Impact of an N-terminal extension on the stability and activity of the GH11 xylanase from Thermobacillus xylanilyticus
Résumé
To understand structure-function relationships in the N-terminal region of GH11 xylanases, the 17 N-terminal amino acids of the GH11 xylanase from Neocallimastix patriciarum (Np-Xyn) have been graftedonto the N-terminal extremity of the untypically short GH11 xylanase from Thermobacillus xylanilyticus(Tx-Xyn), creating a hybrid enzyme denoted NTfus. The hybrid xylanase displayed properties (pH andtemperature optima) similar to those of the parental enzyme, although thermostability was lowered, withthe Tmvalue, being reduced by 5 degrees C. Kinetic assays using oNP-Xylo-oligosaccharides (DP2 and 3) indicatedthat the N-extension did not procure more extensive substrate binding, even when further mutagenesis was performed to promote this. However, these experiments confirmed weak subsite -3 for bothNTfus and the parental enzyme. The catalytic efficiency of NTfus was shown to be 17% higher than thatof the parental enzyme on low viscosity wheat arabinoxylan and trials using milled wheat straw as thesubstrate revealed that NTfus released more substituted oligosaccharide products (Xyl/Ara = 8.97 +/- 0.13 compared to Xyl/Ara = 9.70 +/- 0.21 for the parental enzyme), suggesting that the hybrid enzyme possesseswider substrate selectivity. Combining either the parental enzyme or NTfus with the cellulolytic cocktail Accellerase 1500 boosted the impact of the latter on wheat straw, procuring yields of solubilizedxylose and glucose of 23 and 24% of theoretical yield, respectively, thus underlining the benefits of addedxylanase activity when using this cellulase cocktail. Overall, in view of the results obtained for NTfus, we propose that the N-terminal extension leads to the modification of a putative secondary substratebinding site, a hypothesis that is highly consistent with previous data.