Cross-linked crystals of hydroxynitrile lyase as catalyst for the synthesis of optically active cyanohydrins

Purified hydroxynitrile lyase (HNL) from Manihot esculenta was crystallized by the sitting-drop vapour-diffusion method. The bipyramidal crystals formed (10–20 μm) were cross-linked with different amounts of glutaraldehyde and used as biocatalyst for the synthesis of optically active cyanohydrins. The cross-linked crystals were more stable than Celite-immobilized enzymes when incubated in organic solvents, especially in polar solvents. After six consecutive batch reactions in dibutylether, the remaining activity of the cross-linked crystals was more than 70 times higher than for the immobilized enzymes. Nevertheless, the specific activity of the cross-linked crystals (per milligram protein) was reduced compared to the activity of immobilized enzymes. The product enantiopurity was independent of the type of enzyme preparation used.