Identification of a novel xanthan-binding module of a multi-modular Cohnella sp. xanthanase

A new strain of xanthan-degrading bacteria identified as Cohnella sp. has been isolated from a xanthan thickener for food production. The strain was able to utilize xanthan as the only carbon source and reduce the viscosity of the xanthan-containing medium during cultivation. Xanthan hydrolytic activity was revealed by congo red staining after growth of the strain on agar plates containing xanthan. By analyzing the secretome of Cohnella sp. after growth on different media (lysogeny broth, glucose mineral medium, xanthan mineral medium), a xanthanase designated as CspXan9 was found and its gene was successfully expressed in Escherichia coli Rosetta2. CspXan9 could efficiently degrade the -1,4-glucan backbone of xanthan after previous removal of pyruvylated mannose residues from the ends of the native xanthan side chains by xanthan lyase treatment (XLT-xanthan). Compared with a known xanthanase from Paenibacillus nanensis, the modular xanthanase CspXan9 had a different module composition at the N- and C-terminal ends. High-performance anion-exchange chromatography (HPAEC-PAD) analysis revealed that the main putative end products released from XLT-xanthan by CspXan9 hydrolysis were tetrasaccharides. Deletion derivatives lacking some of the non-catalytic domains (CspXan9-C, CspXan9-N, CspXan9-C-N) were produced in E. coli to explore the functions of the N- and C-terminal regions of the enzyme. Enzyme assays with the purified deletion derivatives, which all contained the catalytic glycoside hydrolase family 9 (GH9) module, resulted in a range of specific activities on XLT-xanthan between 10.31 ± 0.29 U/mg (CspXan9-C) and 1.38 ± 0.05 U/mg (CspXan9-C-N). Mobility shift assays performed by native affinity polyacrylamide gel electrophoresis (NAPAGE) in the presence of different polysaccharides indicated that the C-terminal module of CspXan9 represents a novel carbohydrate-binding module of CBM66 with binding affinity for XLT-xanthan. The only previously known binding function of a member of the CBM66 family is exo-type binding to the non-reducing fructose ends of the -fructan polysaccharides inulin and levan.