Cleaving of S-mandelonitrile catalyzed by S-hydroxynitrile lyase from Hevea brasiliensis--a kinetic investigation based on the rate curve method.

The progress curves of conversion of racemic-mandelonitrile (R-MN) at different concentrations to benzaldehyde (BA) and hydrogen cyanide (HCN), catalyzed by S-hydroxynitrile lyase from Hevea brasiliensis, together with the enantiomeric excess curves of R-MN are converted into individual progress curves of the S- and R-enantiomers. They reveal that, under the prevailing experimental conditions, the non-enzymatic conversion is sufficiently slow compared to its enzyme-catalyzed counterpart, so that it can be ignored in a simplified analysis. Tikhonov regularization is then used to convert the progress curves of the S-enantiomer into reaction rate curves. These curves are used to determine the rate constants in the rate expression based on a three-step reversible ordered Uni-Bi reaction scheme that describes this enzyme-catalyzed reaction. The resulting rate constants are compared against published data. Some of the problems encountered and their solution are briefly discussed.