AbstractThe cloning, expression and characterization of a nitrile reductase (NRed) from the thermophile Geobacillus kaustophilus is reported. The enzyme shows a 12‐fold increase in activity in response to a temperature change from 25 °C to 65 °C. The substrate scope regarding its biocatalytic applicability was investigated by testing a range of common nitriles. The narrow substrate range observed for the wild‐type enzyme prompted the rational design of GkNRed active site mutants based on a previously published homology model from Bacillus subtilis. The activities of the mutants and the wild‐type enzyme were investigated in their structure‐function relationship regarding the natural substrate 7‐cyano‐7‐deazaguanine (preQ0) as well as a range of synthesized preQ0‐like substrate structures. A distinct dependence of the wild‐type enzyme activity on specific structural modifications of the natural substrate was observed. Two non‐natural nitriles derived from preQ0 could be reduced to their corresponding amino compounds.