GtHNL from Granulicella tundricola is a Mn(II) containing hydroxynitrile lyase with a cupin fold. The quasi-octahedral manganese is pentacoordinated by the enzyme. It catalyzes the enantioselective addition of HCN to aldehydes, yielding R-cyanohydrins. On the Lewis acidic vacant coordination site the Mn binds either substrate or the product, leading to a hexacoordinated 17 electron complex. EPR spectra of the active enzyme are unusually wide with a zero-field splitting approximately equal to the X-band microwave energy. A spectral change is induced by incubation with either one of the substrates/products HCN, benzaldehyde, and/or mandelonitrile. This points toward Mn(II) catalyzed cyanohydrin synthesis.@en