Site-Directed Mutagenesis and Thermostability of Xylanase XYNB from Aspergillus niger 400264

Improvement in thermostability of metagenomic GH11 endoxylanase (Mxyl) by site-directed mutagenesis and its applicability in paper pulp bleaching process

An attempt has been made for enhancing the thermostability of xylanase (Mxyl) retrieved from a compost-soil-based metagenomic library. The analysis of the structure of xylanase by molecular dynamics simulation revealed more structural fluctuations in β-sheets. When the surface of β-sheets was enriched with arginine residues by substituting serine/threonine by site-directed mutagenesis, the enzyme with four arginine substitutions (MxylM4) exhibited enhanced thermostability at 80 °C. The T 1/2 of MxylM4 at 80 °C, in the presence of birchwood xylan, increased from 130 to 150 min at 80 °C without any alteration in optimum pH and temperature and molecular mass. Improvement in thermostability of MxylM4 was corroborated by increase in T m by 6 °C over that of Mxyl. The K m of MxylM4, however, increased from 8.01 ± 0.56 of Mxyl to 12.5 ± 0.32 mg ml(-1), suggesting a decrease in the affinity as well as specific enzyme activity. The Mxyl as well as MxylM4 liberated chromophores and lignin-derived compounds from kraft pulp, indicating their applicability in pulp bleaching.

Molecular approaches for ameliorating microbial xylanases

In industrial processes, chemical catalysis is being replaced by enzyme catalysis, since the latter is environmentally benign, non-persistent and cost effective. Microbial xylanases have significant applications in textile, baking, food and feed industries, and in paper and pulp industries for reducing the chlorine requirement. The hazardous chlorine required for bleaching can be reduced up to 25-30% by including an enzymatic step in the pulp bleaching process. The paper pulp bleaching requires xylanases that are active at alkaline pH and elevated temperatures. The enzymes from the cultured microbes do not perform optimally in the paper industry due to their inadequate stability under the process conditions of high temperature and alkaline pH. This review, therefore, deals with the rationale of molecular approaches such as protein engineering for designing xylanases with improved characteristics to suit the process conditions in industries, and prospects and problems.