Enhancement of engineered trifunctional enzyme by optimizing linker peptides for degradation of agricultural by-products

Fusion proteins composed of a xylosidase–arabinosidase (Xar) from Thermoanaerobacter ethanolicus and Thermomyces lanuginosu xylanase (XynA) were construted using different peptide linkers. The aim was to create optimal linkers that were able to join the functional modules without disturbing their function. Five fusion variants with linker containing different length and ratio of Ala/Gly were produced in Escherichia coli and purified for characterization of their enzymatic and kinetic properties. The fusion proteins with linkers had similar temperature and pH profiles to those without linkers, being well in accordance with their parents. However, the fusion variant containing SAGSSAAGSGSG had greater stability for xylosidase–arabinosidase activity than other fusion variants at 65–80 °C and pH 6.6–8.2. An obvious increase in activity, which occurred in the first 60 min at 65–75 °C for xylosidase–arabinosidase, appeared for xylanase and arabinosidase at pH 6.6–7.8, and for xylosidase at pH 7.8–9.5. The reducing sugar release efficiency of fusion variants containing SAGSSAAGSGSG and SGGSSAAGSGSG were obviously higher compared with those containing other linkers. These results indicate that introducing Ala in monotonous peptides consisting of only Gly and Ser would make the best linkers for the Xar–XynA fusion protein.