Characterization of a tannin acyl hydrolase from Streptomyces sviceus with substrate preference for digalloyl ester bonds.

The search for new tannases with novel enzymatic properties suitable for industrial applications has been a continuous effort since the first discovery of the enzyme more than a century ago. A tannase gene (Ss-Tan) from the Gram-positive bacterium Streptomyces sviceus was identified, chemically synthesized, and cloned into a C-terminal His-tagged vector for expression in Escherichia coli. The tannase possesses the active site motif of GXSXG that is conserved for serine hydrolases. The residues that constitute the catalytic triad and galloyl binding site in bacterial tannases are found conserved in Ss-Tan, which include Ser209, Asp452, His484 and Lys370, Glu384, Asp454, respectively. Ss-Tan was overexpressed in E. coli BL21-AI cells with high productivity. Enzymatic assay revealed that the enzyme displays tannase activities to hydrolyze both the ester bonds and depside bonds in hydrolyzable tannins. Kinetic analysis indicated that the enzyme preferentially acts on depside bonds with considerably higher substrate affinity and catalytic efficiency. The enzyme showed maximum activity around pH 8.0 and at 50 °C with the highest melting temperature close to 70 °C. The high depsidase activity and thermostablility of Ss-Tan may make the enzyme suitable for potential industrial applications to achieve complete digestion of hydrolyzable tannins.