Tài liệu Báo cáo khoa học: Helicases ) feasible antimalarial drug target for Plasmodium falciparum pptx

MINIREVIEW
Helicases
)
feasible antimalarial drug target for
Plasmodium falciparum
Renu Tuteja
Malaria Group, International Centre for Genetic Engineering and Biotechnology, New Delhi, India
The universal presence of helicases in eukaryotes and
prokaryotes, including parasites, reflects their funda-
mental importance in DNA and RNA metabolic pro-
cesses and the maintenance of genomic stability [1–4].
The emerging evidence demonstrates that helicases are
indispensable enzymes because a growing number of
human genetic disorders are attributed to mutations in
helicase genes [1–4]. Helicases act on double-stranded
nucleic acid substrate and thus can be designated
DNA–DNA, RNA–DNA or RNA–RNA helicases
depending on the composition of the substrate. They
are also known as motor proteins because to unwind
the duplexes they require energy, which is provided by
their intrinsic nucleic acid-dependent ATPase activity.
These enzymes act as necessary molecular tools for cel-
lular machinery and significantly contribute to normal
cellular metabolism. In general, helicases require a sin-
gle-stranded nucleic acid region to bind and start their
action of strand separation and once loaded onto the
strand, they display a directional bias and translocate
in either a 3¢ to 5¢ or 5¢ to 3¢ direction, however, a few
bidirectional helicases have also been reported [5,6].
A typical helicase reaction occurs in three successive
steps: (a) binding of the enzyme to the nucleic acid

substrate, (b) NTP binding and hydrolysis, and (c)
NTP-hydrolysis-dependent unwinding of the duplex
substrate (Fig. 1). Various studies have shown that the
unwinding activity of a helicase is tightly coupled to its
intrinsic NTP-hydrolyzing (NTPase) activity [7]. There-
fore, if the NTPase activity is inhibited, this will inhibit
the helicase activity. An alternative approach, i.e. the
reduction of NTP binding by blocking the NTP-bind-
ing site with NTP analogs may also be a possible way
to inhibit the NTPase and subsequently the helicase
activity (Fig. 1). This binding results in the uncoupling
of NTPase and helicase activities and hence functions
through interaction with the enzyme. It is important
to mention here that because the substrate- and
NTP-binding regions are probably highly similar and
conserved between various helicases, specifying the
blockade through these sites will be immensely tough,
although probably not impossible. Helicases form part
of macromolecular complexes and contain discrete
domains responsible for protein–protein interactions,
Keywords
DEAD-box; DNA unwinding; DNA-dependent
ATPase; DNA-interacting compounds; drug
target; helicase; inhibitors; malaria parasite;
molecular motor; Plasmodium falciparum
Correspondence
R. Tuteja, Malaria Group, International
Centre for Genetic Engineering and
Biotechnology, Aruna Asaf Ali Marg,
New Delhi-110067, India

Fax: +91 11 2674 2316
Tel: +91 11 2674 1358
E-mail:
(Received 23 April 2007, revised 23 May
2007, accepted 19 July 2007)
doi:10.1111/j.1742-4658.2007.06000.x
Of the four Plasmodium species that cause human malaria, Plasmodium fal-
ciparum is responsible for the most severe form of the disease and this par-
asite is developing resistance to the major antimalarial drugs. Therefore, in
order to control malaria it is necessary to identify new drug targets. One
feasible target might be helicases, which are important unwinding enzymes
and required for almost all the nucleic acid metabolism in the malaria
parasite.
FEBS Journal 274 (2007) 4699–4704 ª 2007 The Author Journal compilation ª 2007 FEBS 4699
therefore, in some cases strategies to block this inter-
action will also lead to inhibition of the activity.
Helicase signature motifs and
Plasmodium falciparum helicases
Based on sequence comparison and the presence of
characteristic ‘helicase motifs’, three helicase superfam-
ilies (SF1–3) have been identified [7–9]. SF1 and SF2
contain helicases that share a set of nine and SF3 con-
tains only a set of three highly conserved ‘helicase
motifs’, respectively [7–9]. The DEAD (Asp-Glu-Ala-
Asp), DEAH (Asp-Glu-Ala-His), DExH and DExD-
box helicases are ubiquitous and are the most common
members of SF2 [8,9]. The various ‘helicase motifs’
have been named Q, I, Ia, Ib, II, III, IV, V and VI
and, based on the mutational analysis and structural
data in a variety of systems, specific roles have been

