INCREASE IN CATALYTIC ACTIVITY AND THERMOSTABILITY OF THE XYLANASE-A OF STREPTOMYCES-LIVIDANS-1326 BY SITE-SPECIFIC MUTAGENESIS

The xylanase A gene from Streptomyces lividans was modified by site-directed mutagenesis, selecting for mutations that improved the catalytic activity and thermostability of the enzyme. Mutant notation uses the one-letter abbreviation for amino acids. The first and the last letters represent, respectively, the residue to be changed and the replacing residue. The number indicates the position of the substitution. The mutant enzymes F155Y: R156E, R156K and N173D were respectively 28, 10, 50, and 25% more active than the wild-type enzyme. In addition, the half-lives at 60 degrees C of the R156E and N173D xylanases were respectively 6 and 40 min longer than that of the wild-type enzyme even in the absence of substrate. The favorable single mutations were combined to generate the double mutants E156/173D and K156/173D, which were 22 and 47% less active than the wild type. However the activity half-life of the E156/173D enzyme at 60 degrees C was twice that of the xylanase A. The pH-activity profiles bf all the mutant xylanases were similar to that of the wild-type enzyme.