Lyase-catalyzed reactions

Figure 14.10 Hydroxynitrile lyase-catalyzed reactions for production of pharma intermediates used in the synthesis of cardiovascular drugs...

Lyases are the enzymes responsible for catalyzing addition and elimination reactions. Lyase-catalyzed reactions involve the breaking of a bond between a carbon atom and another atom such as oxygen, sulfur, or another carbon atom. They are found in cellular processes, such as the citric acid cycle, and in organic synthesis, such as in the production of cyanohydrins. 

Lyases. Lyases catalyze reactions in which groups (e.g., H20, COz, andNH3) are removed to form a double bond or are added to a double bond. Decarboxylases, hydratases, dehydratases, deaminases, and synthases are examples of lyases. 

Lyases - catalyze reactions involving a double bond ... 

Two particular problems which are often encountered in hydroxynitrile lyase-catalyzed reactions are the spontaneous nonenzymatic formation of racemic cyanohydrin and racemization of the product due to equilibration of the reaction. As a result, the optical purity of the product is decreased. Bearing in mind that both the chemical formation and the racemization of cyanohydrins are pH-dependent and require water, three different techniques have been developed in order to suppress the reduction of the optical purity of the product. 

Immobilization. The fixing property of PEIs has previously been discussed. Another appHcation of this property is enzyme immobilization (419). Enzymes can be bound by reactive compounds, eg, isothiocyanate (420) to the PEI skeleton, or immobilized on soHd supports, eg, cotton by adhesion with the aid of PEIs. In every case, fixing considerably simplifies the performance of enzyme-catalyzed reactions, thus faciHtating preparative work. This technique has been appHed to glutaraldehyde-sensitive enzymes (421), a-glucose transferase (422), and pectin lyase, pectin esterase, and endopolygalacturonase (423). 

Enzymes are classified into six categories depending on the kind of reaction they catalyze, as shown in Table 26.2. Oxidoreductases catalyze oxidations and reductions hansferases catalyze the transfer of a group from one substrate to another hydrolases catalyze hydrolysis reactions of esters, amides, and related substrates lyases catalyze the elimination or addition of a small molecule such as H2O from or to a substrate isomerases catalyze isomerizalions and ligases catalyze the bonding together of two molecules, often coupled with the hydrolysis... 

Griengl reported the first example of hydroxynitrile lyase-catalyzed cyanohydrin formation in a mixed solvent system of [bmim][BF4] and buffer (pH 3.7) (1 1) (Fig. 19). In the reaction, a mixed solvent system was essential, but excellent results were obtained. 

The PNL exhibits an optimum pH of 9.5 (figure 6) and an optimum temperature of 55" C (figure 7). Lyases catalyze the reaction in an alkaline or in a neutral medium at high temperatures [32] pectin lyase from Phoma medicaginis var. pinodella showed an optimum pH of 7.5 [25], endopectate lyase from Fusarium solani f sp. pisi showed an optimum pH of 9.4 [31], and pectate lyase fi om Rhizoctonia solani showed an optimum pH of 8.0 [34]. 

Gruber-Khadjawi, M., Purkarthofer, T. Skranc, W. et al. (2007) Hydroxynitrile lyase-catalyzed enzymatic nitroaldol (Henry) reaction. Advanced Synthesis and Catalysis, 349, 1445-1450. 

Lyases (class 4, often also referred to as synthases ) catalyze reactions involving either the cleavage or formation of chemical bonds, with double bonds either arising or disappearing. 

This enzyme (also known as DNA-(apurmic or apyrimid-inic site) lyase, AP endonuclease class I, E. coli endonuclease III, Phage-T4 UV endonuclease, and Micrococcus luteus UV endonuclease) catalyzes the cleavage of the phosphodiester bond in a lyase-type reaction, not hydro-... 

Adenylosuccinate lyase, malyl coenzyme A lyase , and NADase are reported to have this mechanism. It should also be recognized that the termed ordered Uni Bi is often used to refer to enzyme-catalyzed reactions which are actually ping pong Bi Bi mechanisms in which water is the second substrate. See Multisubstrate Mechanisms... 

This enzyme [EC 4.1.99.1], also known as L-tryptophan indole-lyase, catalyzes the hydrolysis of L-tryptophan to generate indole, pyruvate, and ammonia. The reaction requires pyridoxal phosphate and potassium ions. The enzyme can also catalyze the synthesis of tryptophan from indole and serine as well as catalyze 2,3-elimination and j8-replacement reactions of some indole-substituted tryptophan analogs of L-cysteine, L-serine, and other 3-substituted amino acids. 

Ammonia lyases catalyze the enantioselective addition of ammonia to an activated double bond. A one-pot, three-step protocol was developed for the enantioselective synthesis of L-arylalanines 50 using phenylalanine ammonia lyase (PAL) in the key step (Scheme 2.20). After formation of the unsaturated esters 48 in situ via a Wittig reaction from the corresponding aldehydes, addition of porcine Ever esterase and basification of the reaction mixture resulted in hydrolysis to the carboxylic acids 49. Once this reaction had gone to completion, introduction of PAL and further addition of ammonia generated the amino acids 50 in good yield and excellent optical purity [22]. 

Fig. 25-15). In every case it is NH3 or an amine, rather than an OH group, that is eliminated. However, the mechanisms probably resemble that of fumarate hydratase. Sequence analysis indicated that all of these enzymes belong to a single fumarase-aspartase family.64 65 The three-dimensional structure of aspartate ammonia-lyase resembles that of fumarate hydratase, but the catalytic site lacks the essential HI 88 of fumarate hydratase. However, the pKa values deduced from the pH dependence of Vmax are similar to those for fumarase.64 3-Methylaspartate lyase catalyzes the same kind of reaction to produce ammonia plus czs-mesaconate.63 Its sequence is not related to that of fumarase and it may contain a dehydroalanine residue (Chapter 14).66... 

Arginine is synthesized from aspartate and ornithine during urea formation. Argininosuccinate synthetase and -lyase catalyze the condensation and cleavage reactions, respectively, that result in the formation of arginine (Sec. 15.5). 

There is a fifth bifunctional enzyme which catalyzes reactions 8 and 12 of the purine pathway (Fig. 15-16) but adenylosuccinate lyase has one active site with dual specificity, catalyzing both reactions (SA1CAR—> AICAR, sAMP—> AMP Fig. 15-16). All 14 enzymatic activities of Fig. 15-16 are cytosolic and there is a variety of evidence for association of subsets of these activities in vivo. The existence of a pathway particle or metabolon" for de novo purine biosynthesis in intact cells has been proposed. 

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