suggested for a number of the conserved motifs [10–
12]. For example, motif I (A ⁄ GxxGxGKT), motif II
(VLDEAD), motif III (SAT) and motif VI
(HRIGRxxR) are responsible for ATP binding and
hydrolysis, nucleic acid binding and ATP-hydrolysis-
dependent nucleic acid unwinding, respectively [7]. It
has been reported that Arabidopsis thaliana contains
55 members of the DEAD-box family of helicases,
humans contain 38 and Saccharomyces cerevisiae con-
tains 25 [13]. In addition to the ‘helicase core region’,
which harbors the conserved motifs and functions as
an ATP-dependent motor or switch, most helicases
contain divergent amino and ⁄ or C-terminal extensions
that confer substrate specificity and provide the basis
for protein–protein interaction [14]. These structurally
different domains are also responsible for targeting the
helicase to a specific cellular process.
Although helicases have been reported from a vari-
ety of other systems there are very few reports of
helicases from P. falciparum. Since completion of the
P. falciparum genome in 2002, new opportunities for
research have arisen [15]. The P. falciparum genome
has 14 chromosomes, a linear mitochondrial genome
and a circular plastid-like genome [15,16]. A full set of
helicases was identified in the original genome
sequence of P. falciparum during annotation (http://
www.plasmodb.org), but detailed analysis using a
bioinformatic approach revealed that the genome con-
tains at least 22 full-length putative DEAD-box heli-
cases, as well as a few other putative helicases [15–18].

These 22 P. falciparum helicases contain all the con-
served domains, but the length and sequence of the
N- and C-terminal extensions and the intervening
sequences are variable [17]. Based on the crystal struc-
ture of human DEAD-box helicase, a model for the
structure of p68 (a well-characterized 68 kDa protein
of the DEAD-box protein family, which is conserved
from yeast to human) homolog of P. falciparum
(P. falciparum DNA helicase 60, PfDH60) was
created, which suggests that although there are minor
variations in length and sequence between the
conserved domains these two structures are highly
superimposable (Fig. 2). These observations further
suggest that although these proteins most likely act
through related mechanisms the parasite-specific
sequences could still be specifically targeted because
the antibodies to PfDH60 do not cross-react with the
human p68 [17,19–21].
To the best of our knowledge only a few helicases
have been characterized from P. falciparum. These
include two members of the DEAD-box family namely
PfDH60 and P. falciparum DNA helicase 45 (PfDH45)
[20,21] (A. Pradhan and R. Tuteja, unpublished) and
PfDHA, a 90 kDa DNA helicase which has been puri-
fied from P. falciparum [18]. Our studies indicated
that PfDH60 contains helicase and ssDNA-dependent
Fig. 1. Schematic representation of the three successive steps involved in a typical helicase reaction. The details of steps a–c are written
above the arrows. A particular helicase inhibitor ⁄ drug most probably acts at the substrate or enzyme level via one or more of the following
processes: (i) modulates enzyme–substrate binding, (ii) inhibits helicase activity by obstructing NTP binding, (iii) inhibits NTPase activity via
an undefined or allosteric mechanism, (iv) inhibits the coupling of NTP hydrolysis with the unwinding reaction, and (v) inhibits translocation

of the helicase on the nucleic acid substrate due to the steric blockade.
Helicases as an antimalarial drug target R. Tuteja
4700 FEBS Journal 274 (2007) 4699–4704 ª 2007 The Author Journal compilation ª 2007 FEBS
ATPase activities and is expressed in schizont stages of
the development of parasite [20,21]. It has also been
reported that PfDH60 is a unique dual, bipolar heli-
case and its enzyme activities are modulated by phos-
phorylation [21]. PfDH45 is a homolog of eukaryotic
initiation factor 4A contains helicase and ssDNA-
dependent ATPase activities and is expressed in all the
developmental stages of the parasite (A. Pradhan and
R. Tuteja, unpublished). PfDHA moves in the 3¢ to 5¢
direction and prefers a fork-like substrate for its
unwinding activity [18].
Helicases as drug and therapeutic
target
Resistance to the most efficient, reasonably priced and
safe antimalarials has called for the search for new
drug targets and ultimately new drugs. Because heli-
cases contain multiple functional domains and a vari-
ety of enzymatic activities, and have essential roles in
the metabolism of DNA and RNA, helicase inhibitors
might offer a feasible route towards the development
of novel drugs. Various studies have shown that heli-
cases are indispensable enzymes and in yeast the loss
of one DEAD-box gene cannot be supplemented by
overexpression of another family member, which fur-
ther suggests that each helicase gene is independently
essential [22,23]. Some helicases are required for the
proliferation of bacteria and viruses, therefore, inhibi-

