Z.L. Liu, S.A. Weber
Keywords: beta-glucosidase, cellobiose, cellulosic ethanol
Summary:For ethanol production from cellulosic materials, there are generally two major steps needed including enzymatic hydrolysis to break down biomass sugars and microbial fermentation to convert available simple sugars into ethanol. It often requires two different kinds of microorganisms since ethanologenic microbes usually do not producing cellulolytic enzymes while the enzyme producing microorganisms are commonly unable to produce ethanol. Genetic engineering efforts for improved enzyme production of ethanologenic agent have been taken but outcomes are yet unsatisfactory. High enzyme cost remains as a significant challenge for sustainable cellulosic ethanol production. ARS scientists discovered and developed a new yeast strain Clavispora NRRL Y-50464 that is able to produce sufficient innate β-glucosidase to digest cellobiose and produce ethanol. Beta-glucosidase is a key enzyme component in cellulase for decomposition of lignocellulosic biomass materials. Elimination of the addition of external β-glucosidase would reduce the enzyme cost for cellulosic ethanol production using simulteneous saccharification and fermentation (SSF). We isolated and characterized three forms of β-glucosidase, BGL1, BGL2, and BGL3, from Clavispora NRRL Y-50464 that confirmed its dual function of cellobiose digest and ethanol production capability. We demonstrated its highest conversion rate of 0.088 g/L/h with an ethanol production of 32 g/L ethanol from 20% solids loading of corn stover in 48 h, applying cellulase alone without addition of β-glucosidase by SSF. Using a pure commercial available cellulose, it produced 40.44 g/L ethanol within 72 h that reached an industrial standard. The desirable characteristics of this US patented yeast strain including tolerance to inhibitors, fast growth rate, β-glucosidase producing capability, and quick ethanol conversion rate, made it a strong candidate for lower-cost cellulosic ethanol production using SSF.