The amino acid sequences of xylanase B (XlnB) and xylanase C (XlnC) from
Streptomyces lividans show significant homology. However, the temperature optima and stabilities of the two enzymes are quite different. XlnB exhibits
an optimum temperature of 40 °C and retains 50% of its maximum activity at 43 °C, whereas the corresponding values for XlnC
are 60 and 70 °C. To analyze these properties further, as well as to study the effect of the
exchange of homologous segments in the C-terminal region, four chimeras designated as BSC, BFC, CSB, and CFB were constructed by substituting
segments from the C-terminal homologous region of XlnB gene with that of XlnC and in turn substituting XlnC gene with that
of XlnB. The purified chimeric enzymes were characterized with respect to pH/temperature activity, stability, and kinetic
parameters. Most of enzymatic properties of chimeras were admixtures of those of the two parents. The chimeric enzymes were
optimally active at 45–55 °C and pH 7.0. Both
K
m and
k
cat values of chimeric enzymes for
p-nitrophenyl-β-
d-cellobioside were admixtures of both parental enzymes, except that the
k
cat value of chimeric BFC (2.79 s
−1) was higher than that of parental XlnC (1.99 s
−1). Notably,
thermal stability of chimeric BSC and BFC was increased by 25 and 13 °C separately, as compared to one of parental XlnB, whereas
the thermal stability of chimeric CSB and CFB was decreased by 23 and 21 °C, respectively, as compared to another parental
XlnC. These results suggest that homologous C-terminal region in
S. lividans GH11 xylanase appears to play an important role in determining enzyme characteristics, and
exchanging of different segments of gene in this region might significantly alter or improve the enzymatic properties such as thermal
stability.
Keywords Xylanase - Chimeric enzyme - Thermostability -
Streptomyces lividans