there are many cases of biological addition reactions to alkenes. For example, Che enzyme fumarase... 

STEPS 7-8 Regeneration of oxaloacetate. Catalyzed by the enzyme fumarase. conjugate miclcophilic addition of w ater to fumarate yields t-malate in a reaction simUar to tliat of step 2 in the fatty acid j3>oxidation pathway. Oxida-i m with NAD then, gives oxaloacetate in a step catalyzed by malate dehydrogenase, and the citric acid cycle has return to ita starting point, ready to revolve again. 

The hydration of fumaric acid [( )-butenedioic acid, 1 R = H] to (S)-2-hydroxybutanedioic acid (2) is catalyzed by the enzyme fumarase. This reaction can be run even on an industrial scale, exploiting the fumarase activity of immobilized microorganisms77. Unfortunately, the substrate spectrum of fumarase is very narrow. Nevertheless, (Z)-2-chlorobutenedioic acid (3, R = Cl) could be diastereo- and enantioselectively hydrated to (2S,3/ )-2-chloro-3-hydroxybu-tanedioic acid (4) on a 50-gram scale, employing commercially available pig heart fumarase [EC 4.2.1.2.]78. 

FIGURE 14.16. Stereochemistry of the enzymatic conversion of fumarate to malate (Ref. 56). Shown are (a) the crystal structure of the complex, and (b) the deduced steric course of the reaction catalyzed by the enzyme fumarase. 

Enzyme-catalyzed reactions are usually much more chemically selective than their laboratory counterparts. Fumarase, for example, is completely inert toward maleic acid, the cis isomer of fumaric acid. Nevertheless, the fundamental processes of oi nic chemistry are the same in the living cell and in the laboratory. 

The X-ray crystal structure of P. putida muconate lactonizing enzyme (cycloisomerase) was determined in 1987, and was found to contain an a/(i barrel fold, also found in triosephosphate isomerase and enolase. Remarkably, the structure of P. putida mandelate racemase, which catalyzes a mechanistically distinct reaction earlier in the same pathway, was found in 1990 to have a homologous structure, indicating that the structural fold of the enolase superfamily is able to support a range of enzyme-catalyzed reactions. The P. putida 3-carboxy- r, x-muconate lactonizing enzyme, in contrast, shares sequence similarity with a class II fumarase enzyme, and determination of its structure in 2004 has shown that it shares the same fold as the class II fumarase superfamily, hence these two catalysts of similar reactions have evolved from different ancestors. ... 

The organic reactions that occur in biological systems are catalyzed by enzymes. Enzyme-catalyzed reactions are almost always completely stereoselective. In other words, enzymes catalyze reactions that form only a single stereoisomer. For example, the enzyme fumarase, which catalyzes the addition of water to fumarate, forms only... 

Dehydration reactions are known to occur in many important biological processes. Instead of being catalyzed by strong acids, which would not be available to a cell, they are catalyzed by enzymes. Fumarase, for example, is an enzyme that catalyzes the dehydration of malate in the... 

Figure 11 The neutron diffraction results of the enzymatic reaction of Scheme 3. The known absolute configuration of the (+)(R)a-phenylethylammonium cation (a) showed that the product malate anion, produced enzymatically by the enzyme fumarase (b), had the absolute configuration (—)(2S,3R), meaning that the enzymatic addition of D2O was stereospecifically trans. (Published in Bau, Brewer, Chiang, Fujita, Hoffman, Watkins and Koetzle, Elsevier 1983)...

The turnover number of the enzyme fumarase that catalyzes the reaction,... 

Step 7. Formation of L-Malate In Step 7, which is catalyzed hy the enzyme fumarase, water is added across the double bond of fumarate in a hydration reaction to give malate. Again, there is stereospecihcity in the reaction. Malate has two enantiomers, L- and D-malate, but only L-malate is produced. 

In general, enzymes exhibit maximum catalytic activity at a definite pH. This optimum pH is generally in the vicinity of pH 7 ( 1), although exceptions are well known. The dependence of the kinetic parameters of the fumarase reaction on pH is illustrated in Figs. 9-2 and 9-3. Several different possible effects of pH on the reaction must be distinguished. In the first place, many substrates may have ionizable groups and only one of the ionized forms of the substrate may be acted upon by the enzyme. Since substrate ionization constants can be determined quite easily and precisely,... 

The hydration of alkenes provides a convenient method for preparing alcohols on a large scale. The reaction is also important in living organisms, but the catalyst is an enzyme rather than sulfuric acid. For example, one of the steps in the body s utilization of carbohydrates for energy involves the hydration of fumaric acid, which is catalyzed by the enzyme fumarase ... 

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