Biomedical Engineering Reference
In-Depth Information
Co4 was found to reduce the parasite load in a murine infection of P. c. cha-
baudi by 92% compared to controls. 45,46 The phosphinate dipeptide analogs
were shown to be fast, tight-binding inhibitors in contrast to the slow tight-
binding kinetics displayed by bestatin. 46 Resolution of the X-ray crystal
structures of both PfA-M1 and PfA-M17 bound to Co4 (3KR5.pdb,
3EBI.pdb) confirmed that the central phosphate group of the ligand provides a
hexavalent coordination of the active site metals, presumably accounting for its
increased potency. 30,42 Overall, it was found that Co4 and bestatin interact with
the active site of each enzyme in a similar fashion. The Phe-rings of Co4 pro-
vided more favorable hydrophobic interactions with both enzymes, also con-
tributing to the potency of the inhibitor.
7.3.2 Structure-Activity Relationships and Future Rational
Drug Design
A detailed understanding of the precise pharmacophore of the PfA-M1 and
PfA-M17 active sites is essential for the future development of 'drug-like'
compounds. Detailed atomic knowledge of how inhibitors bind, intractable and
flexible residues of each enzyme, and the location of non-catalytic inhibitory
sites provides a platform of SAR data for the future that may not be rationally
envisaged. A recent study 49 demonstrated the value of these data using a
combined structure-function-based approach. Not identified in the earlier
structures of bestatin or Co4 bound PfA-M1, 30 Velmourougane et al. 49 showed
that the S1 pocket of PfA-M1 had considerable flexibility upon inhibitor
binding. By screening a diverse library of bestatin derivatives, it was shown that
extended aromatic side-chains at the P1 position resulted in potent inhibition of
PfA-M1. After examination of the bound structures, it was clear that the S1
pocket could be deformed to permit such extended inhibitors.
Although the common chelation mechanism used by both bestatin and Co4
is effective, neither ligand represents a lead compound for further optimization.
Bestatin is rapidly eliminated from serum, and the phosphinic acid chemistry is
not favorable for further optimization. However, SAR data, from enzyme
kinetics and structural analysis of these inhibitors, have clearly shown that the
composition and coordination of the metal cores of the proteases affect enzyme
activity/inhibitor potency. Optimizing the interactions with the metal cores will
be a key design requirement for the chemistry of lead compounds. Our
understanding of the role of the loosely bound metal in PfA-M17 still requires
further investigation to determine exactly why the alteration of this site from
zinc to cobalt enhances activity. The physiological metal has been proposed to
be magnesium or zinc, so the role of cobalt is unclear. 43 SAR data from
enzymes bound with Zn/Co may provide some insight
into the exact
mechanism of enzyme modulation.
The precise in vivo role and requirement of each of the neutral aminopepti-
dases still remain to be determined. Research continues in many laboratories to
identify and generate enzyme-specific inhibitors (that can inhibit one enzyme
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