Creation of an Engineered Amide Synthetase Biocatalyst by the Rational Separation of a Two-Step Nitrile Synthetase

The synthesis of amides through acid and amine coupling is one of the most commonly used reactions in medicinal chemistry, yet still requires atom-inefficient coupling reagents. There is a current demand to develop greener, biocatalytic approaches to amide bond formation. The nitrile synthetase (NS) enzymes are a small family of ATP-dependent enzymes which catalyse the transformation of a carboxylic acid into the corresponding nitrile via an amide intermediate. The Bacillus subtilis QueC (BsQueC) is an NS involved in the synthesis of 7-cyano-7-deazaguanine (CDG) natural products. Through sequence homology and structural analysis of BsQueC we identified three highly conserved residues, which could potentially play important roles in NS substrate binding and catalysis. Rational engineering led to the creation of a NS K163A/R204A biocatalyst that converts the CDG acid into the primary amide, but does not proceed to the nitrile. This study suggests that NSs could be further developed for coupling agent-free, amide-forming biocatalysts.