Ethanolic fermentation of pentoses in lignocellulose hydrolysates

Abstract

In the fermentation of lignocellulose hydrolysates to ethanol, two major problems are encountered: the fermentation of the pentose sugar xylose, and the presence of microbial inhibitors. Xylose can be directly fermented with yeasts, such as Pachysolen tannophilus, Candida shehatae, and Pichia stipis, or by isomerization of xylose to xylulose with the enzyme glucose (xylose) isomerase (XI; EC 5.3.1.5), and subsequent fermentation with bakers' yeast, Saccharomyces cerevisiae. The direct fermentation requires low, carefully controlled oxygenation, as well as the removal of inhibitors. Also, the xylose-fermenting yeasts have a limited ethanol tolerance. The combined isomerization and fermentation with XI and S. cerevisiae gives yields and productivities comparable to those obtained in hexose fermentations without oxygenation and removal of inhibitors. However, the enzyme is not very stable in a lignocellulose hydrolysate, and S. cerevisiae has a poorly developed pentose phosphate shunt. Different strategies involving strain adaptation, and protein and genetic engineering adopted to overcome these different obstacles, are discussed.

Alcoholic fermentation of carbon sources in biomass hydrolysates by Saccharomyces cerevisiae: current status

“…The nature and availability of lignocellulosic feedstocks in different parts of the world depends on climate and other environmental factors, agricultural practice and technological development (Claassen et al 1999). Lignocellulose consists of an intermeshed and chemically bonded complex of three main polymers (Table 1): cellulose, hemicellulose, lignin and depending on the feedstock, pectin (Hahn-Hä gerdal et al 1991;Ingram et al 1999;Perez et al 2002;Zaldivar et al 2001). …”

“…Lignin, constituting 10-20% of biomass dry weight (Table 1), is an aromatic polymer containing phenolic residues such as trans-qcoumaryl alcohol, trans-q-coniferyl alcohol and trans-q-sinapyl alcohol (Hahn-Hä gerdal et al 1991;Ingram et al 1999;Klinke et al 2004;Perez et al 2002;Zaldivar et al 2001). While the lignin fraction does not contribute fermentable carbon sources, it is relevant as a potential source of microbial inhibitors (see below).…”

Functional Survey for Heterologous Sugar Transport Proteins, Using Saccharomyces cerevisiae as a Host

“…These pentose sugars cannot be readily metabolized by nonrecombinant versions of common fermentative host organisms such as the bacterium Zymomonas mobilis or the yeast Saccharomyces cerevisiae. While native pentose-utilizing organisms exist, a lack of well-developed genetic tools and low product tolerances (13) limit their utility as hosts for industrial scale lignocellulosic conversion processes. As a result, a significant effort has focused on the metabolic engineering of pentose catabolic pathways in the yeast S. cerevisiae to enable xylose and arabinose fermentation (12,18).…”

Shuffling of Promoters for Multiple Genes To Optimize Xylose Fermentation in an Engineered Saccharomyces cerevisiae Strain

“…In the present study, we developed a multiple-gene-promoter-shuffling (MGPS) method that can achieve optimal levels of overexpression for several genes at a time. This technique improved xylose fermentation in recombinant S. cerevisiae (11).…”