Multiple reuses of Rhodococcus ruber TH3 free cells to produce acrylamide in a membrane dispersion microreactor

N-terminal loops at the tetramer interface of nitrile hydratase act as "hooks" determining resistance to high amide concentrations

Nitrile hydratase (NHase) has been extensively utilized in industrial acrylamide production. However, the vulnerability to high concentrations of acrylamide limits its further application. Herein, we redesigned the N-terminal loop at the tetramer interface of a thermophilic NHase from Pseudonocardia thermophila JCM3095 (PtNHase), and its catalytic activity, resistance to high acrylamide concentrations, and thermostability were improved. Amino acid residues located in the N-terminal loop of the tetramer interface that are responsible for enhancing the resistance to high acrylamide concentrations were identified via static structural analysis and molecular dynamics simulations. A variant library was used to fine-tune the tetramer interface. Variant αL6T exhibited 3.5-fold greater resistance to 50% (v/v) acrylamide, whereas its activity was 1.2-fold higher than that of the wild-type (WT) enzyme, revealing no activity-stability trade-off. Compared to the use of Escherichia coli harboring the WT enzyme, the use of E. coli harboring αL6T increased the acrylamide concentration from 398.1 g/L to 500 g/L. Crystal structure-guided analysis of αL6T and molecular dynamics simulations revealed that increased enzyme surface hydration and the introduction of positive cross-correlation into the N-terminal loop of the tetramer interface caused the two loop regions to hook to each other, thus improving the resistance to high acrylamide concentrations.

Improving stress tolerance and cell integrity of Rhodococcus ruber by overexpressing small-shock-protein Hsp16 of Rhodococcus

Rhodococcus species have been successfully used as cell catalysts for valuable chemicals production due to their well-characterized resistance to harmful factors. An understanding of how they respond to stress is of great interest, which will enable the identification of engineering strategies for further improving their resistance and maintaining cell integrity and viability. Here, we assessed the transcriptome response of R. ruber TH3 to heat shock. Approximately, 376 genes were up-regulated in heat-shocked TH3. Among all the up-regulated functional genes, the small heat-shock-protein (Hsp16) with maximal enhanced transcript (463-fold) was identified, and its function was investigated. Results showed that overexpressed Hsp16 has no significant promotive effect on stress tolerance of in-cell enzyme. Interestingly, compared to the control TH3, a little fewer pores and folds on the surface of TH3(Hsp16) and more intact TH3(Hsp-GFP) cells under AM treatment were observed by SEM and LCSM, respectively. Moreover, survival test showed that more (about 501–700) TH3(Hsp16) colonies were observed while only 1–100 TH3 colonies after 50% AM treatment, and this trend is also found in high-temperature cultivation experiments. These results indicate that Hsp16 does great contributions to preventing cell leakage, maintaining cell integrity and viability of R. ruber under stress conditions.