The QS system is a cell-cell communication mechanism in bacteria that is used to synthesize, secrete and detect small signal molecules in order to perceive the population density and regulate the expression of specific genes in response to a changing environment. Bacteria use the QS system to regulate gene expression and thus a diverse array of physiological activities. Many pathogenic bacteria that infect humans, animals and plants rely on the 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 AHL. Structural study of N-Acyl homoserine lactonases will help us better understand its AHL degradingmechanism.
Professor LIANG Dongcai is a renowned structural biologist. He works at the Institute of Biophysics, Chinese Academy of Sciences, a leading biological institution in China. Professor LIANG’s group recently resolved the high-resolution structrues ofN-acyl homoserine lactonase complexes . This is the first structural study of a metal-independent AHL-lactonase.
In this study, LIANG’s group resolved the structures of the AHL-lactonase from Ochrobactrum sp. (AidH) in complex with N-hexanoyl homoserine lactone, N-hexanoyl homoserine and N-butanoyl homoserine. The high-resolution structures together with biochemical analyses reveal convincing details of AHL degradation. Accurate electron-density maps indicate no metal ion is bound in the active site, which is different from other AHL-lactonases, which have a dual Lewis acid catalysis mechanism. In addition, AidH has no acyl-chain length or C3 substituent preference, it is a broad catalytic spectrum AHL-lactonase. Moreover, AidH contains an approximately 14 A ° long narrow tunnel that connects the bulk solvent and the active site through which the substrates access and bind to the active site. The size of the entrance varies with different substrate acyl-chain lengths. This shows that the tunnel participates in all aspects of substrate selection, substrate binding, catalysis and product release, and plays an important role in the enzymatic reaction. Taken together, these results reveal the novel catalytic mechanism of the metal-independent AHL-lactonase, which is a typical acid–base covalent catalysis.
This study will help us to thoroughly understand the novel catalytic mechanism of AHL degradation and should be important in developing therapeutic strategies for the control and prevention of infectious bacterial diseases.
This work was published in the Acta Crystallogr D Biol Crystallogr (http://journals.iucr.org/d/issues/2013/01/00/dw5029/index.html). It was supported by grants from the Ministry of Science and Technology of China, the National Natural Science Foundation of China and the Chinese Academy of Sciences.
Figure. Substrate-binding tunnel of AidH and the conformational change of the tunnel entrance. (a) Slicedsurface view of the substrate-binding tunnel. The tunnel lying between the core and cap domains is lined by hydrophobic residues and has an overall positive charge. (b) The entrance of the tunnel is located on the cap domain and surrounded by hydrophobic residues. In (a) and (b), the molecular surface is coloured according to the electrostatic potential. Positive and negative potentials are shown in blue and red, respectively. (c) The distance between Phe189 and Phe192 changes upon substrate/product binding. In panel 1, free AidH, C4-bonded AidH and C6-bonded AidHS102G (and AidHE219G) are shown in purple, cyan and yellow (and green), respectively. Panel 2 shows that there is no distance change in free-form AidHS102G (orange) and AidHE219G (grey).
