Abstract
The baker’s yeast Saccharomyces cerevisiae is currently the dominant organism for industrial ethanol production due to its inherent general robustness and its long history of successful usage in the fermentation industry. It demonstrates a high rate of fermentation of hexose sugars, very good tolerance to ethanol and to inhibitors in lignocellulosic hydrolysates. On the other hand, baker’s yeast is unable to metabolize pentose sugars, particularly D-xylose, and L-arabinose, which account for more than one third of the total sugars in lignocellulosic biomass. As a result, it cannot be used for efficient lignocellulose based ethanol production. Great progress has been made to develop pentose-fermenting strains of S. cerevisiae through expression of two distinct heterologous pathways. The first pathway relies on expression of the fungal redox pathway that converts D-xylose or L-arabinose to D-xylulose. This approach suffers from the problem of cofactor imbalance, resulting in unnecessary byproduct formation and therefore lower ethanol yield. The second pathway utilizes xylose isomerase that directly isomerizes D-xylose to D-xylulose or a multistep bacterial pathway that converts L-arabinose to D-xylose-5-P. Expression of the latter pathways is proven superior due to higher ethanol yield per consumed sugar. However, the expression of a bacterial pathway especially into industrial yeast strains has been a challenge a.o. due to the lower activities of the heterologous enzymes in yeast. This challenge has been addressed using various strain engineering approaches, including inverse metabolic engineering and evolutionary engineering. No single strain development approach outshines alone. Thus, successful strain development strategies should encompass a combination of the different engineering strategies.
Keywords: Arabinose fermentation, arabinose uptake, bioethanol, evolutionary adaptation, genome shuffling, metabolic engineering, pentose phosphate pathway, redox balance, Saccharomyces cerevisiae, xylose fermentation, xylose uptake.
