Biomedical Engineering Reference
In-Depth Information
disease.
9
In this context, the emergence of resistance to the few available
effective anti-malarial drugs, including artemisinin,
5,10
is more worrying, par-
ticularly given the lack of diversity among new agents in advanced clinical
development.
11
Interestingly, our most effective traditional anti-malarials have been identi-
fied serendipitously. The quinones and the artemisinins are both derived from
long-established herbal remedies, and their molecular targets are still not
definitely understood.
12
However, as our knowledge and understanding of
molecular targets increase, high-throughput- and structure/activity-based
approaches to the design of new and potent inhibitors of enzymes are now a
major component of many modern drug-discovery programs. The release of the
Pf genome sequence and the subsequent structural data on Pf proteins has
encouraged many target-based anti-malarial development programs,
12
while
recent advances in high-throughput in vitro assays have delivered many com-
pounds with anti-malaria activity, but unknown targets, to the malaria scien-
tific community.
13
In these circumstances, structure-based drug design can
provide valuable structural detail to facilitate both the selection of suitable
targets and the development of compounds to provide drug candidates.
12
The intra-erythrocytic stage of Pf infection represents a stage during which
many metabolic pathways switch on, and consequently has been the focus of the
majority of anti-malaria drug-development strategies.
14,15
One essential path-
way that has been of particular interest for anti-malaria drug discovery is the
catabolism of erythrocyte hemoglobin; 65-75% of the host-cell hemoglobin is
degraded in a process that ultimately results in the liberation of free amino
acids.
16,17
The initial steps in the catabolism of host hemoglobin take place
within a specialized organelle of the parasite, the food or digestive vacuole (DV).
The breakdown of hemoglobin is achieved by a range of proteases present in the
DV, including aspartic acid proteases (Plasmepsins), cysteine proteases (Falci-
pains), and metalloproteases (Falcilysin).
14,18
The roles of many of these DV
endopeptidases appear to be largely redundant, and so enthusiasm for targeting
them for new anti-malarial drugs has decreased considerably.
11,17
The final stage of hemoglobin digestion is mediated by the metalloamino-
peptidases (MAPs) that act to degrade small peptide fragments into free amino
acids. This final step of digestion is thought to occur predominantly within the
cytosol of the cell. The MAP enzymes are exopeptidases and act to remove
amino acids from the N-termini of peptides and proteins. Their active site
coordinates essential metal ions that activate a water molecule to form a
hydroxide nucleophile which attacks the scissile peptide bond. The MAPs are
classified into families based on structural patterns and substrate specificity.
19
Each MAP uses its amino acids to create a unique active site pocket, which
interacts with the peptide substrate. The MAP enzymes are widely distributed
throughout all cellular biology and are of fundamental importance to normal
cellular catabolic and anabolic functions. Disruption of MAP activity can result
in cancer-cell proliferation and abnormalities in neuropeptide regulation.
20
To date, 10 MAPs have been identified in the Pf genome (http://www.plas-
modb.org). Four of these are methionine aminopeptidases that presumably have
