Structural modeling and further improvement in pH stability and activity of a highly-active xylanase from an uncultured rumen fungus

Rumen fungi are a rich source of enzymes degrading lignocelluloses. XynR8 is a glycosyl hydrolase family 11 xylanase previously cloned from unpurified rumen fungal cultures. Phylogenetic analysis suggested that xynR8 was obtained from a Neocallimastix species. Recombinant XynR8 expressed in Escherichia coli was highly active and stable between pH 3.0 and 11.0, and displayed a V-max of 66,672 mu mol min(-1) mg(-1), a k(cat) of 38,975 s(-1), and a K-m of 11.20 mg/mL towards soluble oat spelt xylan. Based on molecular modeling, residues N41 and N58, important in stabilizing two loops and the structure of XynR8, were mutated to D. Both mutant enzymes showed higher tolerance to pH 2.0. The V-max, k(cat) and K-m of the N41D and N58D mutant enzymes were 79,645 mu mol min(-1) mg(-1), 46,493 s(-1), 29.29 mg/mL, and 96,689 mu mol min(-1) mg(-1), 56,503 s(-1), and 21.24 mg/mL, respectively. Thus, they are good candidates for application, including biofuel production. (C) 2012 Elsevier Ltd. All rights reserved.