Correction: Li, F.-L. et al. Specific Immobilization of Escherichia coli Expressing Recombinant Glycerol Dehydrogenase on Mannose-Functionalized Magnetic Nanoparticles. Catalysts 2019, 9, 7

Immobilization of psychrophile Psychrobacter sp. ANT206 onto novel reusable magnetic nanoparticles and its application for nitro-aromatic compounds biodegradation under low temperature

Efficient biodegradation may offer a solution for the treatment of nitro-aromatic compounds (NACs) with toxicity, mutagenicity and persistence in the environment. In this study, dopamine (DA) functionalized magnetic nanoparticles with biocompatibility and hydrophilicity were synthesized and utilized for the immobilization of nitro-aromatic compounds degrading psychrophile Psychrobacter sp. ANT206 harboring the cold-adapted nitroreductase. The prepared nanocarriers were characterized using multiple methods. The highest immobilization yield of cells immobilized by Fe3O4@SiO2@DA was 90.67% under the optimized conditions of 10 °C, pH 7.5, 2 h and cell/support 1.2 mg/mg, and the activity recovery was 89.41%. In addition, the obtained immobilized cells displayed excellent salinity stability and reusability. Moreover, immobilized P. sp. ANT206 strains showed remarkable biodegradation capability on nitrobenzene and p-nitrophenol. This study introduced those novel Fe3O4@SiO2@DA nanoparticles could be applied as ideal and low-cost nanocarriers for the immobilization of cells and large-scale bioremediation of hazardous NACs with perspective applications under low temperature.

Optimal pH shift of the NADH oxidase from Lactobacillus rhamnosus with a single mutation

OBJECTIVE

To improve the activity of a water-forming NADH oxidase from Lactobacillus rhamnosus under neutral or alkaline pH for coupling NAD⁺-dependent dehydrogenases with an alkaline optimal pH.

RESULTS

The water-forming NADH oxidase from Lactobacillus rhamnosus was engineered by replacing the aspartic acid or glutamic acid with arginine on the surface. The mutant D251R improved the activity with a 112%, 111%, and 244% relative activity to the wild-type at pH 6.5, pH 7.0, and pH 7.5, respectively. Docking substrate into the D251R mutant reveals that the NADH is access to the substrate-binding site with a larger substrate loop due to the enhanced electrostatic repulsion between ARG-251 and ARG-243. In the D251R-NADH complex, the carboxyl of NADH additionally forms two hydrogen bonds (2.6 and 2.9 Å) with G154 due to the changed interaction of substrate and the residues in the catalytic sites, and the hydrogen bond with the oxygen of carbonyl in P295 is shortened from 2.9 to 2.0 Å, which could account for the enhanced specific activity.

CONCLUSIONS

The D251R mutant displayed higher catalytic activity than the wild-type in the pH range 6.5-7.5, and further insight into those shorter and newly formed hydrogen bonds in substrate docking analysis could account for the higher bind affinity and catalytic efficiency of D251R mutant.

Cloning, expression, and characterization of a novel heparinase I from Bacteroides eggerthii

Heparinase I (Hep I) specifically degrades heparin to oligosaccharide or unsaturated disaccharide and has been widely used in preparation of low molecular weight heparin (LMWH). In this work, a novel Hep I from Bacteroides eggerthii VPI T5-42B-1 was cloned and overexpressed in Escherichia coli BL21 (DE3). The enzyme has specific activity of 480 IU·mg^-1 at the optimal temperature and pH of 30 °C and pH 7.5, and the K_m and V_max were 3.6 mg·mL^-1 and 647.93 U·mg^-1, respectively. The Hep I has good stability with t_1/2 values of 350 and 60 min at 30 and 37 °C, respectively. And it showed a residual relative activity of 70.8% after 21 days incubation at 4 °C. Substrate docking study revealed that Lys99, Arg101, Gln241, Lys270, Asn275, and Lys292 were mainly involved in the substrate binding of Hep I. The shorter hydrogen bonds formed between heparin and these residues suggested the higher specific activity of BeHep I. And the minimum conformational entropy value of 756 J·K^-1 provides an evidence for the improved stability of this enzyme. This Hep I could be of interest in the industrial preparation of LMWH for its high specific activity and good stability.