Microbial xylanases have attracted considerable research interest because of their various applications in biotechnology including the biobleaching of kraft pulp, to increase the nutritional value of foods and animal feed as well as for their potential use in the production of ethanol and methane. In the paper and pulp industry, the bleaching process involves the use of toxic chemicals and in the interim produces harmful gases that have a negative impact on the environment. The application of enzymes for this process will potentially reduce the environmental pollution by this industry. In addition, using an enzyme that is thermostable and alkali tolerant means that they will remain active under the required processing conditions. The xylanase gene, xynA derived from Thermomyces lanuginosus DSM 5826, was previously evolved to produce a number of xylanase variants, which were further enhanced for increased thermostability and alkalinity. In this study, these variants were cloned in Pichia pastoris using the pBGP1 vector to achieve extracellular production of the recombinant proteins. The xylanase genes were isolated using PCR. Both vector and DNA inserts were linearized with restriction enzymes EcoRI and XbaI and ligated. Electroporation was employed to transform the yeast with the recombinant plasmids. This was followed by the expression of the enzymes in P. pastoris grown in yeast peptone glucose (YPD) medium. Enzyme activity was thereafter assessed and the yeast was found to produce 164, 78, 96 and 142 IU/ml of S325, S340, G41 and G53 xylanase respectively, higher levels than bacterial hosts. The enzymes were then characterized and it was established that the optimum temperatures and pH for maximum xylanase activity were, 60°C, pH 6 for S325; 40°C, pH 5 for S340; 60°C, pH 6 for G41 and 60°C, pH 7 for G53. i The pH and temperature stabilities of the respective enzymes were investigated, the S325 variant was exceptionally stable at a pH between 5 and 7 and temperature range of 40-80°C and retained a minimum of 40% of activity at higher pH and temperature after an incubation period of 90 min. The S340 variant was the least thermostable and alkali stable from all four variants, it however retained 40% of activity when subjected to conditions of pH 9, 80°C after 90 min. The G41 and G53 were highly stable under the pH and temperature conditions that they were subjected to. Thus being suitable for potential application in the pulp and paper industry. The enzymes were able to retain 80% of activity at pH 9, 80°C after 120 min. P. pastoris has been proven to be a more suitable protein expression vector than E. coli for a number of reasons, including; the ability to perform complex post-translational modifications and grow to high densities in minimal media resulting in the production of a high yield of heterologous proteins.
Optimized Expression of a Thermostable Xylanase from Thermomyces lanuginosus in Pichia pastoris
