Abstract
The aim of the current study was to optimize the production of xylanase, and its application for ethanol production using the lignocellulosic biomass. A highly thermostable crude xylanase was obtained from the Geobacillus sp. strain DUSELR13 isolated from the deep biosphere of Homestake gold mine, Lead, SD. Geobacillus sp. strain DUSELR13 produced 6 U/mL of the xylanase with the beechwood xylan. The xylanase production was improved following the optimization studies, with one factor at a time approach, from 6 U/mL to 19.8 U/mL with xylan. The statistical optimization with response surface methodology further increased the production to 31 U/mL. The characterization studies revealed that the crude xylanase complex had an optimum pH of 7.0, with a broad pH range of 5.0–9.0, and an optimum temperature of 75 °C. The ~45 kDa xylanase protein was highly thermostable with t1/2 of 48, 38, and 13 days at 50, 60, and 70 °C, respectively. The xylanase activity increased with the addition of Cu+2, Zn+2, K+, and Fe+2 at 1 mM concentration, and Ca+2, Zn+2, Mg+2, and Na+ at 10 mM concentration. The comparative analysis of the crude xylanase against its commercial counterpart Novozymes Cellic HTec and Dupont, Accellerase XY, showed that it performed better at higher temperature, hydrolyzing 65.4% of the beechwood at 75 °C. The DUSEL R13 showed the mettle to hydrolyze, and utilize the pretreated, and untreated lignocellulosic biomass: prairie cord grass (PCG), and corn stover (CS) as the substrate, and gave a maximum yield of 20.5 U/mL with the untreated PCG. When grown in co-culture with Geobacillus thermoglucosidasius, it produced 3.53 and 3.72 g/L ethanol, respectively with PCG, and CS. With these characteristics the xylanase under study could be an industrial success for the high temperature bioprocesses.
Highlights
The environmental, and economic incentives associated with the utilization of lignocellulosic biomass at industrial scale drive the quest for finding new microorganisms capable of producing enzymes with better characteristics
The preliminary experiments showed that Geobacillus sp. strain DUSELR13 was capable of producing xylanase
Geobacillus sp. strain DUSELR13 had been previously known for producing thermostable cellulase [24], and this study presents the production of a thermostable endoxylanase by this organism
Summary
The environmental, and economic incentives associated with the utilization of lignocellulosic biomass at industrial scale drive the quest for finding new microorganisms capable of producing enzymes with better characteristics. The abundant, economic, and ubiquitously available lignocellulosic biomass (LCB), composed of cellulose and hemicellulose can act as an ideal substrate for the industrial bioprocesses. The complex chemical composition of xylan requires a concerted action of several enzymes, collectively known as hemicellulases, including endo-β-D-xylanases, β-D-xylosidases, α-L-arabinofuranosidases, α-D-glucuronidases, acetylxylan esterases, ferulic and p-coumaric acid esterases. These enzymes act synergistically on the linear chain as well as on the side chains giving xylose and xylooligosaccharides (XOS) as the end products [4,5]. Among the various enzymes required for the effective hydrolysis of xylan component of biomass, endo-1,4-β-xylanase ( referred to as xylanase), is an essential enzyme that acts on the xylan backbone of the hemicellulose with high specificity, negligible substrate loss, and side products [4] compared to the prevalent chemical methods of hydrolysis
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