Enhancement in affinity of Aspergillus niger JMU-TS528 α-L-rhamnosidase (r-Rha1) by semiconservative site-directed mutagenesis of (α/α)6 catalytic domain

Abstract

α-L-Rhamnosidase has attracted lots of attention due to its industrial potential applications. The applicability of α-L-rhamnosidase, however, was limited by their low ligand affinity on the industrial scale. In order to improve the affinity of α-L-rhamnosidase for industrial use, we investigated the variation of its affinity by amino acid replacement. Particularly, the enzyme affinity of a α-L-rhamnosidase from Aspergillus niger JMU-TS528 (rRha1) was measured with the semi-conservative amino acid (homology between 30% -80%) replaced. As a result, the enzyme affinity of the two mutants, R404S and N578D, were increased by 1.45-fold and 2.3-fold, respectively, showing that these two mutants could be the promising candidates for industrial use. To test if these mutations bring negative effect on the enzyme properties, we also determined the other enzymatic properties of these mutants and showed no negative effect. To understand the improvement of enzyme affinity, the conformational flexibility of (α/α)6-barrel catalytic domain were examined by molecular dynamics (MD) simulation, and demonstrated that the conformations of these mutants are more flexible, which could influence the affinity of substrates to the enzyme and hence the enzyme activity. This work not only enhanced the enzyme affinity of a α-L-rhamnosidase, making rRha1 a promising candidate for industrial processes, but also provided an effective technical strategy for improving affinity of other enzymes.