The active site of hydroxynitrile lyase from Prunus amygdalus: Modeling studies provide new insights into the mechanism of cyanogenesis

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Abstract
The FAD‐dependent hydroxynitrile lyase from almond (Prunus amygdalus, PaHNL) catalyzes the cleavage of R‐mandelonitrile into benzaldehyde and hydrocyanic acid. Catalysis of the reverse reaction—the enantiospecific formation of α‐hydroxynitriles—is now widely utilized in organic syntheses as one of the few industrially relevant examples of enzyme‐mediated C–C bond formation. Starting from the recently determined X‐ray crystal structure, systematic docking calculations with the natural substrate were used to locate the active site of the enzyme and to identify amino acid residues involved in substrate binding and catalysis. Analysis of the modeled substrate complexes supports an enzymatic mechanism that includes the flavin cofactor as a mere “spectator” of the reaction and relies on general acid/base catalysis by the conserved His‐497. Stabilization of the negative charge of the cyanide ion is accomplished by a pronounced positive electrostatic potential at the binding site. PaHNL activity requires the FAD cofactor to be bound in its oxidized form, and calculations of the pKa of enzyme‐bound HCN showed that the observed inactivation upon cofactor reduction is largely caused by the reversal of the electrostatic potential within the active site. The suggested mechanism closely resembles the one proposed for the FAD‐independent, and structurally unrelated HNL from Hevea brasiliensis. Although the actual amino acid residues involved in the catalytic cycle are completely different in the two enzymes, a common motif for the mechanism of cyanogenesis (general acid/base catalysis plus electrostatic stabilization of the cyanide ion) becomes evident.