1. Targetable Metabolic Changes in ​Plasmodium falciparum​ During
Parasitic Takeover of Mammalian Erythrocytes
Mini-Review
Daniel Haines
Other group members:
Elizabeth Barrett
Samuel Del’Olio
Group topic:
LDH and MDH in ​Plasmodium falciparum
1

2. Abstract
Plasmodium falciparum is a protozoan parasite that infects red blood cells in order to
exploit their resources to replicate and survive. In order to do this it must remodel the erythrocyte
to streamline the metabolites it needs, and breakdown anything it does not into building blocks
that it can use. It must also remodel its own form for uptake of these nutrients. The parasite’s
heavy reliance on glycolysis and fermentation via lactate dehydrogenase suggest that going
forward LDH may serve as a strong potential candidate for targeting for inhibition by
antimalarial drugs.
Introduction
P. falciparum is a protozoan parasite responsible for malaria in humans. Transmitted to
humans by female mosquitoes, ​P. falciparum exists in four stages: sporozoite, merozoite,
trophozoite, and gametocyte.
Upon the bite of a carrier
mosquito, sporozoites in the saliva are
injected into the bloodstream, where
they travel to the liver to infect cells.
Using the nutrients provided by liver
cells, they produce large levels of
merozoites, eventually rupturing the
liver cell and releasing themselves back
into the bloodstream, where they will go
2

3. on to attack erythrocytes. Once the parasite has infected an erythrocyte, it exists as a trophozoite,
growing and replicating through asexual reproduction, at the expense of the erythrocytes. Most
of these erythrocytes eventually rupture, releasing merozoites into the bloodstream, beginning
the cycle anew. The trophozoite and merozoite stages are the largest areas of focus for
researchers, as they are when the host first begins to show symptoms of malaria, and are
therefore also the focus of this review.
It is also of note that a small amount of these infected cells will also leave this cycle,
instead producing gametocytes. When a mosquito bites an infected human, these infected
erythrocytes will rupture inside the gut of the mosquito, releasing gametocytes and beginning the
cycle of transmission again.​1
Knowledge of ​P. falciparum metabolism at the trophozoite/merozoite stages is important
for the creation of anti-malarial drugs. By knowing which enzymes play the largest role in
providing energy to the parasite at these stages, drugs can be created that selectively target them
in order to cut off the parasite’s energy source and help support the host’s immune response.​2
Metabolic Changes
Once a ​P. falciparum merozoite has taken over an erythrocyte, it then has to change itself
and the host cell as to avoid an immune response and utilize existing metabolic pathways.​3
It
does this in a variety of ways.
3

4. Metabolic Needs
P. falciparum has functioning mitochondria, however, due to the lack of oxygen
consumption, evidence suggests the Kreb’s cycle is occurring at relatively low levels at this stage
in its life cycle.​4
Studies furthered this question to determine if the Kreb’s cycle is necessary for
survival at the trophozoite stage. Six of the eight genes encoding for the enzymes of the Kreb’s
cycle were successfully knocked out with no change in growth of the parasite.​5
The two
exceptions to this are the genes encoding fumarate hydratase and malate quinone oxidoreductase,
suggesting they may in fact be necessary for survival at the trophozoite stage.​5
This, coupled with intermediates such as citrate, aconitate, and a-ketoglutarate having
been observed during the trophozoite stage, suggest that Kreb’s cycle enzymes are not acting as a
cycle and may play a still unknown role in metabolism.​4
Upon entry into an erythrocyte, glucose fermentation to lactate accounts for almost all the
energy metabolism in ​P. falciparum. In fact 60-70% of all glucose in the red blood cell is
converted to lactate.​6
This is supported by high NAD+ levels in infected red blood cells, which is
consistent with high consumption of glucose and production of lactate.​4
In turn, this suggests that
lactate dehydrogenase (LDH) plays a large role in the production of ATP for the parasite, and
therefore may serve as a potential target for inhibition.
Erythrocyte Remodeling
The streamlining of nutrients requires remodeling of the erythrocyte in order to
better suit the parasite. ​P. falciparum will digest 75% of a red blood cell’s hemoglobin to supply
amino acids for protein synthesis and clear space within the host cell.​4​
Digestion of hemoglobin
4

