An extreme halophilic xylanase from camel rumen metagenome with elevated catalytic activity in high salt concentrations

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 47kDa showed maximum activity at pH 6 and 55 degrees C. The enzyme activity was significantly stimulated by NaCl in 1-5M concentrations. Interestingly, the optimum temperature was not influenced by NaCl but the K-cat of the enzyme was enhanced by 2.7-folds at 37 degrees C and 1.2-folds at 55 degrees C. The K-m value was decreased with NaCl by 4.3-folds at 37 degrees C and 3.7-folds at 55 degrees C resulting in a significant increase in catalytic efficiency (K-cat/K-m) by 11.5-folds at 37 degrees C and 4.4-folds at 55 degrees C. Thermodynamic analysis indicated that the activation energy (E-a) 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.