Hydroxynitrile Lyases, Interesting Biocatalysts in Stereoselective Organic Syntheses

Hydroxynitrile lyases (HNLs) from Prunus amygdalus (PaHNL), Sorghum bicolor (SbHNL), Manihot esculenta (MeHNL) and Hevea brasiliensis (HbHNL) are excellent biocatalysts for the preparation of optically active cyanohydrins. (R)as well as (S)-cyanohydrins are obtained in high optical and chemical yields. The synthetic potential of the now easily available optically pure cyanohydrins is demonstrated by stereoselective transformations of the cyanohydrins to biologically active compounds with one or two stereogenic centers. The release of HCN (cyanogenesis) as a defence mechanism against herbivores, is not only widely distributed in higher plants, including important food products like cassava and sorghum, but is in addition found in several species of ferns, bacteria, fungi and insects (ref. 1). During cyanogenesis cyanohydrin-O-glycosides are decomposed to a sugar, HCN and a carbonyl compound by a two step mechanism. In the first step β-glycosidase catalyzed hydrolysis of the glycosides into carbohydrates and the corresponding cyanohydrins occur. In the second step the cyanohydrins are cleaved to give HCN and aldehydes or ketones, respectively (ref. 2). The latter step occurs (base catalyzed) spontaneously or enzymatically by action of an α-hydroxynitrile lyase (HNL) (ref. 1). Hydroxynitrile lyases not only catalyze the cleavage of cyanohydrins, but also their formation from carbonyl compounds and HCN. CHARACTERIZATION, MOLECULAR CLONING AND OVEREXPRESSION OF HYDROXYNITRILE LYASES HNLs have been purified and characterized from almost a dozen cyanogenic plants. In all cases where the cyanohydrins have a chirality center, only one stereoisomer (enantiomer) is found in naturally occuring cyanohydrin glycosides (ref. 1). Traditionally, HNLs have been divided in two groups, flavoprotein HNLs and nonflavoprotein HNLs. Flavoprotein HNLs are major seed proteins of the rosaceous stone fruits and have (R)-mandelonitrile as natural substrate, they are monomeric glycoproteins. Nonflavoprotein HNLs have been isolated from three families of dicotyledons (Linaceae, Euphorbiaceae, Olacaceae), one family of monocotyledons (Gramineae) and one family of ferns (Polypodiaceae) (ref. 1). In contrast to the flavoprotein group of HNLs, the nonflavoprotein HNLs comprise enzymes forming a rather heterogeneous group regarding their biochemical properties.