Metabolic Engineering of Saccharomyces cerevisiae for Vitamin B5 Production

Engineered E. coli for D-pantothenic acid production with an acetolactate isomeroreductase mutant

D-Pantothenic acid, as a momentous vitamin, is extensively applied to feed, medicine, cosmetics and other fields. However, there are still limitations to produce D-pantothenic acid by microbial fermentation at present. In this paper, we constructed a recombinant strain for D-pantothenic acid production by blocking the organic acid pathway, boosting pyruvate biosynthesis, relieving feedback inhibition of acetolactate synthase, improving glucose intake capacity, and modifying essential genes in the metabolic pathway. In addition, a new acetolactate isomeroreductase mutant V412A origin from Escherichia coli (EcAHAIR) encoded by ilvC was obtained to explore its substrate promiscuity. Compared with the wild type, the variant EcAHAIR-V412A has reduced steric hindrance and enhanced intermolecular forces, resulting in a high affinity for 2-acetolactate. Eventually, the fermentation production of the final strain DPAN19/trc-ilvCV412A reached 4.65 g/L, increased by 192.5% compared with strain DPA8 in shake flask cultivation and produced 62.82 g/L D-pantothenic acid in a 5 L bioreactor. The metabolic engineering strategies and enzyme modification approaches described in this paper provide a particular perspective for the bio-manufacturing of D-pantothenic acid, branched-chain amino acids and its derivates.

Combining Metabolic Engineering and Lipid Droplet Assembly to Achieve Campesterol Overproduction in Saccharomyces cerevisiae

Campesterol is a kind of important functional food additive. Therefore, stable and efficient campesterol biosynthesis is significant. Herein, we first knocked out the sterol 22-desaturase gene in Saccharomyces cerevisiae and expressed sterol Δ7-reductase from Pangasianodon hypophthalmus, obtaining a strain that produced 6.6 mg/L campesterol. Then, the modular expression of campesterol synthesis enzymes was performed, and a campesterol titer of 88.3 mg/L was achieved. Because campesterol is a lipid-soluble macromolecule, we promoted lipid droplet formation by exploring regulatory factors, and campesterol production was improved to 169.20 mg/L. Next, triacylglycerol lipase was used to achieve compartment campesterol synthesis. After enhancing the expression of sterol Δ7-reductase and screening cations, the campesterol titer reached 438.28 mg/L in a shake flask and 1.44 g/L in a 5 L bioreactor, which represents the highest campesterol titer reported to date. Metabolic regulation combined with lipid droplet engineering may be useful for the synthesis of other steroids as well.

Discovery and Engineering of a Novel Bacterial L-Aspartate α-Decarboxylase for Efficient Bioconversion

L-aspartate α-decarboxylase (ADC) is a pyruvoyl-dependent decarboxylase that catalyzes the conversion of L-aspartate to β-alanine in the pantothenate pathway. The enzyme has been extensively used in the biosynthesis of β-alanine and D-pantothenic acid. However, the broad application of ADCs is hindered by low specific activity. To address this issue, we explored 412 sequences and discovered a novel ADC from Corynebacterium jeikeium (CjADC). CjADC exhibited specific activity of 10.7 U/mg and Km of 3.6 mM, which were better than the commonly used ADC from Bacillus subtilis. CjADC was then engineered leveraging structure-guided evolution and generated a mutant, C26V/I88M/Y90F/R3V. The specific activity of the mutant is 28.8 U/mg, which is the highest among the unknown ADCs. Furthermore, the mutant displayed lower Km than the wild-type enzyme. Moreover, we revealed that the introduced mutations increased the structural stability of the mutant by promoting the frequency of hydrogen-bond formation and creating a more hydrophobic region around the active center, thereby facilitating the binding of L-aspartate to the active center and stabilizing the substrate orientation. Finally, the whole-cell bioconversion showed that C26V/I88M/Y90F/R3V completely transformed 1-molar L-aspartate in 12 h and produced 88.6 g/L β-alanine. Our study not only identified a high-performance ADC but also established a research framework for rapidly screening novel enzymes using a protein database.