Improved catalytic efficiency and active site modification of 1,4-beta-D-glucan glucohydrolase A from Thermotoga neapolitana by directed evolution

Thermotoga neapolitana 1,4-beta-d-glucan glucohydrolase A preferentially hydrolyzes cello-oligomers, such as cellotetraose, releasing single glucose moieties from the reducing end of the cello-oligosaccharide chain. Using directed evolution techniques of error-prone PCR and mutant library screening, a variant glucan glucohydrolase has been isolated that hydrolyzes the disaccharide, cellobiose, at a 31% greater rate than its wild type (WT) predecessor. The mutant library, expressed in Escherichia coli, was screened at 85 degrees C for increased hydrolysis of cellobiose, a native substrate rather than a chromogenic analog, using a continuous, thermostable coupled enzyme assay. The V(max) for the mutant was 108 +/- 3 units mg(-1), whereas that of the WT was 75 +/- 2 units mg(-1). The K(m) for both proteins was nearly the same. The k(cat) for the new enzyme increased by 31% and its catalytic efficiency (k(cat)/K(m)) for cellobiose also rose by 31% as compared with the parent. The nucleotide sequence of two positive clones and two null clones identified 11 single base shifts. The nucleotide transition in the most active clone caused an isoleucine to threonine amino acid substitution at position 170. Structural models for I170T and WT proteins were derived by sequence homology with Protein Data Bank code 1BGA from Paenibacillus polymyxa. Analysis of the WT and I170T model structures indicated that the substitution in the mutant enzyme repositioned the conserved catalytic residue Asn-163 and reconfigured entry to the active site.