A cold-adapted carbohydrate esterase, CEST, belonging to the carbohydrate esterase family 6, was cloned from Microbulbifer thermotolerans DAU221. CEST was composed of 307 amino acids with the first 22 serving as a secretion signal peptide. The calculated molecular mass and isoelectric point of the mature enzyme were 31,244 Da and pH 5.89, respectively. The catalytic triad consisted of residues Ser37, Glu192, and His281 in the conserved regions: GQSNMXG, QGEX(D/N), and DXXH. The

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... structure of CEST revealed that CEST belongs to the α/β-class of protein consisted of a central six-stranded β-sheet flanked by eight α-helices. The recombinant CEST was purified by His-tag affinity chromatography and the characterization showed its optimal temperature and pH were 15°C and 8.0, respectively. Specifically, CEST maintained up to 70% of its enzyme activity when preincubated at 50°C or 60°C for 6 h, and 89% of its enzyme activity when preincubated at 70°C for 1 h. The results suggest CEST belongs to group 3 of the cold-adapted enzymes. The enzyme activity was increased by Na + and Mg 2+ ions but was strongly inhibited by Cu + and Hg 2+ ions, at all ion concentrations. Using p-nitrophenyl acetate as a substrate, the enzyme had a K m of 0.278 mM and a k cat of 1.9 s -1 . Site-directed mutagenesis indicated that the catalytic triad (Ser37, Glu192, and His281) and Asp278 were essential for the enzyme activity.