Bioconversion of lignocellulose-derived sugars to ethanol by engineeredSaccharomyces cerevisiae

Abstract

Lignocellulosic biomass from agricultural and agro-industrial residues represents one of the most important renewable resources that can be utilized for the biological production of ethanol. The yeast Saccharomyces cerevisiae is widely used for the commercial production of bioethanol from sucrose or starch-derived glucose. While glucose and other hexose sugars like galactose and mannose can be fermented to ethanol by S. cerevisiae, the major pentose sugars D-xylose and L-arabinose remain unutilized. Nevertheless, D-xylulose, the keto isomer of xylose, can be fermented slowly by the yeast and thus, the incorporation of functional routes for the conversion of xylose and arabinose to xylulose or xylulose-5-phosphate in Saccharomyces cerevisiae can help to improve the ethanol productivity and make the fermentation process more cost-effective. Other crucial bottlenecks in pentose fermentation include low activity of the pentose phosphate pathway enzymes and competitive inhibition of xylose and arabinose transport into the cell cytoplasm by glucose and other hexose sugars. Along with a brief introduction of the pretreatment of lignocellulose and detoxification of the hydrolysate, this review provides an updated overview of (a) the key steps involved in the uptake and metabolism of the hexose sugars: glucose, galactose, and mannose, together with the pentose sugars: xylose and arabinose, (b) various factors that play a major role in the efficient fermentation of pentose sugars along with hexose sugars, and (c) the approaches used to overcome the metabolic constraints in the production of bioethanol from lignocellulose-derived sugars by developing recombinant S. cerevisiae strains.

Heterologous expression of cellulase genes in natural Saccharomyces cerevisiae strains

“…In this context, the ideal host for CBP would hydrolyse lignocellulose, hemicellulose and cellulose and utilise all available sugars to produce ethanol, or a similar valuable product, at high rates and titers (159). Hexose sugars such as glucose, mannose, and galactose can be readily fermented to ethanol by most yeast species, however, only a small number of yeast species that can naturally metabolise xylose or arabinose have been identified (163). Examples of xylose and arabinose -utilising species include Candida spp.…”

“…and isolates of Pichia spp. and several genes required for uptake and metabolism have been characterised in these strains (163). Despite their potential for diverse sugar metabolism, these species have not advanced for development as cell factories for bioethanol production because of low ethanol yields (163).…”

Conversion of biomass platform molecules into fuel additives and liquid hydrocarbon fuels

“…However nowadays, an important effort is being done in order to obtain bioethanol from sugars coming from nonedible lingnocellulosic biomass. [8][9][10] In the last years the consumption of these biofuels has increased exponentially 11 due mainly to their production is based on simple and well-known technologies and their partial compatibility with existing transportation infrastructure of gasoline and diesel. However these biofuels possess several drawbacks that limit their use as transportation fuels.…”

Combined production of biogas and ethanol at high solids loading from wheat straw impregnated with acetic acid: experimental study and techno-economic evaluation

“…However, wild-type S. cerevisiae strains are unable to metabolize pentoses (C5 sugars), rendering the utilization of pentoses a significant challenge in biomass-to-ethanol conversion. Considerable research has been devoted to developing genetically modified organisms that co-ferment C6 sugars (hexoses) and C5 sugars to ethanol at high yields [2][3][4][5][6], which has led to commercial-scale co-fermentation being implemented at Poet-DSM's cellulosic ethanol plant in Emmetsburg, Iowa, and the Beta Renewables plant in Crescentino.…”

“…In most of these studies, sulfur dioxide or sulfuric acid were used as acid catalysts in the steam pretreatment. The main drawback of using sulfur-containing compounds is that they impede the downstream process and must be removed or recycled as acid or SO 2 .…”