Expression of Cellulolytic Enzymes in Yeast

Abstract

Lignocellulose is the most abundantly sustainable raw material on the earth. To increase the utilization of sustainable lignocellulosic biomass, yeast, especially Saccharomyces cerevisiae, is used as a microbial cell factory to produce renewable chemicals like ethanol, butanol, or other fermentative products from lignocellulosic biomass. The cost of cellulolytic enzymes including cellulase and hemi cellulase is one of the limitations for industrial applications. Combining the cellulolytic enzyme expression and chemical production is a promising strategy to improve cost efficiency. Although cellulolytic enzyme-encoding genes have been expressed in yeast for enzyme characterization and protein engineering, co-expression of multiple cellulolytic enzyme genes is required to enable the non-cellulolytic yeast to utilize lignocellulose. Types of co-expressed systems include free cellulolytic enzyme, surface-displayed enzyme, or artificial minicellulosome. However, a lignocellulosic biomass utilizing strain with practical applications has been not constructed so far. The low efficiency of cellulolytic enzyme expression is the major roadblock hampering the generation of the desired strain. Although strong promoter, multiple gene copies, and codon optimization improve the gene expression, the protein misfolding, glycosylation, and vesicle transportation during secretion are more critical. Co-expression of multiple cellulolytic enzymes in an optimal ratio is also a challenging in cellulolytic yeast construction. Other than S. cerevisiae, nonconventional yeasts such as Kluyveromyces marxianus and Yarrowia lipolytica with special properties such as thermotolerance, xylose utilization, and lipid production are also good candidates for lignocellulosic biomass utilizing microbial cell factory construction.