Isolation and Characterization of the Stress-Tolerant Candida tropicalis YHJ1 and Evaluation of Its Xylose Reductase for Xylitol Production From Acid Pre-treatment Wastewater.

Seonghun Kim,Bong Hyun Sung,Jinhyuk Lee

doi:10.3389/fbioe.2019.00138

Abstract

A stress-tolerant yeast was isolated from honey using acid hydrolysate generated from sequential acid-/alkali-pretreatment of empty palm fruit bunch fiber (EPFBF). The isolated yeast was identified molecularly, taxonomically, and morphologically as Candida tropicalis YHJ1, and analyzed for application in xylitol production. The isolated yeast showed stress tolerance toward various chemical reagents and could grow with up to 600 g/L xylose in the culture medium. This yeast also had a broad carbohydrate utilization spectrum, and its xylitol yield was greatest in medium supplemented with xylose as the sole carbon source. In batch fermentation for xylitol production, the yeast could convert xylose prepared from acidic EPFBF pretreatment wastewater into xylitol. Interestingly, C. tropicalis YHJ1 xylose reductase, containing a Ser279 residue, exhibited more effective xylitol conversion compared to orthologous Candida enzymes containing Leu279 or Asn279; this improvement was associated with NADPH binding, as predicted through homologous structure modeling and enzyme kinetic analysis. Taken together, these results show a novel stress-tolerant yeast strain that may be applicable to xylitol production from toxic lignocellulosic byproducts.

Highlights

Lignocellulosic biomass is a renewable resource for production of biofuels and chemical monomers through the biorefinery process (Brethauer and Studer, 2015)
Honey was sampled with a loop and inoculated through streaking onto minimal medium supplemented with neutralized empty palm fruit bunch fiber (EPFBF) acidic hydrolysate solution (Kim, 2019) as the sole carbon source
The xylose-fermenting yeast C. tropicalis YHJ1 was isolated from honey and molecularly identified based on phylogenetic, taxonomic, and morphological analyses

Summary

Introduction

Lignocellulosic biomass is a renewable resource for production of biofuels and chemical monomers through the biorefinery process (Brethauer and Studer, 2015). Such biomass consists primarily of three polymeric materials, cellulose, hemicellulose, and lignin. These main components interact strongly with each other, and disturbance of their rigid structures is needed to break down the polymeric substances to sugar monomers, oligomers, or other aromatic derivatives (Brethauer and Studer, 2015). Different pretreatment procedures are employed for various types of lignocellulosic biomass, which may influence the enzyme conversion efficiency and productivity of fermentation (Alvira et al, 2010; Silveira et al, 2015; Capolupo and Faraco, 2016)

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Discussion

Conclusion