Genetically Modified Baker’s Yeast Saccharomyces cerevisiae in Chemical Synthesis and Biotransformations

Abstract

Chemical Biology 212 yields from lignocellulosic biomass from agricultural and agro-industrial residues are dependent upon efficient hydrolysis of sugar polymers and utilization of all the available sugars including D-glucose, D-xylose, L-arabinose and other fermentable compounds. S. cerevisiae, which plays a traditional and major role in industrial bioethanol production, has several advantages due to its high ethanol productivity as well as its high ethanol tolerance. However, baker's yeast cannot hydrolyze cellulose and is not able to use pentoses, which constitute up to 20% of lignocellulosic biomass. Many studies regarding the use of S. cerevisiae in metabolic engineering for xylose utilization have been reported and several reviews have been published [Chu & Lee, 2007; Hahn-Hägerdal et al., 2007; Matsushika et al., 2009]. The first step of D-xylose metabolism is its direct isomerization to D-xylulose catalyzed by bacterial xylose isomerase XR (EC 5.3.1.5) or stepwise transformation in yeast cells, firstly to xylitol (xylose reductase XR EC 1.1.1.21) and then to D-xylulose (xylitol dehydrogenase XDH EC 1.1.1.19). After phosphorylation of D-xylulose to D-xylulose-5phosphate (xylulokinase XK EC 2.7.1.17) further metabolizm proceeds via a pentose phosphate pathway. Different strategies have been applied to engineering yeast including the introduction of initial xylose metabolism and xylose transport. However, these change the intracellular redox balance and result in over-expression of xylulokinase and further metabolism via a pentose phosphate pathway. Nevertheless they are insufficient for industrial bioprocesses mainly due to a low rate of reaction as compared with glucose fermentation [Kondo et al. 2010; Young et al., 2010].

Yeast‐Mediated Stereoselective Synthesis

“…Major drawbacks are the rather messy and time-consuming work-up procedures (the product has to be separated from the biomass) and the isolation and purification of the desired product. In specific and isolated cases, reaction times may be long; this problem might be solved by using genetically modified yeasts or recombinant yeast enzymes [44][45][46][47][48][49][50][51][52]. For special systems even the use of a continuous solid-gas bioreactor has been suggested [53].…”