Functional expression of cellobiohydrolases in Saccharomyces cerevisiae towards one-step conversion of cellulose to ethanol

To investigate the possible use of Saccharomyces cerevisiae as a candidate for consolidated bioprocessing (CBP) of cellulose to ethanol, four fungal cellobiohydrolase (CBH) encoding genes (Trichoderma reesei cbh1 and cbh2, Aspergillus niger cbhB and Phanerochaete chrysosporium cbh1–4) were expressed in this yeast. All four CBHs were successfully expressed and similar extracellular activity was demonstrated on phosphoric acid swollen cellulose (PASC) and bacterial microcrystalline cellulose (BMCC) using a modified affinity digestion procedure. Our results suggest that although heterologous CBHs can be produced in S. cerevisiae the titers of functionally secreted CBH are relatively low. However, the specific activity of recombinant CBH1 on PASC and BMCC, determined by a sensitive ELISA-based technique, was found not to differ significantly from that of the native T. reesei enzyme. Given this similarity in specific activity, but the disparity between current levels of CBH expression relative to expression levels required to enable growth on crystalline cellulosic substrates with concomitant ethanol production, future studies should aim to increase the expression levels of CBHs.