The Impact of Self-Cycling Fermentation on the Production of Shikimic Acid in Populations of Engineered Saccharomyces cerevisiae

Abstract

Shikimic acid is an intermediary metabolite in the biosynthesis of aromatic amino acids in plants and microorganisms. It is also a precursor for the production of specialty chemical ingredients used in pharmaceutical, cosmetic, and healthcare products. However, the current commercial method of obtaining shikimic acid is a tedious and expensive process. The present study focuses on using Saccharomyces cerevisiae engineered to overproduce shikimic acid under a self-cycling fermentation (SCF) process to increase the yield, productivity, and specific productivity of this compound. SCF is an automated, unsteady state, semi-continuous mode of operation which has been shown to increase the specific productivity of biomolecules and induce synchrony in microbial populations. The engineered S. cerevisiae was grown in batch cultures to quantify the increase in production of shikimic acid as compared to the unmodified strain. Shikimic acid titers between 0.08 to 0.13 g/L were obtained during glucose metabolism. Subsequently, the engineered yeast strain was characterized in a 1-L batch fermenter for maximum shikimic acid production in preparation for SCF operation. It was determined that the most suitable time point at which cycling should be initiated is shortly after the depletion of glucose. In addition, the coupled readings of carbon dioxide evolution rate (CER) and its first derivative (dCER) proved to be an appropriate feedback control parameter to trigger cycling and to achieve stable SCF operation. SCF was then implemented and a shikimic acid titer of 0.39 g/L was obtained after 23 cycles. This is a three-fold increase as compared to titers obtained in shake flask cultures during glucose metabolism and a four-fold increase as compared to the titer obtained after the first cycle. Moreover, the yield and after 21 cycles was 0.022 mol/mol-glucose. Furthermore, the integrated specific productivity after 23 cycles was 2.4 ×10^-15 mol/(cell∙h). This is a four-fold increase as compared to values obtained in batch studies. It is important to note that the use of the SCF system greatly improved the yield and specific productivity of shikimic acid in the engineered yeast strain. This shows great promise for an environmentally friendly alternative for the sustainable manufacture of this chemical