Homology model of a novel xylanase: molecular basis for high-thermostability and alkaline stability.

Xylanases form enzymes of considerable interest to a variety of biotechnological industries. Their industrial usage is especially attractive since they can replace some of the environmental pollutants, and are economically viable. Those with higher thermostability and optimal activity at alkaline pH are of particular importance to the paper and pulp industry due to the demands of conditions under which the enzymatic reactions are carried out. We have earlier isolated a xylanase from Bacillus sp. NG-27, which is active both at high temperature as well as at alkaline pH. In order to find out factors responsible for the adaptation of this enzyme to the extreme conditions, three dimensional structure of NG-27 xylanase has now been obtained by homology modelling. The tertiary structure shows TIM barrel fold consisting of 8 parallel beta-strands surrounded by alpha-helices. The active site is located at the carboxy terminal end of the TIM barrel. Factors which contribute to the thermostability of the enzyme are increased number of salt bridges. The salt bridges occur remarkably on one face of alpha-helices, with oppositely charged residues occupying i, i+4, i+7 positions. A solvent shielded salt bridge interaction is also observed, which is absent in the mesophilic homologous xylanases. Solvent shielding may enhance electrostatic interaction through lowering of the dielectric, and contribute to increased stability of the enzyme.