5. Aspartic proteinases PF14_0075, PF14_0076, PF14_0077, PF14_0078
Falcipain proteinases PF11_0161, PF11_0162, PF11_0165
Metallopeptidase PF13_0322
Table 1. ​Enzymes involved in the catabolism of hemoglobin in ​P. falciparum food vacuole. They include members
of the plasmepsin, falcipain, and falcilysin families. (Data from Florens ​et al., 2002.)
occurs in the parasite’s food vacuole via proteinases and a metallopeptidase (Table 1).​7
This need
for space is not surprising considering that during the trophozoite stage, the parasite, existing
inside a parasitophorous vacuole membrane (PVM), will occupy 50% of the volume of the red
blood cell as it digests the host cell cytoplasm.​8
The digestion of hemoglobin is a major catabolic
process that is largely important because the parasite cannot produce its own amino acids. It will
need them to produce the proteins that will make up the new merozoite parasites.​7
Furthermore, the membrane of infected red blood cells has shown to have an increase in
permeability to many solutes needed by the parasite that cannot be provided by the red blood cell
itself. This increased permeability may be caused by the targeting of membrane transporters by
parasitic protein kinases.​3
Parasite Remodeling
While remodeling the erythrocyte, ​P. falciparum exports over 10% of all its proteins into
the cytosol of the erythrocyte.​9
The parasite therefore must create a pathway to do this in an
orderly and controlled way. As a result, ​P. falciparum creates and releases structures called
Maurer’s clefts (figure 2), which are responsible for sorting these proteins so they can travel to
their final destination.​9
Although the pathway by which Maurer’s clefts are created is still widely
debated, they are structurally similar to the Golgi apparatus of eukaryotes.​10
5

6. The parasite also anchors
itself to the erythrocyte membrane.
To do this it releases a protein called
erythrocyte surface protein 1
(​PfEMP1), which exists in the knobs
of a ​Plasmodium infected
erythrocyte (figure 2). The protein is
also largely responsible for the
immune evasion and pathogenicity
experienced by ​P. falciparum.​11
Like
most proteins released from the
PVM, it is temporarily localized in
the Maurer’s cleft before being
embedded in the erythrocyte membrane, once again highlighting the importance of this protein
trafficking mechanism.​9
Modifications must also be made to the parasite itself in order to absorb its newly
obtained nutrients. Because ​P. falciparum exists in a parasitophorous vacuole membrane, it has
been suggested that the parasite creates a tubulovesicular membrane that extends into the cytosol
of the host cell in order to obtain these nutrients (Figure 2).​3
6

7. Lactate Dehydrogenase as a Target
P. falciparum infected Lactate dehydrogenase (​PfLDH) is a promising target enzyme for
antimalarial drugs. This is largely because the majority of ATP production is a result of the
conversion of pyruvate to lactate via ​PfLDH. By inhibiting ​PfLDH, ​P. falciparum does not have
an energy source. The structures of LDH
and ​PfLDH also have unique amino acid
sequences at their active sites, meaning
targeting one would not likely inhibit the
other. Furthermore: LDH and ​PfLDH, both
isolated and in complex with many
structures, have widely been studied and
sequenced by researchers. As a result,
researchers can take this data to try and
discover potential targets for inhibition.​2
A
promising example of this kind of inhibition
can be seen in figure 3.
Conclusions
P. falciparum is a protozoan parasite that hijacks the red blood cells of mammals and
alters them to rob them of their resources. A byproduct of its destruction is sickness and often
death for those who are affected. For this reason, the development of antimalarial drugs through
targeting metabolic processes is becoming increasingly important.
7

8. Various studies have shown that, although some enzymes are still necessary, the Kreb’s
cycle provides little energy to the cell at the trophozoite stage of the life cycle. Instead glucose
fermentation is the main source of metabolism for the parasite, raising the possibility of LDH as
a target for inhibition by antimalarial drugs.
P. falciparum goes on to wreak havoc in the erythrocyte in other ways too as it remodels
it to be more efficient. It does this by breaking down things it cannot use, like hemoglobin, into
building blocks it can repurpose, or by affecting the permeability of the erythrocyte membrane to
allow solutes it needs to come into the cell.
In order to do all these things effectively, the parasite must remodel itself, especially by
releasing proteins into the host cell to change the erythrocyte. ​P. falciparum creates Maurer’s
clefts, which effectively sort proteins heading to various parts of the host cell. One of these
proteins that is sorted is ​PfEMP1, which embeds itself in the erythrocyte membrane and helps to
prevent being targeted by the immune system.
Because most glucose in the erythrocyte eventually is converted to lactate, LDH must
play a large role in the metabolism of ​P. falciparum. As a result, targeting by inhibitors can be
seen as a possible way for future antimalarial drugs to cut off the parasites energy supply and
help create cures for the disease. Attempting to discover novel inhibitors that lack that side
effects of those like Gossypol should be at the forefront of research into antimalarial treatments.
Through understanding the mechanisms behind the parasite's metabolism and the changes
it causes in its host, antimalarial drugs can be created to selectively target and inhibit the
enzymes that are most heavily involved. For this reason, moving forward, research into
inhibition of LDH could take us one step closer to stopping the spread of malaria.
8

