Microbial esterases play important roles in deep-sea organic carbon degradation and cycling. Though sharing similar catalytic triads and oxyanion holes, esterases are hydrolases, and homoserine transacetylases (HTAs) are transferases. Because two HTA homologs were identified as acetyl esterases, the HTA family was divided into the bonafide acetyltransferase subfamily and the acetyl esterase subfamily recently. Here, we identified and characterized a novel HTA-like esterase, Est22, from a deep-sea sedimentary metagenomic library. Est22 could efficiently hydrolyze esters with acyl length of up to six carbon atoms, but had no transacetylase activity, which is different from HTAs and HTA-like acetyl esterases. Phylogenetic analysis also showed that Est22 and its homologs form a separate branch in the HTA family. We solved the structures of Est22 and its mutant L374D and modeled the structure of L374D with p-nitrophenyl butyrate. Based on structural, mutational and biochemical analyses, Phe71 and Met176 in the oxyanion hole and Arg294 are revealed to be the key substrate-binding residues. A detailed structural comparison indicated that differences in their catalytic tunnels lead to the different substrate specificities of Est22 and the other two HTA subfamilies. Biochemical and sequence analyses suggested that Est22 homologs may share the same substrate recognition and catalysis mechanism with Est22. Due to the significant differences in sequence, structure and substrate specificity between Est22 (and its homologs) and the other two HTA subfamilies, we suggest that Est22 and its homologs represent a new subfamily in the HTA family.IMPORTANCE Microbial esterases play important roles in the turnover of organic carbon in deep sea. Esterases and homoserine transacetylases (HTAs) represent two groups of alpha/beta hydrolases. Esterases catalyze the hydrolysis of simple esters and are widely used in pharmaceutical and agrochemical industries, while HTAs catalyze the transfer of acetyl group from acetyl-CoA to homoserine and are essential for microbial growth. Here, we reported a novel HTA-like esterase, Est22, from a deep-sea sediment. Because of the significant differences in sequence, structure and substrate specificity from HTAs and HTA-like acetyl esterases, Est22 together with its homologs represent a new subfamily in the HTA family. This study offers new knowledge on marine esterases.
Li PY, Yao QQ, Wang P, Zhang Y, Li Y, Zhang YQ, Hao J, Zhou BC, Chen XL, Shi M, Zhang YZ, Zhang XY (2017) A Novel Subfamily Esterase with a Homoserine Transacetylase-like Fold but no Transferase Activity Applied Environmental Microbiology83: e00131""
Li PY, Yao QQ, Wang P, Zhang Y, Li Y, Zhang YQ, Hao J, Zhou BC, Chen XL, Shi M, Zhang YZ, Zhang XY (2017) Applied Environmental Microbiology83: e00131""