Enhanced biotransformations and product recovery in a membrane bioreactor through application of a direct electric current

Improvement of multi-catalytic properties of nitrilase from Paraburkholderia graminis for efficient biosynthesis of 2-chloronicotinic acid

Nitrilase-catalyzed hydrolysis of nitriles is the promising approach for green and efficient biosynthesis of high value-added carboxylic acids. However, undesirable catalytic efficiency toward non-natural substrates restricts their wide-spread applications. Until now, very few robust nitrilases have been reported for 2-chloronicotinic acid (2-CA) production since the enzymes always show low activity and sometimes with poor reaction specificity. Herein, a nitrilase from Paraburkholderia graminis (PgNIT) was engineered to improve its catalytic properties. We identified the beneficial residues via computational analysis and constructed the mutant library. A series positive mutants were obtained and the "best" mutant F164G/I130L/N167Y/A55S exhibited 6000-folds higher catalytic efficiency to 2-chloronicotinonitrile (2-CN). Its reaction specificity was improved with elimination of hydration activity and meanwhile, the half-lives (t_1/2 ) against different temperatures were increased. Molecular docking and molecular dynamics simulation revealed that the steric hindrance, conformational flexibility, as well as nucleophilic attack distance between the enzyme and substrate were the main reason alternating the catalytic properties of PgNIT. With the mutant as biocatalyst, a product yield of 130 g/L 2-CA was produced from 2-CN after 60 h, laying the foundation for constructing the nitrilase-catalyzed route of 2-CA. This article is protected by copyright. All rights reserved.

Electro-extractive fermentation for efficient biohydrogen production

Electrodialysis, an electrochemical membrane technique, was found to prolong and enhance the production of biohydrogen and purified organic acids via the anaerobic fermentation of glucose by Escherichia coli. Through the design of a model electrodialysis medium using cationic buffer, pH was precisely controlled electrokinetically, i.e. by the regulated extraction of acidic products with coulombic efficiencies of organic acid recovery in the range 50-70% maintained over continuous 30-day experiments. Contrary to previous reports, E. coli produced H(2) after aerobic growth in minimal medium without inducers and with a mixture of organic acids dominated by butyrate. The selective separation of organic acids from fermentation provides a potential nitrogen-free carbon source for further biohydrogen production in a parallel photofermentation. A parallel study incorporated this fermentation system into an integrated biohydrogen refinery (IBR) for the conversion of organic waste to hydrogen and energy.

Responses of soil microbial communities to weak electric fields

Electrokinetically stimulated bioremediation of soils (electro-bioremediation) requires that the application of weak electric fields has no negative effect on the contaminant degrading microbial communities. This study evaluated the hypothesis that weak direct electric current (DC) fields per se do not negatively influence the physiology and composition of soil microbial communities given that secondary electrokinetic phenomena such as soil pH changes and temperatures are minimized. Mildly buffered, water-saturated laboratory mesocosms with agricultural soil were subjected for 34 days to a constant electric field (X=1.4 V cm(-1); J approximately 1.0 mA cm(-2)) and the spatiotemporal changes of soil microbial communities assessed by fingerprints of phospholipids fatty acids (PLFA) and terminal restriction fragment length polymorphisms (T-RFLP) of bacterial 16S rRNA genes. DC-induced electrolysis of the pore water led to pH changes (<1.5 pH units) in the immediate vicinity of the electrodes and concomitant distinct soil microbial community changes. By contrast, DC-treated bulk soil distant to the electrodes showed no pH changes and developed similar PLFA- and T-RFLP-fingerprints as control soil in the absence of DC. Our data suggest that the presence of an electric field, if suitably applied, will not influence the composition and physiology of soil microbial communities and hence not affect their potential to biodegrade contaminants.

The effect of whole cell immobilisation on the biotransformation of benzonitrile and the use of direct electric current for enhanced product removal

The nitrilase of Rhodococcus rhodochrous performs a one-step biotransformation of nitriles to their corresponding carboxylic acids. Application of a direct electric current moves the charged carboxylic acid towards an anode, across an anion exchange membrane, into a separate compartment. Cells encapsulated within alginate beads (2.9 mm diameter) for protection against the current biotransformed benzonitrile to benzoic acid with a 26% reduction in the biotransformation rate, from 0.054 mmol/min/g dcw with free cells to 0.040 mmol/min/g dcw with immobilised cells. When the electric current was applied, the biotransformation rate increased to 0.047 mmol/min/g dcw and product recovery increased from 19% to 79%.