High‐resolution structures of AidH complexes provide insights into a novel catalytic mechanism for N ‐acyl homoserine lactonase

Many pathogenic bacteria that infect humans, animals and plants rely on a quorum‐sensing (QS) system to produce virulence factors. N ‐Acyl homoserine lactones (AHLs) are the best‐characterized cell–cell communication signals in QS. The concentration of AHL plays a key role in regulating the virulence‐gene expression and essential biological functions of pathogenic bacteria. N ‐Acyl homoserine lactonases (AHL‐lactonases) have important functions in decreasing pathogenicity by degrading AHLs. Here, structures of the AHL‐lactonase from Ochrobactrum sp. (AidH) in complex with N ‐hexanoyl homoserine lactone, N ‐hexanoyl homoserine and N ‐butanoyl homoserine are reported. The high‐resolution structures together with biochemical analyses reveal convincing details of AHL degradation. No metal ion is bound in the active site, which is different from other AHL‐lactonases, which have a dual Lewis acid catalysis mechanism. AidH contains a substrate‐binding tunnel between the core domain and the cap domain. The conformation of the tunnel entrance varies with the AHL acyl‐chain length, which contributes to the binding promiscuity of AHL molecules in the active site. It also supports the biochemical result that AidH is a broad catalytic spectrum AHL‐lactonase. Taken together, the present results reveal the catalytic mechanism of the metal‐independent AHL‐lactonase, which is a typical acid–base covalent catalysis.