354
14.3 Quorum Quenching Strategies
Some of the quorum quenching strategies are described below:
(a) Amide bond hydrolysis: AHL acylases are the enzymes required for the complete
and irreversible degradation of AHLs. They undergo hydrolysis and releases
homoserine lactone and the relevant fatty acid. This enzymatic activity was
described in both prokaryotes and eukaryotes. Leadbetter and Greenburgin
(2000) first demonstrated this type of enzymatic activity in Variovorax paradoxus (Leadbetter and Greenberg 2000; Uroz et al. 2009). Six genes that encode
AHL-acylases have been characterized and all of these AHL-acylases degrade
long chain AHLs more expeditiously than the short chain forms. Enzymes such
as Aac, AhIM, PvdQ, QuiP are unable to degrade AHL that has an acyl chain
composed of less than eight (8) carbons (Park et al. 2005; Uroz et al. 2009).
Porcine kidney acylase which was previously known for its ability to hydrolyze
a variety of N-acyl-L-amino acids has been shown to degrade AHLs with an acyl
chain that had four (4) to eight (8) carbons (Xu et al. 2003; Uroz et al. 2009).
(b) Lactone hydrolysis: In this case AHL lactonases induce the hydrolysis of the
homoserine lactone ring of the AHLs which in turn leads to the generation of acyl
homoserine. This reaction resembles the pH-mediated lactonolysis, and thus it
can be reversed by the acidification of the medium. Lactonase activities have
been identified in several bacterial genera and also in eukaryotic cells. AHLlactonase activity was first demonstrated in Bacillus sp. (Dong et al. 2001; Uroz
et al. 2009). This lactonase has been found to hydrolyze short and longer (C4- and
C14-HSL), with or without a substitution at the C3 position (e.g., Acidobacteria).
Crystal structure of AHL lactonase shows that the enzyme has two zinc (Zn
+
) ions
in its active site and thus is a metalloprotein. Those residues which are involved
in metal coordination directly are completely conserved in all AHL-lactonases.
The catalytic mechanism that occurs in AHL-lactonase, described as follows: It
starts with the attack of the substrate’s carbonyl carbon by a nucleophilic water/
hydroxide that is bridging the two (Zn
+
) ions. Second step involves the formation
of negatively charged intermediate. Finally an open ring product is formed by
breaking of C-O bond of lactone ring of AHL inactivation enzymes.
(c) Paraoxonase enzymes and Quorum Quenching: AHL inactivation by paraoxonase enzyme was performed in human epithelial cells. Later, this AHL inactivation ability was found to be widely conserved in the sera of 6 mammalian
species – human, bovine, sheep, horse, mouse, and rabbit. As far as the characteristics of these AHL inactivation enzymes are concerned they are dependent
on Ca
2+
ion and lactonase like activity. Physiologically important hydrolytic
activities are carried out by these paraoxonase enzymes (PONs). These paraoxonase enzymes include PON1, PON2, and PON3. These enzymes play a vital
role in drug metabolism and organophosphate detoxification. PON enzymes,
PON1 particularly, are known to catalyze the hydrolysis of many synthetic
chemicals which include organophosphate-based insecticides, nerve agents aromatic carboxylic acid esters, cyclic carbonate esters, aromatic lactones, and
alkyl lactone.
G. Seghal Kiran et al.
14.3 Quorum Quenching Strategies
Some of the quorum quenching strategies are described below:
(a) Amide bond hydrolysis: AHL acylases are the enzymes required for the complete
and irreversible degradation of AHLs. They undergo hydrolysis and releases
homoserine lactone and the relevant fatty acid. This enzymatic activity was
described in both prokaryotes and eukaryotes. Leadbetter and Greenburgin
(2000) first demonstrated this type of enzymatic activity in Variovorax paradoxus (Leadbetter and Greenberg 2000; Uroz et al. 2009). Six genes that encode
AHL-acylases have been characterized and all of these AHL-acylases degrade
long chain AHLs more expeditiously than the short chain forms. Enzymes such
as Aac, AhIM, PvdQ, QuiP are unable to degrade AHL that has an acyl chain
composed of less than eight (8) carbons (Park et al. 2005; Uroz et al. 2009).
Porcine kidney acylase which was previously known for its ability to hydrolyze
a variety of N-acyl-L-amino acids has been shown to degrade AHLs with an acyl
chain that had four (4) to eight (8) carbons (Xu et al. 2003; Uroz et al. 2009).
(b) Lactone hydrolysis: In this case AHL lactonases induce the hydrolysis of the
homoserine lactone ring of the AHLs which in turn leads to the generation of acyl
homoserine. This reaction resembles the pH-mediated lactonolysis, and thus it
can be reversed by the acidification of the medium. Lactonase activities have
been identified in several bacterial genera and also in eukaryotic cells. AHLlactonase activity was first demonstrated in Bacillus sp. (Dong et al. 2001; Uroz
et al. 2009). This lactonase has been found to hydrolyze short and longer (C4- and
C14-HSL), with or without a substitution at the C3 position (e.g., Acidobacteria).
Crystal structure of AHL lactonase shows that the enzyme has two zinc (Zn
+
) ions
in its active site and thus is a metalloprotein. Those residues which are involved
in metal coordination directly are completely conserved in all AHL-lactonases.
The catalytic mechanism that occurs in AHL-lactonase, described as follows: It
starts with the attack of the substrate’s carbonyl carbon by a nucleophilic water/
hydroxide that is bridging the two (Zn
+
) ions. Second step involves the formation
of negatively charged intermediate. Finally an open ring product is formed by
breaking of C-O bond of lactone ring of AHL inactivation enzymes.
(c) Paraoxonase enzymes and Quorum Quenching: AHL inactivation by paraoxonase enzyme was performed in human epithelial cells. Later, this AHL inactivation ability was found to be widely conserved in the sera of 6 mammalian
species – human, bovine, sheep, horse, mouse, and rabbit. As far as the characteristics of these AHL inactivation enzymes are concerned they are dependent
on Ca
2+
ion and lactonase like activity. Physiologically important hydrolytic
activities are carried out by these paraoxonase enzymes (PONs). These paraoxonase enzymes include PON1, PON2, and PON3. These enzymes play a vital
role in drug metabolism and organophosphate detoxification. PON enzymes,
PON1 particularly, are known to catalyze the hydrolysis of many synthetic
chemicals which include organophosphate-based insecticides, nerve agents aromatic carboxylic acid esters, cyclic carbonate esters, aromatic lactones, and
alkyl lactone.
G. Seghal Kiran et al.