Hydrolase catalyzed reaction

Next to reactions catalyzed by transaminases, hydrolase-catalyzed reactions also lead to limitations regarding the equilibrium. This problem occurs during ester synthesis, because this condensation reaction produces water. The equilibrium is shifted by high amounts of water towards the reactants therefore, an efficient removal is necessary to reach high conversions. Here, two process setups of Unichema Chemie B V will be discussed illustrating in situ product removal [41]. The first setup is based on azeotropic distillation of the water produced... 

The chemical diversity of carboxylic acid esters (R-CO-O-R ) originates in both moieties, i.e., the acyl group (R-CO-) and the alkoxy or aryloxy group (-OR7). Thus, the acyl group can be made up of aliphatic or aromatic carboxylic acids, carbamic acids, or carbonic acids, and the -OR7 moiety may be derived from an alcohol, an enol, or a phenol. When a thiol is involved, a thioester R-CO-S-R7 is formed. The model substrates to be discussed in Sect. 7.3 will, thus, be classified according to the chemical nature of the -OR7 (or -SR7) moiety, i.e., the alcohol, phenol, or thiol that is the first product to be released during the hydrolase-catalyzed reaction (see Chapt. 3). Diesters represent substrates of special interest and will be presented separately. 

Fig. 7. Enzyme-coupled assay in which the hydrolase-catalyzed reaction releases acetic acid. The latter is converted by acetyl-CoA synthetase (ACS) into acetyl-CoA in the presence of (ATP) and coenzyme A (CoA). Citrate synthase (CS) catalyzes the reaction between acetyl-CoA and oxaloacetate to give citrate. The oxaloacetate required for this reaction is formed from L-malate and NAD in the presence of L-malate dehydrogenase (l-MDH). Initial rates of acetic acid formation can thus be determined by the increase in adsorption at 340 nm due to the increase in NADH concentration. Use of optically pure (Ry- or (5)-acetates allows the determination of the apparent enantioselectivity i app i81)-...

Two typical hydrolase-catalyzed reactions are shown in Scheme 4.3. It is important to note that these reactions are reversible, and in water the equilibrium of course favors hydrolysis. However, the use of hydrophobic organic solvents allows the acylation to give e.g., esters and amides using hydrolases as catalysts. Many of these enzymes are commercially available and can be used in hydrophobic organic solvents as received, and because they are insoluble in the reaction medium they can easily be recovered by filtration and used again. 

Enantiomeric purity. In order to assess the efficiency of an enantioselective hydrolase-catalyzed reaction, it is imperative that one can accurately measure at least the conversion and the enantiomeric excesses of either the substrate or the product (see equations Equation 1, Equation 2, and Equation 3). Although optical rotation is sometimes used to assess enantiomeric excess, it is not recommended. Much better alternatives are various chromatographic methods. For volatile compounds, capillary gas chromatography on a chiral liquid phase is probably the most convenient method. Numerous commercial suppliers offer a large variety of columns with different chiral liquid phases. Hence it is often easy to find suitable conditions for enantioselective GC-separations that yield ee-values in excess of... 

Many 1-heteroaryl-1-alkanols are also easily resolved by hydrolase-catalyzed reactions in organic solvents. Thus, some racemic l-(2-pyridyl)-l-alkanols and isoquinonyl-l-ethanols are easily resolved by CALB (Novozym 435) and vinyl acetate in diisopropyl ether to give the acetates 38—44 (Scheme 4.18) [75]. 

Because of the minimal steric difference between the substituents surrounding the alcohol moiety in simple 2-cyclohexenols, these are not easy to resolve by hydrolase-catalyzed reactions. However if a sterically bulky group can be temporarily introduced and then removed, such resolution will be facilitated. One example of this approach is the preparation of enantiomerically enriched 4-hydroxycyclohex-2-enone (64) (Scheme 4.25) [85]. A similar approach was used for the preparation of enantiomerically enriched cryptone (65) (Scheme 4.25) [86]. 

One of the most promising new screening systems is the monitoring of a change in pH for hydrolase-catalyzed reactions that produce an acid such as esterases. Several research groups including our own have explored this type of assay at several levels of sophistication and for screening purposes. Scheme 4... 

Scheme 4. pH-shift based assays for hydrolase catalyzed reactions... 

Hydrolases. Hydrolases catalyze reactions in which the cleavage of bonds is accomplished by adding water. The hydrolases include the esterases, phosphatases, and peptidases. 

