Cellulosic Ethanol Production Using a Dual Functional Novel Yeast.

Z. Lewis Liu,Bruce S. Dien

doi:10.1155/2022/7853935

Abstract

Reducing the cost of cellulosic ethanol production, especially for cellulose hydrolytic enzymes, is vital to growing a sustainable and efficient cellulosic ethanol industry and bio-based economy. Using an ethanologenic yeast able to produce hydrolytic enzymes, such as Clavispora NRRL Y-50464, is one solution. NRRL Y-50464 is fast-growing and robust, and tolerates inhibitory compounds 2-furaldehyde (furfural) and 5-hydroxymethyl-2-furaldehyde (HMF) associated with lignocellulose-to-fuel conversion. It produces three forms of β-glucosidase isozymes, BGL1, BGL2, and BGL3, and ferment cellobiose as the sole carbon source. These β-glucosidases exhibited desirable enzyme kinetic parameters and high levels of enzyme-specific activity toward cellobiose and many oligosaccharide substrates. They tolerate the product inhibition of glucose and ethanol, and are stable to temperature and pH conditions. These characteristics are desirable for more efficient cellulosic ethanol production by simultaneous saccharification and fermentation. NRRL Y-50464 provided the highest cellulosic ethanol titers and conversion rates at lower cellulase loadings, using either pure cellulose or agricultural residues, as so far reported in the literature. This review summarizes NRRL Y-50464 performance on cellulosic ethanol production from refined cellulose, rice straw, and corn stover processed in various ways, in the presence or absence of furfural and HMF. This dual functional yeast has potential to serve as a prototype for the development of next-generation biocatalysts. Perspectives on continued strain development and process engineering improvements for more efficient cellulosic ethanol production from lignocellulosic materials are also discussed.

Highlights

Renewable cellulosic ethanol as an advanced biofuel is an attractive alternative for transportation use to reduce fossil fuel consumption and a cleaner environment
In a conventional simultaneous saccharification and fermentation (SSF) process for cellulosic ethanol production, cellulase hydrolyzes cellulose into oligoglucans and cellobiose, and additional β-glucosidase converts cellobiose into glucose for yeast fermentation
TM fermented purified cellulose substrate Avicel or SigmaCell fermented well in SSFs under the following conditions: a freshly prepared overnight culture of NRRL Y-50464 cells was added at a ratio of 60 mg/mL wet weight to make a total volume of 25 mL for fermentation

Summary

Introduction

Renewable cellulosic ethanol as an advanced biofuel is an attractive alternative for transportation use to reduce fossil fuel consumption and a cleaner environment. Since most ethanol-fermenting microbes do not synthesize cellulose hydrolytic enzymes, engineering efforts have been made to enable ethanologenic yeast to produce β-glucosidase [8–15] These strains have insufficient β-glucosidase activity for efficient cellulosic ethanol production by SSF. Strain NRRL Y-50464 has the potential to lower the cost of producing cellulosic ethanol because it is a fast-growing yeast, produces sufficient native β-glucosidase enzyme activity for SSF, and can utilize cellobiose as a sole carbon source to produce ethanol. E adapted strain of Clavispora NRRL Y-50464 is a fastgrowing yeast with a growth rate that surpasses Saccharomyces cerevisiae on glucose It can utilize cellobiose as sole source of carbon and produces sufficient native β-glucosidase activity for ethanol production by SSF [17, 18]. Strain NRRL Y-50464 grows vigorously at 37°C, which is a suitable temperature for ethanol production using SSF

Basic Assessment

Pure Cellulose versus Corn Stover

Standard Corn Stover versus Delignified Corn Stover

Industrial Processed Corncob Residue

DOE-NREL-Pretreated Corn Stover

Conventional Corn Stover

Rice Straw

Summary of Cellulosic Conversion Performance

Expression

Characterization of BGLs from NRRL Y-50464

Substrate Specificity

Perspectives