Abstract
An extreme halophilic xylanase, designated as XylCMS, was characterized by cloning and expression of the encoding gene from a camel rumen metagenome. XylCMS proved to be a GH11 xylanase with high identity to a hypothetical glycosyl hydrolase from Ruminococcus flavefaciens. XylCMS with a molecular weight of about 47 kDa showed maximum activity at pH 6 and 55 °C. The enzyme activity was significantly stimulated by NaCl in 1–5 M concentrations. Interestingly, the optimum temperature was not influenced by NaCl but the Kcat of the enzyme was enhanced by 2.7-folds at 37 °C and 1.2-folds at 55 °C. The Km value was decreased with NaCl by 4.3-folds at 37 °C and 3.7-folds at 55 °C resulting in a significant increase in catalytic efficiency (Kcat/Km) by 11.5-folds at 37 °C and 4.4-folds at 55 °C. Thermodynamic analysis indicated that the activation energy (Ea) and enthalpy (∆H) of the reaction were decreased with NaCl by 2.4 and threefold, respectively. From the observations and the results of fluorescence spectroscopy, it was concluded that NaCl at high concentrations improves both the flexibility and substrate affinity of XylCMS that are crucial for catalytic activity by influencing substrate binding, product release and the energy barriers of the reaction. XylCMS as an extreme halophilic xylanase with stimulated activity in artificial seawater and low water activity conditions has potentials for application in industrial biotechnology.
Highlights
Xylanase is an important enzyme in the degradation of plant biomass by hydrolysing xylan as a main constituent of the plant cell wall
The extreme halophilic xylanase was tolerant of salt and its activity was stimulated in high salt concentrations
Bioinformatic analysis A homology search using BlastP program at the National Center for Biotechnology Information (NCBI) database revealed that XylCMS exhibited 99% identity with a hypothetical protein of glycosyl hydrolase family 11 from Ruminococcus flavefaciens
Summary
Xylanase is an important enzyme in the degradation of plant biomass by hydrolysing xylan as a main constituent of the plant cell wall. Xylanase has application in various industries where plant biomass hydrolysis is required for the production of human food, animal feed, paper, pulp, and biofuels (Adigüzel and Tunçer 2016; Kaur et al 2016; Kumar et al 2016; Yegin et al 2018). Halophilic xylanase can be suitable for applications in low water activity conditions such as food industries and biorefineries. It is estimated that 1.9–5.9 m3 water is required for the production of 1 m3 biofuel from plant biomass (Fang et al 2015). Extreme halophilic xylanase, designated as XylCMS, was discovered by cloning and recombinant expression of the encoding gene from a camel rumen metagenome, in Escherichia coli. The halophilic xylanase may be interesting due to its potential application in high salt conditions
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