<i>Helicobacter pylori</i>
colonises the human gastric mucosa. <i>H</i>. <i>pylori </i>infection may cause diseases of
the upper gastrointestinal tract, such as chronic gastritis, ulcers and gastric
cancer. The success rate of the current <i>H. pylori</i> treatment regimen has
declined over time, mainly due to antibiotic resistance. Therefore, there is a
clear need for identification of novel targets that can be used in the
development of alternative treatment strategies for <i>H</i>. <i>pylori </i>infections. To
address this, the present study focuses on two enzymes from <i>H. pylori</i> that
represent novel potential targets for drug design: M17 aminopeptidase (HpM17AP)
and α-carbonic anhydrase (HpαCA). HpM17AP plays a role in bacterial defence
against human innate immune response and contributes to the mechanism of drug
resistance, whereas HpαCA is important for buffering periplasmic pH of <i>H.
pylori</i> close to neutral as a means of survival in the harsh acidic environment
of human stomach. Due to their crucial role in colonisation and survival within
the host, HpM17AP and HpαCA need to be evaluated as potential drug targets.<br> <br>
In order to understand the structural basis of catalysis and
inhibition of HpM17AP, the crystal structures of HpM17AP and its complex with
its inhibitor have been determined and analysed. In addition, the specificity of
HpM17AP towards the N-terminal amino acid of its peptide substrates has been
established. The inhibitor-bound structure of HpM17AP revealed that the
D-phenylalanine moiety of the inhibitor binds in the S1 subsite of the enzyme.
At the end of the S1 subsite, HpM17AP was found to harbor a hydrophilic pocket,
which is a unique feature of the enzyme compared to very similar homologues
from other bacteria. This pocket is flanked by a sodium ion and its
coordinating water molecules. Furthermore, structural analysis revealed that
variable loops at the entrance to and in the middle of the substrate-binding
channel are important determinants of the substrate specificity of M17
aminopeptidases. In fact, the study has demonstrated that HpM17AP has broad
substrate specificity and, in contrast to most characterised M17
aminopeptidase, HpM17AP displays preference for L-Arg over L-Leu residues in
the peptide substrate.<br> <br>
In order to understand the molecular details of catalysis and
the structural basis for the inhibition of HpαCA by sulfonamides, the crystal
structures of HpαCA in complex with a series of sulfonamide inhibitors have
been determined and analysed. This study revealed that HpαCA and human carbonic
anhydrase likely follow the same catalytic mechanism, where HCO<sub>3</sub>- is generated
from CO<sub>2</sub> via a nucleophilic attack on CO<sub>2</sub> by the zinc-bound hydroxide ion. In
addition, the study has demonstrated that sulfonamides act as site-directed
inhibitors by mimicking the transition state of the CO<sub>2</sub> hydration reaction.
Analysis of the structures revealed that the binding mode of sulfonamide correlates
well with their inhibitory activities. Furthermore, the study has identified
two pockets near the active site in HpαCA that are distinctly different from
the corresponding regions in the structure of human carbonic anhydrase II.
Thus, our analysis identified major structural features that can be exploited
in the design of selective and more potent inhibitors of HpαCA that may lead to
novel antimicrobials.<br> <br>
Finally, this study has shown that sulfonamides RSO<sub>2</sub>NH<sub>2</sub> kill
<i>H. pylori</i> via mechanisms that are different from the mechanisms of action of
common antibiotics used to treat <i>H. pylori</i> infections. Despite having been
chosen for this study due to their known activity as inhibitors of carbonic
anhydrases, whose function is essential at acidic pH, these compounds also
displayed bactericidal activity at neutral pH. As neutral pH approximates the
conditions under which <i>H. pylori</i> persists in the stomach, we have analysed the
mechanisms by which spontaneous mutations give rise to sulfonamide resistance
at pH close to 7.0. The carbonic anhydrase inhibitors acetazolamide,
methazolamide and ethoxzolamide produce a complex phenotype, likely involving
the combination of different mechanisms which involve modifications in cellular
proteins and systems other than α- and β-carbonic anhydrases themselves. The
frequency of spontaneous mutations resulting in <i>H. pylori</i> resistance to
sulfonamides was 2.5×10-8 or less, indicating that resistance does not develop
easily. Furthermore, this and other studies have shown that the antimicrobial
activity of sulfonamides is specific to <i>H. pylori</i> and several other bacteria,
including <i>Streptococcus pneumoniae</i>, <i>Neisseria</i> spp. <i>Brucella suis</i>. This suggests
that this class of sulfonamides can be developed into selective anti-<i>H. pylori</i>
agents with a novel mechanism of action, which can be used for <i>H. pylori
</i>infections management
