In enzymology, a hydroxymandelonitrile lyase (EC 4.1.2.11) is an enzyme that catalyzes the chemical reaction

(S)-4-hydroxymandelonitrile cyanide + 4-hydroxybenzaldehyde

This enzyme belongs to the family of lyases, specifically the aldehyde-lyases, which cleave carbon-carbon bonds. The systematic name of this enzyme class is (S)-4-hydroxymandelonitrile 4-hydroxybenzaldehyde-lyase (cyanide-forming). Other names in common use include hydroxynitrile lyase, oxynitrilase, Sorghum hydroxynitrile lyase, and (S)-4-hydroxymandelonitrile hydroxybenzaldehyde-lyase. This enzyme participates in cyanoamino acid metabolism.

As of late 2007, only one structure has been solved for this class of enzymes, with the PDB accession code 1GXS.

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