Mechanism of cyanogenesis: The crystal structure of hydroxynitrile lyase from Hevea brasiliensis

Background: Over three thousand species of plants, including important food crops such as cassava, use cyanogenesis, the liberation of HCN upon tissue damage, as a defense against predation, Detoxification of cyanogenic food crops requires disruption of the cyanogenic pathway, Hydroxynitrile lyase is one of the key enzymes in cyanogenesis, catalyzing the decomposition of an alpha-cyanohydrin to form HCN plus the corresponding aldehyde or ketone. These enzymes are also of potential utility for industrial syntheses of optically pure chiral cyanohydrins, being used to catalyze the reverse reaction, We set out to gain insight into the catalytic mechanism of this important class of enzymes by determining the three-dimensional structure of hydroxynitrile lyase from the rubber tree, Hevea brasiliensis, Results: The crystal structure of the enzyme has been determined to 1.9 Angstrom resolution. It belongs to the alpha/beta hydrolase superfamily, with an active site that is deeply buried within the protein and connected to the outside by a narrow tunnel. The catalytic triad is made up of Ser80, His235 and Asp207. By analogy with known mechanisms of other members of this superfamily, catalysis should involve an oxyanion hole formed by the main chain NH of Cys81 and the side chains of Cys81 and Thr11. Density attributed to a histidine molecule or ion is found in the active site. Conclusions: By analogy with other alpha/beta hydrolases, the reaction catalyzed by hydroxynitrile lyase involves a tetrahedral hemiketal or hemiacetal intermediate formed by nucleophilic attack of Ser80 on the substrate, stabilized by the oxyanion hole. The SH group of Cys81 is probably involved in proton transfer between the HCN and the hydroxynitrile OH. This mechanism is significantly different from the corresponding uncatalyzed solution reaction.