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.
