Tyrosine Residue 300 Is Important for Activity and Stability of Branching Enzyme from Escherichia coli

Branching enzyme belongs to the α-amylase family, which includes enzymes that catalyze hydrolysis or transglycosylation at α-(1,4)- or α-(1,6)-glucosidic linkages. In the α-amylase family, four highly conserved regions are proposed to make up the active site. From amino acid sequence analysis a tyrosine residue is completely conserved in the α-amylase family. In Escherichia coli branching enzyme, this residue (Y300) is located prior to the conserved region 1. Site-directed mutagenesis of the Y300 residue in E. coli branching enzyme was used in order to study its possible function in branching enzymes. Replacement of Y300 with Ala, Asp, Leu, Ser, and Trp resulted in mutant enzymes with less than 1% of wild-type activity. A Y300F substitution retained 25% of wild-type activity. Kinetic analysis of Y300F showed no effect on the Km value. The heat stability of Y300F was analyzed, and this was lowered significantly compared to that of the wild-type enzyme. Y300F also showed lower relative activity at elevated temperatures compared to wild-type. Thus, these results show that Tyr residue 300 in E. coli branching enzyme is important for activity and thermostability of the enzyme.