Optimizing CBP yeasts for lignocellulose conversion

The biological conversion of cellulose to ethanol is initiated by the enzymatic conversion of the substrate to soluble sugars that can be assimilated by a fermentative organism, adding additional costs to the process.  One strategy for developing a microorganism capable of consolidated bioprocessing of cellulosic feedstocks to commodity products involves engineering the yeast Saccharomyces cerevisiae so that it expresses a heterologous enzyme system enabling crystalline cellulose utilization. 

We have illustrated proof of concept in the laboratory through the successful expression of the three major cellulase activities (β‑glucosidase, endoglucanase and cellobiohydrolase) in S. cerevisiae laboratory strains but secreted titers were low in some cases.  We have illustrated that the production of different cellulases by S. cerevisiae can be enhanced by overexpressing genes encoding native target proteins involved in the secretion pathway or stress relief response in yeast.

Optimum hydrolysis of cellulose at minimum enzyme loading is achieved by the synergistic actions of endoglucanases, exoglucanases and β-glucosidases.  We have used purified cellulases produced heterologously in the yeast Saccharomyces cerevisiae to evaluate different mixtures of the enzymes at a range of loadings to determine the optimum levels of synergy.  A mathematical model was generated to simulate the interactions of these enzymes on crystalline cellulose, both under isolated and synergistic conditions.  The optimum synergy between recombinant enzyme cellulases produced in S. cerevisiae and accurate simulation with a mathematical model as tool for future CBP yeast development will be presented.