Biochemical and structural studies of two tetrameric nucleoside 2'-deoxyribosyltransferases from psychrophilic and mesophilic bacteria: Insights into cold-adaptation

Molecular evolution of nucleoside deoxyribosyl transferase to enhance the activity toward 2'-fluoro-2'-deoxynucleoside

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- 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.

ONE-SENTENCE SUMMARY

The nucleoside deoxyribosyl transferase mutant were engineered to enhance biological activity and physical resistance for production of fluorinated deoxynucleoside as a raw material of oligonucleotide therapeutics.

Activation of Thoeris antiviral system via SIR2 effector filament assembly

To survive bacteriophage (phage) infections, bacteria developed numerous anti-phage defence systems1-7. Some of them (for example, type III CRISPR-Cas, CBASS, Pycsar and Thoeris) consist of two modules: a sensor responsible for infection recognition and an effector that stops viral replication by destroying key cellular components8-12. In the Thoeris system, a Toll/interleukin-1 receptor (TIR)-domain protein, ThsB, acts as a sensor that synthesizes an isomer of cyclic ADP ribose, 1''-3' glycocyclic ADP ribose (gcADPR), which is bound in the Smf/DprA-LOG (SLOG) domain of the ThsA effector and activates the silent information regulator 2 (SIR2)-domain-mediated hydrolysis of a key cell metabolite, NAD+ (refs. 12-14). Although the structure of ThsA has been solved15, the ThsA activation mechanism remained incompletely understood. Here we show that 1''-3' gcADPR, synthesized in vitro by the dimeric ThsB' protein, binds to the ThsA SLOG domain, thereby activating ThsA by triggering helical filament assembly of ThsA tetramers. The cryogenic electron microscopy (cryo-EM) structure of activated ThsA revealed that filament assembly stabilizes the active conformation of the ThsA SIR2 domain, enabling rapid NAD+ depletion. Furthermore, we demonstrate that filament formation enables a switch-like response of ThsA to the 1''-3' gcADPR signal.

Rational Design of a Thermostable 2'-Deoxyribosyltransferase for Nelarabine Production by Prediction of Disulfide Bond Engineering Sites

One of the major drawbacks of the industrial implementation of enzymatic processes is the low operational stability of the enzymes under tough industrial conditions. In this respect, the use of thermostable enzymes in the industry is gaining ground during the last decades. Herein, we report a structure-guided approach for the development of novel and thermostable 2′-deoxyribosyltransferases (NDTs) based on the computational design of disulfide bonds on hot spot positions. To this end, a small library of NDT variants from Lactobacillus delbrueckii (LdNDT) with introduced cysteine pairs was created. Among them, LdNDTS104C (100% retained activity) was chosen as the most thermostable variant, displaying a six- and two-fold enhanced long-term stability when stored at 55 °C (t1/255 °C ≈ 24 h) and 60 °C (t1/260 °C ≈ 4 h), respectively. Moreover, the biochemical characterization revealed that LdNDTS104C showed >60% relative activity across a broad range of temperature (30−90 °C) and pH (5−7). Finally, to study the potential application of LdNDTS104C as an industrial catalyst, the enzymatic synthesis of nelarabine was successfully carried out under different substrate conditions (1:1 and 3:1) at different reaction times. Under these experimental conditions, the production of nelarabine was increased up to 2.8-fold (72% conversion) compared with wild-type LdNDT.

Some Clues about Enzymes from Psychrophilic Microorganisms

Enzymes purified from psychrophilic microorganisms prove to be efficient catalysts at low temperatures and possess a great potential for biotechnological applications. The low-temperature catalytic activity has to come from specific structural fluctuations involving the active site region, however, the relationship between protein conformational stability and enzymatic activity is subtle. We provide a survey of the thermodynamic stability of globular proteins and their rationalization grounded in a theoretical approach devised by one of us. Furthermore, we provide a link between marginal conformational stability and protein flexibility grounded in the harmonic approximation of the vibrational degrees of freedom, emphasizing the occurrence of long-wavelength and excited vibrations in all globular proteins. Finally, we offer a close view of three enzymes: chloride-dependent α-amylase, citrate synthase, and β-galactosidase.