Biobutanol produced in acetone-butanol-ethanol fermentation at batch mode cannot compete with chemically derived butanol because of the low reactor productivity. Continuous fermentation can dramatically enhance productivity and lower capital and operating costs but are rarely used in industrial fermentation because of increased risks in culture degeneration, cell washout, and contamination. In this study, cells of the asporogenous Clostridium acetobutylicum ATCC55025 were immobilized in a single-pass fibrous-bed bioreactor (FBB) for continuous production of butanol from glucose and butyrate at various dilution rates. Butyric acid in the feed medium helped maintaining cells in the solventogenic phase for stable continuous butanol production. At the dilution rate of 1.88 h -1, butanol was produced at 9.55 g/L with a yield of 0.24 g/g and productivity of 16.8 g/L∙h, which was the highest ever achieved for biobutanol fermentation and an 80-fold improvement over the conventional ABE fermentation. The extremely high productivity was attributed to the high density of viable cells (~100 g/L at >70% viability) immobilized in the fibrous matrix, which also enabled the cells to better tolerate butanol and butyric acid. The FBB was stable for continuous operation for an extended period of over one month.

Inducible lysine decarboxylases (LDCs) are essential in various cellular processes of microorganisms and plants, especially under acid stress, which induces expression of genes encoding LDCs. In this study, a novel Serratia marcesenes LDC (SmcadA) was successfully expressed in E. coli and structural modeling showed the protein as a decamer with a molecular weight of approximately 75 kDa. Further structural analysis of the protein network interactions showed that specific N-terminal and C-terminal residues with significant node size-degree correlation and high color intensity possessed high values of betweenness, suggesting they played a key role in protein function and activity. Molecular dynamics simulations further revealed that hydrogen bonds were vital interactions for stabilization of cofactor PLP and formation of intermediates, hence improved biocatalysis. Furthermore, mutations coffered structural changes of the protein residues and PLP hence altered the type of interacting residues with the ligand, causing changes in conformation of PLP. Moreover, temperature also induced changes in orientation of cofactor PLP and amino acid residues. This work therefore demonstrates the role of protein-ligand interactions in altering cofactor and binding site residue conformations thus contributing to improved biocatalysis.