N-Acyl homoserine lactones (Abbreviated as AHLs or N-AHLs) are a class of signaling molecules involved in bacterial quorum sensing, a means of communication between bacteria enabling behaviors based on population density. The first AHL (N-3-(oxo-hexanoyl)-homoserine lactone) was found as the natural inducer of bioluminescence in the bacterium Vibrio fischeri. Quorum sensing by the means of AHLs contributes to regulate the transcription of specific genes and therefore expression of specific phenotypes, including growth, virulence, biofilm formation, bioluminescence, production of exopolysaccharide (EPS). Over 50 gram-negative bacteria species (including several pathogenic species) use AHLs as autoinducers and the means of their communication in quorum sensing. In one study, AHL was shown to interact with eukaryotic cells, and mitigate an immune response and facilitates infection. AHLs are one of the major groups of the autoinducer (AI) molecules which are found primarily in gram-negative proteobacteria but also in some Bacteroidetes, Cyanobacteria, and archaea. The other two major groups are oligopeptides AIs in gram-positive bacteria; and autoinducer-2 (AI-2), as a universal signal for interspecies communications.
Formation It arises by the reaction of acyl carrier proteins react with S-adenosylmethionine. The latter donates the equivalent of α-amino-γ-butyrolactone. Methylthioadenosine is a coproduct. Homoserine lactones are also a product of the proteolytic reaction of cyanogen bromide (CNBr) with a methionine residue. This reaction is important for chemical sequencing of proteins.
Structure AHLs have hydrophobic and hydrophilic sections. The hydrophilic section consists of the homoserine lactone ring and the amide group. The hydrophobic section has a strain-specific hydrocarbon chain with varieties in length and level of oxygenation with a 3-oxo group. The length of the acyl chain generally ranges from 4 to 18 carbons. The length of R-group side-chain variable. Chain lengths vary from 4 to 18 carbon atoms and in the substitution of a carbonyl at the third carbon. The hydrophilic sections form a hydrogen bonded network within the receptor binding site, while the hydrophobic sites contribute to diffusional and binding properties within the hydrophobic pocket. Studies have not yet demonstrated a correlation between the AHL synthase enzymes and AHL type. LuxI protein synthesizes an acylated homoserin-lactone molecule. The LuxI gene is highly conserved, which indicates that although diverse, there are a limited number of AHL-type signals that are produced by bacteria. However, in the AHL synthase enzyme family, the C-terminal region, which determines the type of substrates the synthetase can recognize and the subsequent acyl-chain length, is not conserved. Moreover, there is no evidence as of now that the distribution of AHL synthase and the species are correlated. Contrary to LuxI genes, the receptors of AHLs, LuxR protein and their genes, are highly variable among species.
Signalling
Mechanism Bacterial quorum signaling begins with N-AHL secretion into the environment. In the process of quorum sensing, first the LuxI protein synthesizes an acylated homoserine lactone molecule which can pass through cell membrane along the gradient through diffusion to the environmental space. When the concentration of these autoinducers in the environment is lower than inside the cell, they will move down the gradient and will leave the cell, therefore, they will not attach to their receptor, LuxR, which is in the cytoplasm. When the population of bacteria reaches a threshold, and the concentration of the autoinducers in the environment is higher than inside the cell, they will move along the gradient into the cell and will attach to the receptor. Thus, the LuxR-AHL complex will be formed. This complex will bind to a 20 base pair (bp) section of DNA, called the lux box. This region is in or near the lux promoter region, which is located ~40 bp upstream of the regulated gene. Because LuxR is bound to the promoter, RNA polymerase is recruited to this promoter region and the gene expression is induced. Moreover, the LuxR-AHL complex will upregulate luxI transcription, which will increase the production of AHLs (positive feedback loop). The transcription of the target genes will be regulated, as gene expression of the microbial population will be coordinated. Several studies have been investigating on the potential AHLs effective in infection and resistance to antibiotics. The LuxR–LuxI system mediated by AHLs is the best screened QS system in multi-drug resistant bacteria species.
Quorum Quenching As opposed to quorum sensing, quorum quenching, prevents bacterial communication and influences their gene expression. Targets of the quorum quenching are the signal molecules, the biosynthetic machinery of signal molecules, and the regulatory proteins that perceive these signal molecules with the AHL degradation via AHL degrading enzymes and limiting signal accumulation being the main mechanism. The AHLs are degraded by enzymes through three mechanisms: lactone hydrolysis, amide bond hydrolysis, and acyl chain modification. Lactone hydrolysis occurs when AHL lactonase hydrolyzes homoserine lactone rings. This process was first observed in Bacillus species. AHL acylases catalyze the complete and irreversible destruction of AHLs through the hydrolysis of amide bonds. AHL oxidase and reductase, first discovered in Rhodococcus erythropolis, catalyze a change in the chemical structure of signals, which affects AHL signal recognition and interferes with quorum sensing regulated processes. The second AHLase is a Bacillus megaterium P450 monooxygenase that oxidizes fatty acids and N-fatty acyl amino acids. Lactonases and acylases are the two pioneers of quorum quenching mechanisms. Lactonases break down the lactone bonds in autoinducers, making them unable to bind to target transcriptional regulators and thereby increasing disease resistance.
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