Biochemical routes for uptake and conversion of xylose by microorganisms

Abstract

Xylose is a major component of lignocellulose and the second most abundant sugar present in nature. Efficient utilization of xylose is required for the development of economically viable processes to produce biofuels and chemicals from biomass. However, there are still some bottlenecks in the bioconversion of xylose, including the fact that some microorganisms cannot assimilate xylose naturally and that the uptake and metabolism of xylose are inhibited by glucose, which is usually present with xylose in lignocellulose hydrolysate. To overcome these issues, numerous efforts have been made to discover, characterize, and engineer the transporters and enzymes involved in xylose utilization to relieve glucose inhibition and to develop recombinant microorganisms to produce fuels and chemicals from xylose. Here we describe a recent advancement focusing on xylose-utilizing pathways, biosynthesis of chemicals from xylose, and engineering strategies used to improve the conversion efficiency of xylose.

Keywords: Carbon catabolite repression; Chemicals produced from xylose; Escherichia coli; Lignocellulose; Saccharomyces cerevisiae; Xylose; Xylose catabolic pathways; Xylose transporter.

Fig. 1

The natural metabolic pathways of xylose in various microorganisms. PK phosphoketolase, XI xylose isomerase, XR xylose reductase, XDH xylitol dehydrogenase

Fig. 2

Biosynthetic pathways of chemicals from xylose. DHAP, dihydroxyacetone phosphate; EG, ethylene glycol; KDX, 2-keto-3-deoxy-d-xylonate; DHB, d-3,4-dihydroxybutanal; BT, d-1,2,4-butanetriol; HBA, 4-hydroxybutyraldehyde; BDO, 1,4-butanediol. a d-xylose dehydrogenase, XlyB from C. crescentus; b xylonolactonase, XylC from C. Crescentus; c d-xylonate dehydratase, YjhG/YagF from E. coli, XylD from C. crescentus; d decarboxylase, KivD from Lactococcus lactis, MdlC from Pseudomonas putida; e alcohol dehydrogenase, YqhD/AdhP from E. coli, ADH2 from S. cerevisiae; f diol dehydratase, engineered PpdA-C-B fusion from Klebsiella oxytoca; g alcohol dehydrogenase, YqhD from E. coli; h 2-keto-3-deoxy-d-pentanoate aldolase, YjhH/YagE from E. coli; i d-xylose isomerase, XylA from E. coli; j d-xylulose-1-kinase, Khk-C from human; k d-tagatose 3-epimerase, DTE from Pseudomonas cichorii; l d-xylulose-1-phosphate aldolase, Aldo-B from human; m l-fuculokinase, FucK from E. coli; n l-fuculose-1-phosphate aldolase, FucA from E. coli; o aldehyde dehydrogenase, AldA from E. coli; p alcohol dehydrogenase, FucO/YqhD from E. coli