[Regulation of intracellular level of ATP and NADH in Escherichia coli to promote succinic acid production].

Succinic acid is an important C4 platform chemical that is widely used in food, chemical, medicine sectors. The bottleneck of fermentative production of succinic acid by engineered Escherichia coli is the imbalance of intracellular cofactors, which often leads to accumulation of by-products, lower yield and low productivity. Stoichiometric analysis indicated that an efficient production of succinic acid by E. coli FMME-N-26 under micro-aeration conditions might be achieved when the TCA cycle provides enough ATP and NADH for the r-TCA pathway. In order to promote succinic acid production, a serial of metabolic engineering strategies include reducing ATP consumption, strengthening ATP synthesis, blocking NADH competitive pathway and constructing NADH complementary pathway were developed. As result, an engineered E. coli FW-17 capable of producing 139.52 g/L succinic acid and 1.40 g/L acetic acid in 5 L fermenter, which were 17.81% higher and 67.59% lower than that of the control strain, was developed. Further scale-up experiments were carried out in a 1 000 L fermenter, and the titer of succinic acid and acetic acid were 140.2 g/L and 1.38 g/L, respectively.

Functional characterization, structural basis, and regio-selectivity control of a promiscuous flavonoid 7,4'-di-O-glycosyltransferase from Ziziphus jujuba var. spinosa.

A highly efficient and promiscuous 7,4'-di-O-glycosyltransferase ZjOGT3 was discovered from the medicinal plant Ziziphus jujuba var. spinosa. ZjOGT3 could sequentially catalyse 4'- and 7-O-glycosylation of flavones to produce 7,4'-di-O-glycosides with obvious regio-selectivity. For 7,4'-dihydroxyl flavanones and 3-O-glycosylated 7,4'-dihydroxyl flavones, ZjOGT3 selectively catalyses 7-O-glycosylation. The crystal structure of ZjOGT3 was solved. Structural analysis, DFT calculations, MD simulations, and site-directed mutagenesis reveal that the regio-selectivity is mainly controlled by the enzyme microenvironment for 7,4'-dihydroxyl flavones and 3-O-glycosylated 7,4'-dihydroxyl flavones. For 7,4'-dihydroxyl flavanones, the selectivity is mainly controlled by intrinsic reactivity. ZjOGT3 is the first plant flavonoid 7,4'-di-O-glycosyltransferase with a crystal structure. This work could help understand the catalytic mechanisms of multi-site glycosyltransferases and provides an efficient approach to synthesise O-glycosides with medicinal potential.