Molecular Evolution of Nucleoside Deoxyribosyl Transferase (NDT) to Enhance the Activity towards 2'-Fluoro-2'-Deoxynucleoside.

Abstract

Nucleoside deoxyribosyl transferase (NDT) is an enzyme that catalyzes the transfer of purine and pyrimidine bases between 2'-deoxyribonucleosides and is widely used for synthesizing nucleoside analogs in various biotechnological applications. While NDT exhibits high activity toward natural nucleosides, its activity toward unnatural nucleoside analogs is significantly lower. Previously, the NDT mutant named fNDT(L59Q) was identified displaying 4.4-fold higher activity toward 2'-fluoro-2'-deoxyuridine (2FDU). In this study, molecular evolution strategies using error-prone PCR were employed to further generate mutant enzymes with enhanced activity toward 2FDU. After two rounds of mutational screening, two mutant clones that exhibited high activity against 2FDU were identified as fNDT-i1 (V52A) and fNDT-i2 (L28I), respectively. A double mutant fNDT-i4, was subsequently constructed by combining the V52A and L28I mutations. Whole-cell-based activity measurements showed that fNDT-i4 exhibited 4.0-fold and 20.6-fold higher activity at 40°C and 50°C, respectively, compared to the wild-type NDT. The detailed characterization of the purified enzymes conducted under various conditions, including temperature, pH, thermal stability, and enzyme kinetics experiments, showed that fNDT-i1 and fNDT-i4 exhibited 3.1- and 3.7-fold higher catalytic efficiency, respectively than wild-type NDT. The L59Q mutation was identified as a key factor in improving the thermal stability, whereas the V52A and L28I mutations were critical for improving substrate affinity and reaction efficiency. These findings provide the potential of fNDT-i1 and fNDT-i4 as highly efficient biocatalysts for developing industrially relevant nucleoside analog synthesis.