tion of the unwinding activity of various helicases
results in a decrease in virus replication in cell cultures
as well as in animal models and this suggests a novel
antiviral strategy [24–27]. Potent antihelicase agents
have been reported for a number of helicases from dif-
ferent viruses [28]. The detailed characterization of two
related DEAD-box helicases, hepatitis C virus NS3
and human eIF-4A has provided evidence for design-
ing specific inhibitors that can be used to target the
viral NS3 helicase and inhibit the viral replication [5].
Table 1 shows a comparison of the inhibitory poten-
tial of some of the helicases from P. falciparum and
helicases from the human host. The comparison clearly
indicates that the IC
50
value for various compounds
tested, including daunorubicin and nogalamycin, is
lowest for helicases from P. falciparum compared with
the other helicases [29,30]. It is interesting to note that
inhibition by nogalamycin is highly variable and
depends on the source of the enzyme [31–34]. In a pre-
vious study it was reported that the IC
50
value for this
compound varied between 0.1 and >650 lm for heli-
cases from different viruses such as hepatitis C virus,
dengue fever virus, Japanese encephalitis virus and
west Nile virus [35,36]. It is possible that some of these
Fig. 2. Structural modeling of PfDH60. The protein sequence of
PfDH60 (GenBank accession number AY700082; PlasmoDB

No. PFL1310c) was subjected to the 3
D-JIGSAW program (version 2.0)
in . This server builds 3D models of proteins
based on known structural homologs [37–40]. The model for PfDH60
was built based on the solved crystal structure of human DEAD-box
helicase (Protein data bank Id:2I4I; Molecular Modeling Database
Id:41213) [39]. The conserved helicase motifs of both proteins are
displayed in different colors using a molecular visualization program
to display, animate and analyze large biomolecule systems using 3D
graphics and built-in scripting (
VMD software; -
c.edu). (A) Template structure, (B) PfDH60 structure. The colors
used for various motifs are: motif I, yellow; motif Ia, green; motif Ib,
red; motif II, light blue; motif III, white; motif VI, white.
R. Tuteja Helicases as an antimalarial drug target
FEBS Journal 274 (2007) 4699–4704 ª 2007 The Author Journal compilation ª 2007 FEBS 4701
compounds, which inhibit helicase activity, could be
utilized to inhibit parasite growth. In fact, it has been
shown that some of these compounds inhibited the
growth of P. falciparum in culture, which further con-
firms that inhibition of the activity of parasite helicase
inhibits the parasite growth [30].
Furthermore, these helicases can be specifically
targeted using the specific antibody and dsRNA
approach. Previous observations have shown that anti-
PfDH60 sera, which recognize only this protein in par-
asite lysate, inhibit parasite growth in culture [30].
Similar results were also obtained for anti-PfDH45
sera (A. Pradhan and R. Tuteja, unpublished).
Regarding the antisense approach, it has been shown

that the specific dsRNA against PfDH60 inhibited par-
asite growth in culture [30]. This inhibition is due to
the degradation of its cognate mRNA, which results in
inhibition of PfDH60 protein synthesis and in turn
inhibition of the parasite growth [30]. These results
collectively indicate that the helicases can be specifi-
cally targeted to inhibit their function. Although these
results are encouraging but overall the data on inhibi-
tor studies of malarial helicases are very limited.
Because helicases belong to a large gene family exten-
sive validation is required before the studies can focus
on a specific malarial helicase that could be used as a
specific target to control malaria. A comparative study
of available inhibitors may help to identify a com-
pound to specifically target and inhibit the parasite
helicase without affecting the host, and thereby could
be used as the potential drug ⁄ drugs to treat malaria.
Conclusions and future perspectives
Antimalarial drug resistance poses a major obstacle to
the control of malaria. Therefore, the development of
suitable and cost-effective drugs for the treatment of
malaria is a significant endeavor. Detailed studies
regarding the mechanism and function of all the
helicases of P. falciparum (including the DEAD-box
helicases) will help to establish their validity as a suitable
target. But extensive evaluation is essential before these
enzymes can be taken as bona fide targets for designing
therapies against malaria. The results summarized in
this article show a ray of hope to control malaria and
further studies should be carried out in this direction.

Acknowledgements
The author is grateful to Dr Narendra Tuteja, ICGEB,
New Delhi for critical comments on the manuscript
and Mr Arun Pradhan for help in preparation of
figures. The author also sincerely thanks the reviewers
for helpful comments. The work in authors’ laboratory
is supported by grants from Defence Research and
Development Organization and Department of Science
and Technology. Infrastructural support from the
Department of Biotechnology, Government of India is
gratefully acknowledged.
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