Covalent Inhibition of Bacterial Quorum Sensing
A R T I C L E S
Figure 1. Bacterial autoinducers and inhibitors. (A) Examples of bacterial autoinducers belonging to distinct structural classes. (B) Examples of synthetic
QS inhibitors in P. aeruginosa (I1-I5). Approximate IC50 values (from different reporter assays) are listed below the compounds. Efficacies of I1 and I2
were reported by Smith et al.,11,12 I3 and I5 by Geske et al.,13 and I4 by Hentzer et al.14
shown that inhibition of even a single QS regulator can lead to
a significant decrease in overall biofilm formation.
P. aeruginosa,12,14,18,21,22 although the number of strong
inhibitors resulting from such efforts remains low. Examples
of moderately potent inhibitors, with their IC50 values, are
shown in Figure 1b.
The primary QS system in P. aeruginosa is regulated through
the synthesis and secretion of 3-oxo-C12-HSL, which, upon
reaching a threshold concentration, binds the transcriptional
activator LasR. This interaction has been proposed to lead to
correct folding, followed by dimerization and binding of the
LasR dimer to its target DNA, resulting in gene expression. In
addition, several other small molecules have been found to play
a role in the regulation of gene expression (e.g., C4-HSL, PQS),
although the signaling events initiated by 3-oxo-C12-HSL
recognition appear to be at the top of the QS hierarchy.15
Because of its medical importance, QS in P. aeruginosa has
been extensively studied. One notable breakthrough in this field
came with the determination of the crystal structure of LasR
bound to its natural ligand (3-oxo-C12-HSL), recently reported
by Bottomley et al.16
One obstacle in the design of more potent P. aeruginosa QS
inhibitors is the apparently high affinity and specificity with
which LasR binds the primary QS molecule, 3-oxo-C12-HSL.
Consequently, small changes in the structure of the ligand can
lead to a large loss in affinity. A potential solution to this
problem lies in the design of structures that present only a
minimal deviation from the parent autoinducer and contain a
small reactive moiety that can covalently bind a residue in the
LasR binding pocket. Such covalent probes would be expected
to compete effectively with 3-oxo-C12-HSL for binding to LasR,
such that their slightly altered occupation of the binding pocket
upon conjugation would likely result in a conformational change
that is less than optimal for effective binding of the transcrip-
tional activator to its target DNA. Use of this type of probe
could also severely affect the regulation and recycling of both
LasR and 3-oxo-C12-HSL. Covalent probes have been widely
used to target, identify, and investigate proteins, enzymes, and
receptors.23 In recent years, the field of activity-based protein
Interfering with QS signaling has been explored in recent
years as a novel approach to combat pathogenesis, with
strategies ranging from receptor antagonism,4,17,18 signaling
molecule binding and degradation,19 to inhibition of signal-
ing molecule biosynthesis.20 Several groups have identi-
fied compounds showing significant inhibition of QS in
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