Enzyme catalyzed reaction oxynitrilases

Since the reaction has been reviewed recently (12) only a few additional facts will be mentioned. Many optically active cyanohydrins can be prepared (33) with e.e. s of 84 to 100% by the use of the flavopnotein D-oxynitrilase adsorbed on special (34) cellulose ion-exchange resins. Although the enzyme is stable, permitting the use of a continuously operating column, naturally only one enantiomer, usually the R isomer, is produced in excess. This (reversible) enzyme-catalyzed reaction is very rapid (34). Nonenzymic catalysts, such as the cinchona alkaloids, permit either enantiomer to be prepared in excess. 

Enzyme-catalyzed reactions in this area include reaction with glycine catalyzed by L-threonine aldolase to afford 164 <2000SL1046> and the use of almond oxynitrilase to catalyze the formation of cyanohydrin 165 by reaction of 161 with acetone cyanohydrin <2001T2213>. 

Cyanohydrin Synthesis. Another synthetically useful enzyme that catalyzes carbon—carbon bond formation is oxynitnlase (EC 4.1.2.10). This enzyme catalyzes the addition of cyanides to various aldehydes that may come either in the form of hydrogen cyanide or acetone cyanohydrin (152—158) (Fig. 7). The reaction constitutes a convenient route for the preparation of a-hydroxy acids and P-amino alcohols. Acetone cyanohydrin [75-86-5] can also be used as the cyanide carrier, and is considered to be superior since it does not involve hazardous gaseous HCN and also virtually eliminates the spontaneous nonenzymatic reaction. (R)-oxynitrilase accepts aromatic (97a,b), straight- (97c,e), and branched-chain aUphatic aldehydes, converting them to (R)-cyanohydrins in very good yields and high enantiomeric purity (Table 10). 

The enzyme-catalyzed cyanohydrin reaction offers new and interesting perspectives for the synthesis of different kinds of chiral cyanohydrins, because over the next few years the continuous development of new genetically modified oxynitrilases will be without any doubt of great utility for the preparation of pharmaceuticals. 

The NHase and amidase were largely nonselective for cyanohydrins and the corresponding 2-hydroxy amides, respectively, but they were suitable for the hydrolysis of enantiopure cyanohydrins prepared from aldehydes and HCN by oxynitrilases [89, 90]. The cascade of NHase and amidase, in which the latter enzyme catalyzed an acyl transfer reaction, was suitable for the preparation of... 

This reaction is known to be catalyzed by the enzyme oxynitrilase to produce the optically pure cyanohydrin 76). Since this reaction proceeds with a base catalyst, Jnoue et al. 75) used cyclic and linear dipeptides containing (S)-histidine. The catalysts employed are as follows benzyloxycarbonyl-R-(S)-histidine methyl ester with R = (S)-alanyl, (R)-alanyl, (S)-phenylalanyl,[Z-(S)-Ala-(S)-His-OCH3, Z-(R)-Ala-(S)-His-OCH3, and Z-(S)-Phe-(S)-His-OCH3] as linear dipeptides, and cyclic (S)-histidine containing dipeptides Gly—(S)—His,... 

For the synthesis of cyanohydrins nature provides the chemist with R- and S-selective enzymes, the hydroxynitrile lyases (HNL) [4-7]. These HNLs are also known as oxynitrilases and their natural function is to catalyze the release of HCN from natural cyanohydrins like mandelonitrile and acetone cyanohydrin. This is a defense reaction of many plants. It occurs if a predator injures the plant cell. The reaction also takes place when we eat almonds. Ironically the benzaldhyde released together with the HCN from the almonds is actually the flavor that attracts us to eat them. 

Several industrial processes using lyases as catalysts have been reported. Perhaps the most prominent lyase-catalyzed process is the production of acrylamide from acrylnitrile. This process is carried out by the Nitto Chemical Company of Japan at a scale of more than 40,000 tons per year. Another example is the use of a fumarase for the production of (5 )-malic acid from fumaric acid. As shown in Fig. 7, a water molecule is added to the double bond in fumarate by means of an addition reaction. The result is a cleavage of the carbon-carbon double bond, and a formation of a new carbon-oxygen bond. A third example is bio-catalytic production of a cyanohydrin from a ketone. This reaction is catalyzed by a lyase called oxynitrilase. It consists of the cleavage of one carbon-oxygen bond, and the addition of a HCN molecule. The chirality of the product is based on the form of the enzyme used (/ -oxynitrilase or 5-oxynitrilase). ... 

A suitable catalyst for the synthesis of (R)-cyanohydrins is the enzyme (R)-oxynitrilase from bitter almonds. It catalyzes exclusively si-face addition of hydrogen cyanide to benzaldehyde or other aldehydes. A competing non-enzymatic parallel reaction lowers the enantiomeric excess of the product155, 56]. 

Oxynitrilase isolated from almonds contains one FAD molecule per monomer, while the enzyme isolated from other sources does not contain a flavin. " The FAD-containing enzyme from almonds catalyzes both the decomposition of mandelonitrile (I cat = 630s , 25 °C) and the reverse reaction, formation of mandelonitrile... 

The enantiomerically pure amino acids also can be produced through a similar synthetic pathway catalyzed by enzymes. For example, reaction of hydrogen cyanide with benzaldehyde catalyzed by either (R)- or (S)-oxynitrilase enzyme yields the enantiomeric cyanohydrins (R)- or (S)-mandelonitrile. Alternatively, by adding carbon dioxide and hydrogen cyanide and ammonia as feedstocks, aldehydes can be converted to hydantoins (4-alkylimidazolidine-2,5-diones), which can be then hydrolyzed with either D- or L- hydan-toinases to produce D- or L-a-amino acids, respectively. 

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