Abstract:
Hydroxynitrile lyases (HNLs) are enzymes that catalyse enantioselective cleavage of the substrate in a reaction and are also used as important industrial biocatalysts for the synthesis of chiral cyanohydrins. The aim of the study was to screen indigenous South African plants for potential hydroxynitrile lyase activity, purify and biochemically characterise the active hydroxynitrile lyase(s) from the selected plants. Several indigenous plants were randomly collected, identified and screened for HNL activity. The plant parts (leaves, seeds or fruits) were processed using established experimental protocols in order to obtain the crude enzyme extracts. The enzymatic conversion of benzaldehyde and potassium cyanide to mandelonitrile was optimised and consequently used for the screening of HNL activity. Enzyme activity was detected in the crude enzyme extracts of Kalanchoe spp and Senecio spp and these were then designated as Ks and Sb, respectively. Ammonium sulphate fractionation, DEAE Toyopearl 650M and Concanavalin A chromatography techniques were then used in the purification process of the active crude enzyme extracts. Subsequently, two purified active fractions were isolated from each plant species with molecular masses estimated at 64.64 kDa and 64.06 kDa for the KsHNL enzymes and 70.60 kDa and 74.04 kDa for SbHNL enzymes. The optimum temperature and pH of all the isolated enzymes were determined as 50°C and pH 5, respectively. The experimental Km and Vmax values of the enzymes were respectively determined to be 0.33 and 0.73 mM and 1.238 and 1.948 μM/min for KsHNL; while that for SbHNL enzymes were 5.86 and 0.22 mM and 9.741 and 1.905 μM/min. The effect of additives and metal ions (viz., DTT, DEP, mercury chloride, magnesium chloride and zinc chloride) was determined. The experimental data obtained alluded to the notion that both KsHNL and SbHNL enzymes may contain the cysteine and serine residues next to their active sites and that a histidine residue may be involved in the catalytic activities of both the isolated KsHNL enzymes and one of the SbHNL enzymes. All the isolated enzymes from the two plant species did not seem to contain an FAD group. These findings compared favourably to the theoretical type II HNLs, although with a slight difference in that they displayed high molecular weights. Kalanchoe spp and Senecio spp are the two indigenous South African plants that were found to contain active HNLs. The isolated HNLs from the two plants have a potential to be xv
purified to homogeneity, cloned and overexpressed into robust recombinant enzymes that can be used for large scale industrial applications.
