Studies on the kinetics of cyanohydrin synthesis and cleavage by the the flavoenzyme oxynitrilase.

Almond oxynitrilase (D-alpha-hydroxynitrile lyase, EC 4.1.2.10) catalyzes the reversible condensation of HCN with aldehydes to form D-alpha-hydroxynitriles. Steady-state kinetic parameters for cleavage and synthesis of mandelonitrile and vanillin cyanohydrin were determined at pH 5.5 which is near the pH optimum of the enzyme. Benzaldehyde and vanillin act as competitive inhibitors of cyanohydrin cleavage while noncompetitive inhibition was observed for HCN. The results are consistent with an ordered uni bi mechanism in which aldehyde is the first substrate bound. Competitive inhibition of cyanohydrin cleavage was observed with various carboxylic acids, alcohols and inorganic anions. The effect of structure on the binding of these inhibitors suggests that the active site of oxynitrilase is located near a hydrophobic region and a positively charged group. Inhibitors which are reasonable analogues for cyanide anion, such as azide and thiocyanate, do not bind to the enzyme-aldehyde complex. This suggests that during cyanohydrin formation the species which binds to the enzyme-aldehyde complex is HCN rather than CN-.