(S) hydroxynitrile lyase

(S)-hydroxynitrile lyase (EC 4.1.2.47, (S)-cyanohydrin producing hydroxynitrile lyase, (S)-oxynitrilase, (S)-HbHNL, (S)-MeHNL, hydroxynitrile lyase, oxynitrilase, HbHNL, MeHNL, (S)-selective hydroxynitrile lyase, (S)-cyanohydrin carbonyl-lyase (cyanide forming), hydroxynitrilase) is an enzyme with systematic name (S)-cyanohydrin lyase (cyanide forming). This enzyme catalyses the following chemical reaction

Contents

• Natural Substrates of Hydroxynitrile Lyases
• Unnatural Substrates
• References

(1) an aliphatic (S)-hydroxynitrile ⇌ cyanide + an aliphatic aldehyde or ketone(2) an aromatic (S)-hydroxynitrile ⇌ cyanide + an aromatic aldehyde

In nature, the liberation of cyanide serves as a defense mechanism against herbivores and microbial attack in plants. Hydroxynitrile lyases of the alpha/beta hydrolase fold are closely related to esterases. All members of the alpha/beta hydrolase fold contain a conserved catalytic triad (nucleophile-histidine-aspartate). The nucleophile in this case is a serine. In contrast to esterases, serine proteases, lipases and other enzymes in this family, the nucleophile in hydroxynitrile lyases functions as a proton acceptor. Key amino acid residues in this reaction are the lysine at position 236 and the threonine at position 11. Lys236 helps to orient the substrate while Thr11 serves to block the oxyanion hole that would convert the enzyme into an esterase.

Commonly studied (S)-selective hydroxynitrile lyases include MeHNL from Manihot esculenta and HbHNL from Hevea brasiliensis. (R)-selective hydroxynitrile lyases have also been found to exist in Arabidopsis thaliana (AtHNL). AtHNL is thought to catalyze this reaction by a different mechanism.

Not all hydroxynitrile lyases belong to the alpha/beta hydrolase family. PaHNL (Prunus amygdalus), (R)-selective like AtHNL, uses a flavin cofactor to catalyze cyanogenesis.