9. References
[1] NIAID Staff. Life Cycle of the Malaria Parasite. ​National Institute of Health.
https://www.niaid.nih.gov/topics/Malaria/Pages/lifecycle.aspx. Published January 4,
2016.
This source was used for an overview of the life cycle of Plasmodium, as well as
for the image that accompanies it in the introduction.
[2] Neeraj Sethiya, Priyadarshan Keluskar, Sanjay Ingle, Shrihari Mishra. Antimalarial
activity of ​Evolvulus alsinoids Linn.-an in vitro ​Plasmodium falciparum specific lactate
dehydrogenase enzyme inhibition assay. ​Asian Pacific Journal of Tropical Disease.
2014; 4(6): 489-491.
This source discussed a possible inhibitor to lactate dehydrogenase. It was used
to discuss why LDH would serve as a good enzyme to inhibit (structure and amino
acid sequence studied, etc). It was also used for figure 3.
[3] Alassane Mbengue, Xue Y. Yam, Catherine Braun-Brenton. Human erythrocyte
remodelling during ​Plasmodium falciparum malaria parasite growth and egress. ​British
Journal of Haematology. February 2012; 157(2): 171-179.
This source discussed changes to the parasite at different stages of the life cycle
and changes to the erythrocyte during the trophozoite stage. For this paper it was
used specifically for changes in membrane permeability and the tubulovesicular
membrane.
9

10. [4] Kellen L. Olszewski, Joanne M. Morrisey, Daniel Wilinski. Host-Parasite Interactions
Revealed by ​Plasmodium ​falciparum Metabolomics. ​Cell Host & Microbe. February
2009; 5(2): 191-199​.
This source widely discussed the metabolism of P. falciparum at various stages of
the life cycle. In this paper the source was used to discuss the Kreb’s cycle, as
well as glucose fermentation.
[5] Ke H, Lewis IA, Morrisey JM, et al. Genetic Investigation of Tricarboxylic Acid
Metabolism during the Plasmodium falciparum Life Cycle. ​Cell Reports 2015; 11(1):
164–174.
This source discussed the role of the Kreb’s cycle during the trophozoite stage of
P. falciparum, using knockouts of different enzymes. It was used in this paper to
discuss whether the Kreb’s cycle is necessary for survival during the trophozoite
stage and what role it plays.
[6] Priyamvada Jain, Babina Chakma, Sanjukta Patra, Pranab Goswami. Potential
Biomarkers and Their Applications for Rapid and Reliable Detection of Malaria. ​BioMed
Research International. April 2014.
This source discussed methods of rapid detection of malaria. For this paper it was
used for the percentage of glucose converted to lactate.
[7] Laurence Florens, Michael P. Washburn, J. Dale Raine, et al. A proteomic view of the
Plasmodium falciparum life cycle. ​Nature. October 2002; 419: 520-526.
This source attempted to use proteomics to help find novel drug and vaccine
targets. For this paper it was used to illustrate another catabolic process in P.
10

11. falciparum, the breakdown of hemoglobin into amino acids that can be used to
build new parasites. It was also used for Table 1.
[8] Krugliak, M., Zhang, J. & Ginsburg, H. Intraerythrocytic ​Plasmodium falciparum utilizes
only a fraction of the amino acids derived from the digestion of host cell cytosol for the
biosynthesis of its proteins. ​Molecular and Biochemical Parasitology. 2002; 119:
249–256.
This source was only used to highlight the size of the parasite and how much
hemoglobin is broken down by the P. falciparum.
[9] Mundwiler-Pachlatko E, Beck H-P. Maurer's clefts, the enigma of Plasmodium
falciparum. ​Proceedings of the National Academy of Sciences 2013; 110(50):
19987–19994.
This source discussed the fact that little is known about Maurer’s clefts and
highlighted the structure and function of them. For this paper it was used to
highlight the volume of proteins that travel through it and for a general
understanding of the structure and its function.
[10] Lanzer M, Wickert H, Krohne G, Vincensini L, Breton CB. Maurer's clefts: A novel
multi-functional organelle in the cytoplasm of Plasmodium falciparum-infected
erythrocytes. ​International Journal for Parasitology 2006; 36(1): 23–36.
This source was found to be used as a reference for what Maurer’s clefts are, but
was less useful. It was used only for the comparison to the Golgi apparatus.
11

12. [11] Pasternak ND, Dzikowski R. PfEMP1: An antigen that plays a key role in the
pathogenicity and immune evasion of the malaria parasite Plasmodium falciparum. ​The
International Journal of Biochemistry & Cell Biology 2009; 41(7): 1463–1466.
This source described the role of PfEMP1. It was used as a background for this as
well as to highlight more information about Maurer’s clefts.
12