My goal in this chapter is to describe the proton transfer reactions that occur during the course of hydrolase-catalyzed reactions and to provide some insight into how they contribute to the catalytic efficiency of these enzymes. Rather than pres-... 

Proton Transfer in Physical Steps of Hydrolase-catalyzed Reactions... 

The hydrolase-catalyzed reactions utilized most for the selective transformation of such substrates are hydrolysis (Schemes 11.1-1, 11.1-2, 11.1-4, 11.1-5 and 11.1-11), acylation (transesterification) (Schemes 11.1-3, 11.1-6 and 11.1-11) and alcoholysis (transesterification) (Schemes 11.1-7,11.1-8 and 11.1-15). Hydrolase-catalyzed esterification of an alcohol with a carboxylic acid, although highly useful in some casesl6Z, has been utilized to a lesser extent. Catalysis of formation and cleavage of the C-O bond of an ester or lactone by pig liver esterase, most lipases, a-chymotrypsin and subtilisin, which are all serine hydrolases, involves the following steps (Scheme... 

The fi-glucan exo- and endo-hydrolases are discussed with reference to newer techniques for the investigation of their specificity and action pattern. Those exo-hydrolases which have been well characterized are described individually. The endo-hydrolases are examined from the point of view of their linkage specificity, action on substituted glucans and their specificity for various monomer units. The significance of more random and less random endo-action patterns is considered in relation to single or multiple attack mechanisms. Certain features of p-glucan endo-hydrolase catalyzed reactions are discussed in relation to current views on the three-dimensional structure and mechanism of action of lysozyme. 

Similarly, the two chemically identical groups X, positioned on carbon atoms of opposite (/ ,5)-configuration in a weso-substrate, can react at different rates in a hydrolase-catalyzed reaction (Scheme 2.4). So, the optically inactive meso-substrate is transformed into an optically active product due to the transformation of one of the reactive groups from X into Y along with the destruction of the plane of symmetry within the substrate. Numerous open-chain or cyclic c/s-weso-diesters have been transformed into chiral monoesters by this technique [30]. Again, for dicarboxylates the reaction usually stops after the first step at the carboxylate monoester stage, whereas two hydrolytic steps are usually observed with diacetoxy esters [31]. The theoretical yield of chiral product from single-step reactions based on an enantioface or enantiotopos differentiation or a desymmetrization of meso-compounds is always 100%. 

The most common application of hydrolase-catalyzed reactions is the preparation of enantiopure compounds. The three possible routes are kinetic resolutions, desymmetrizations and dynamic kinetic resolutions (Figure 5.3 [21-23]). The choice of substrate determines the possible routes. If the substrate is a racemate, then the choice is either a kinetic resolution or a dynamic kinetic resolution. If the substrate is a meso or prochiral compound, then the route is a desymmetrization. Since racemates are more numerous than meso or prochiral compounds, the most common routes are resolutions. 

Three routes to enantiopure compounds using hydrolase-catalyzed reactions, (a) Kinetic resolution starts with racemic substrate and converts one enantiomer into product. This separation yields one enantiomer as the product alcohol and one as the starting acetate, both with a maximum yield of 50%. (b) Desymmetrization of a prochiral compound transforms one of prochiral groups to yield a chiral product with a maximum yield of 100%. (c) A dynamic kinetic resolution combines rapid racemization of racemic starting material with a hydrolase catalyzed acylation of one enantiomer. The maximum yield is 100%. 

Another reason to use organic solvents is to inaease the selectivity of the hydrolase-catalyzed reaction. Changing the solvent changes the solvation of the enzyme-substrate complex. If solvation differs significantly for different substrates, then the enzyme selectivity can change in different solvents. The altered selectivity of subtilisin in water and organic solvents discussed previously is one such example. 

The simplest hydrolase-catalyzed reaction is ester or amide hydrolysis in water or biphasic mixtures of water and an organic solvent. Although proteases and esterases require a soluble substrate, it is enough that some fraction of the substrate dissolve in the aqueous phase. For lipases, a second phase is desirable because it activates most lipases by 10-100-fold, probably due to lid opening as discussed previously. Liquid substrate can also act as the organic phase. 

Note Dry reaction conditions maximize the rate of the hydrolase-catalyzed reaction and minimize the competing hydrolysis of vinyl or isopropenyl acetate, which generates acetic add. Serine hydrolases are active only above pH 6, where the side chain imidazole of the active site histidine is in the uncharged form. Triethylamine prevents addification of the reaction mixture due to hydrolysis and may have other effects [34-36]. 

---