Background3-Hydroxypropionic acid (3-HP) is an important platform chemical which can be produced biologically from glycerol. Klebsiella pneumoniae is an ideal biocatalyst for 3-HP because it can grow well on glycerol and naturally synthesize the essential coenzyme B12. On the other hand, if higher yields and titers of 3-HP are to be achieved, the sustained regeneration of NAD+ under anaerobic conditions, where coenzyme B12 is synthesized sustainably, is required.ResultsIn this study, recombinant K. pneumoniae L17 overexpressing aldehyde dehydrogenase (AldH) was developed and cultured in a bioelectrochemical system (BES) with the application of an electrical potential to the anode using a chronoamperometric method (+0.5 V vs. Ag/AgCl). The BES operation resulted in 1.7-fold enhancement of 3-HP production compared to the control without the applied potential. The intracellular NADH/NAD+ ratio was significantly lower when the L17 cells were grown under an electric potential. The interaction between the electrode and overexpressed AldH was enhanced by electron shuttling mediated by HNQ (2-hydroxy-1,4-naphthoquinone).ConclusionsEnhanced 3-HP production by the BES was achieved using recombinant K. pneumoniae L17. The quinone-based electron transference between the electrode and L17 was investigated by respiratory uncoupler experiments. This study provides a novel strategy to control the intracellular redox states to enhance the yield and titer of 3-HP production as well as other bioconversion processes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0886-x) contains supplementary material, which is available to authorized users.
BACKGROUND: The c-type cytochrome of the CymA of Shewanella oneidensis MR-1 is essential for the anaerobic respiration of Shewanella sp. and transfers electrons from the inner membrane to various terminal electron acceptors, such as soluble redox shuttles and insoluble metal oxides. CymA is believed to be a passage to the outer membrane for dissipating the respiratory electron to the carbon electrode in a microbial fuel cell (MFC) with simultaneous electricity generation. While the deletion and heterologous expression of cymA in Escherichia coli have been studied, there are no reports of the overexpression and its effects on the corresponding bioelectrochemical performance in a MFC.
RESULTS:The cymA gene was overexpressed in Shewanella oneidensis MR-1, and its upregulation was examined under aerobic, anaerobic, and MFC operating conditions by a reverse transcription-polymerase chain reaction (RT-PCR). Overexpression of the CymA protein was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The MFCs inoculated with the engineered strains of MR-1 achieved a higher maximum power of 0.13 mW and specific growth rate of 0.087 h −1 than those of the wild type MR-1 (0.11 mW and 0.043 h −1 , respectively). The higher electrochemical activity of the mutant strains demonstrated by cyclic voltammetry and linear sweep voltammetry, indicates that more respiratory electrons can be transferred to the electrodes through overexpression of the cymA gene of MR-1 in a MFC. CONCLUSION: Overexpression of CymA improves the bioelectrochemical performance of MFCs. This suggests that metabolic engineering of a membrane-associated redox protein, such as CymA, can further improve electricity generation of MFCs and produce an electrochemically enhanced bioprocess. Industry phosphate buffer (pH 7.0) and resuspended with the same buffer.The re-suspended cells were disrupted using a bead beater (Fastprep FP120, Fisher Scientific, Hampton, NH, USA) at a speed of 6.0 for 20 s per cycle for a total of 5 cycles. The disrupted cells
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