Narrowing laccase substrate specificity using active site saturation mutagenesis

The laccase CotA from Bacillus subtilis was converted from a generalist, an enzyme with broad specificity, to a specialist, an enzyme with narrowed specificity. Laccases are members of the multicopper oxidase family and have many applications in biotechnology. To date, it has not been demonstrated that substrate specificity can be tapered for a laccase. Wild-type CotA oxidizes ABTS (ABTS = diammonium 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) and SGZ (SGZ = 4-hydroxy-3,5-dimethoxy-benzaldehyde azine), and it was engineered for increased specificity for ABTS by combining rational and directed evolution approaches. The wild-type was evolved by simultaneously randomizing 19 amino acids found in the substrate-binding pocket. A mutant was identified that had a catalytic efficiency, (kcat/KM)ATBS /(kcat/KM)SGZ, 7.0 times greater when compared to the wild-type after one round of screening. This illustrates that the substrate-binding pocket is highly evolvable for specificity.