Hydroxynitrile_lyase

Family: Hydroxynitrile_lyase

Block: X

Parent Family: Abhydrolase_6

Comment

This entry represents a group of alpha/beta-hydrolase fold-containing proteins mostly from plants, including methylesterase (MES) and hydroxynitrile lyase (HNL) from Arabidopsis. (Bacterial Hydroxynitrile_lyase are grouped in another family. HNLyase_Bact but are very closely related). There are at least 20 Arabidopsis MES members (AtMES1 - AtMES20). The catalytic triad Ser-His-Asp is conserved in 15 of these proteins. However, AtMES19 and AtMES20 lack part of the N-terminal or C-terminal region, respectively, and are therefore likely to be inactive enzymes. AtHNL catalyses the cleavage of cyanohydrins to yield hydrocyanic acid (HCN) and the respective carbonyl compound and are key enzymes in the process of cyanogenesis in plants. Interestingly, HNL homologue from Hevea brasiliensis, known as HbHNL, has opposite enantioselectivities and different reaction mechanisms compared to AtHNL. Cyanogenesis is a defense process of several thousand plant species. Hydroxynitrile lyase, a key enzyme of this process, cleaves a cyanohydrin into hydrocyanic acid and the corresponding aldehyde or ketone. The reverse reaction constitutes an important tool in biocatalysis. Hydroxynitrile lyase activity is performed by a wide variety of enzymes some resemble carboxypetidase (see sorghum hnl) other have completely unrelated sequences and structure and are more related to FAD-dependent oxidoreductases (GMC oxidoreductases family see structure 1JU2 Dreveny et al Structure 2001 9 803) or Zn dependent alcohol dehydrogenases and bacterial cupins. The present family also contains high affinity salicylic acid-binding protein 2 from plants required for innate immunity. Kumar et al.. The structure of Methylketone synthase 1 revealed a new active site for an alpha/beta hydrolase: The nucleophile residue is Alanine located in a pentapeptide GHALG (Auldridge et al.). (a similar pentapeptide GHALD with histidine as nucleophile has been found in E. Coli RutD by Knapik et al in an un related family). PIRSF037175. This family also contains esterases such as Nicotiana tabacum (Common tobacco) salicylic acid-binding protein 2. Few mutations can convert one activity to the other (Padhi et al., Nedrud et al.). Some millipedes are able to exude toxic hydrogen cyanide as a defense, but hydroxynitril lyase from millipedes are not alpha/beta hydrolases. An important review on function and evolution of the plant MES family of methylesterases is published by Chaffin et al.