title: Overcoming inefficient cellobiose fermentation by cellobiose phosphorylase in the presence of xylose.
abstract: BACKGROUND
  Cellobiose and xylose co-fermentation holds promise for efficiently producing biofuels from plant biomass. Cellobiose phosphorylase (CBP), an intracellular enzyme generally found in anaerobic bacteria, cleaves cellobiose to glucose and glucose-1-phosphate, providing energetic advantages under the anaerobic conditions required for large-scale biofuel production. However, the efficiency of CBP to cleave cellobiose in the presence of xylose is unknown. This study investigated the effect of xylose on anaerobic CBP-mediated cellobiose fermentation by Saccharomyces cerevisiae.
  
  
  RESULTS
  Yeast capable of fermenting cellobiose by the CBP pathway consumed cellobiose and produced ethanol at rates 61% and 42% slower, respectively, in the presence of xylose than in its absence. The system generated significant amounts of the byproduct 4-O-β-d-glucopyranosyl-d-xylose (GX), produced by CBP from glucose-1-phosphate and xylose. In vitro competition assays identified xylose as a mixed-inhibitor for cellobiose phosphorylase activity. The negative effects of xylose were effectively relieved by efficient cellobiose and xylose co-utilization. GX was also shown to be a substrate for cleavage by an intracellular β-glucosidase.
  
  
  CONCLUSIONS
  Xylose exerted negative impacts on CBP-mediated cellobiose fermentation by acting as a substrate for GX byproduct formation and a mixed-inhibitor for cellobiose phosphorylase activity. Future efforts will require efficient xylose utilization, GX cleavage by a β-glucosidase, and/or a CBP with improved substrate specificity to overcome the negative impacts of xylose on CBP in cellobiose and xylose co-fermentation.
authors.affiliation: Department of Plant and Microbial Biology, University of California, Berkeley, CA, USA.
authors.forename: Kulika
authors.initials: K
authors.lastname: Chomvong
authors.affiliation: Department of Chemistry, University of California, Berkeley, CA, USA.
authors.forename: Vesna
authors.initials: V
authors.lastname: Kordić
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, CA, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Li
authors.affiliation: Energy Biosciences Institute, University of California, Berkeley, CA, USA.
authors.forename: Stefan
authors.initials: S
authors.lastname: Bauer
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, CA, USA.
authors.forename: Abigail E
authors.initials: AE
authors.lastname: Gillespie
authors.affiliation: Department of Food Science and Human Nutrition, University of Illinois, Urbana, IL, USA ; Institute for Genomic Biology, University of Illinois, Urbana, IL, USA ; Department of Bioengineering and Technology, Kangwon National University, Chuncheon, Republic of Korea.
authors.forename: Suk-Jin
authors.initials: SJ
authors.lastname: Ha
authors.affiliation: Department of Food Science and Human Nutrition, University of Illinois, Urbana, IL, USA ; Institute for Genomic Biology, University of Illinois, Urbana, IL, USA.
authors.forename: Eun Joong
authors.initials: EJ
authors.lastname: Oh
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, CA, USA.
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Galazka
authors.affiliation: Department of Food Science and Human Nutrition, University of Illinois, Urbana, IL, USA ; Institute for Genomic Biology, University of Illinois, Urbana, IL, USA.
authors.forename: Yong-Su
authors.initials: YS
authors.lastname: Jin
authors.affiliation: Department of Chemistry, University of California, Berkeley, CA, USA ; Department of Molecular and Cell Biology, University of California, Berkeley, CA, USA ; Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors.forename: Jamie H D
authors.initials: JH
authors.lastname: Cate
keywords: Cellobiose; Cellobiose phosphorylase; Glucopyranosyl-xylose; Inhibition; Xylose