Organisms acquire organic material from their environments and convert this material into energy or their own substance (i.e., biomolecules). Cells are made of distinct classes of biomolecules with specific functions (Table). These macromolecules are synthesized from small molecular weight precursors, or building blocks. These molecular precursors are components of interconnected metabolic pathways (Figure). (Go to index of subsections.)
The malaria parasite exhibits a rapid growth and multiplication rate during many stages of its life cycle. This necessitates that the parasite, like all other organisms, acquire nutrients and metabolize these various biological molecules in order to survive and reproduce. Obviously, the parasite's metabolism will be intertwined with that of the host's because of the intimate relationship between the host and parasite. These host-parasite interactions are further complicated by the complex life cycle of the parasite involving vertebrate and invertebrate hosts as well as different locations within each of these hosts. A better understanding of the parasite's metabolism may lead to the development of novel therapeutic strategies which exploit the uniqueness of the parasite. [See page on Mechanisms of Drug Action and Resistance.] Below are brief discussions of selected aspects of parasite metabolism--with an emphasis on the unique features of the parasite--according to the classes and functions of biomolecules:
More detailed descriptions can be found in a review article by Lang-Unnash and Murphy (1998) and a website of detailed metabolic pathways of the malaria parasite assembled by Hagai Ginsburg. Several links to these detailed pathways have been made in this document (designated with [HG]). The 'ppt figures' link to powerpoint slides of an introductory lecture on cell biology.
The blood-stage parasite actively ferments glucose as a primary source of energy. The metabolic steps involved in the conversion of glucose to lactate (referred to as glycolysis) are essentially the same as that found in other organisms. All of the enzyme activities have been identified in Plasmodium and some of the genes cloned. The parasite exhibits a high rate of glycolysis and utilizes up to 75 times more glucose than uninfected erythrocytes. Most of the glucose is converted to lactate and the high lactate dehydrogenase (LDH) activity is believed to function in the regeneration of NAD+ from NADH which is produced earlier in the glycolytic pathway by glyceraldehyde-3-phophate dehydrogenase (Figure). (See also detailed diagram of glycolytic pathway [HG]) The net result of glycolysis is to produce ATP which is the energy currency of the cell. In other words, ATP is needed for anabolic and homestatic processes.
Most (approximately 85%) of the glucose utilized by the parasite is converted to lactate. However, some of the glycolytic intermediates may be diverted for synthetic purposes. For example, enzymes of the pentose phosphate pathway [HG] have been identified. This pathway probably provides some of the ribose sugars needed for nucleotide metabolism and provides for the regeneration of reduced NADPH to be used in biosynthesis or defense against reactive oxygen intermediates (see below). Similarly, the further metabolism of pyruvate [HG] may provide intermediates in several biosynthetic pathways.
Aerobic metabolism involves the further catabolism of pyruvate (glycolysis intermediate preceding lactate) to carbon dioxide and hydrogen atoms via the tricarboxylic acid (TCA) cycle. The hydrogen atoms are captured by the reduction of NAD+ to NADH. The electrons from the captured hydrogen are then fed into a chain of electron carriers and ultimately transferred to molecular oxygen to form water. ATP is generated by capturing energy during electron transport by a process known as oxidative phosphorylation. The TCA cycle and oxidative phosphorylation can generate up to 38 molecules of ATP per glucose molecule, whereas glycolysis only produces two molecules of ATP per glucose molecule. Nonetheless, the blood-stages of mammalian malaria parasites do not exhibit a complete TCA cycle. An explanation for this inefficiency is the abundance of glucose in the mammalian blood stream. In contrast, the parasite does appear to exhibit a TCA cycle in the glucose-poor environment of the mosquito host.
The TCA cycle and oxidative phosphorylation are generally carried out in the mitochondria of eukaryotes. These processes are generally assumed to be non-functional in the blood-stage parasite as evidenced by the acristae mitochondria. However, recently a functional electron transport chain and oxidative phosphorylation have been demonstrated in the blood-stage parasite (Uyemura et al, 2000). In addition, the parasite mitochondrion does have a membrane potential and cytochrome oxidase is present. The antimalarial drug atovaquone has been shown to inhibit electron transport and to collapse the mitochondrial membrane potential in malaria parasite. One possible function of the mitochondrion during the blood stage is for pyrimidine synthesis (see nucleotide metabolism). (See also diagram of electron in the mitochondria [HG].)
Lipids are a major component of membranes. The rapidly growing parasite requires large amounts of lipids for this increase in parasite surface area and volume of internal membranes. This huge demand for lipids makes lipid metabolism an attractive target for anti-malarial drugs and several potential drugs targeting lipid metabolism have been identified (Mitamura and Palacpac, 2003).
Membrane lipids are composed of a glycerol (3-carbon unit) backbone which has a polar head group and two long chain fatty acids (see ppt figure). Historically, the parasite has been considered to be incapable of synthesizing fatty acids de novo and restricted to obtaining preformed fatty acids and lipids from the host. However, several enzymes associated with the type II fatty acid synthesis pathway have been identified in Plasmodium and appear to be located in the apicoplast (see box). This type II pathway is found in plants and prokaryotes, whereas the type I fatty acid synthetase is found in yeast and metazoa. (See detailed diagram of acid synthesis in the apicoplast [HG])
Several parasite enzymes involved in lipid synthesis from glycerides and fatty acids, as well as enzymes involved in the remodeling of lipid polar head groups have been identified (Mitamura and Palacpac, 2003). (See detailed diagram of lipid metabolism [HG]). An enzyme capable of activating fatty acids (necessary for incorporation into lipids) has been localized to membranous structures found within the cytoplasm of the infected erythrocyte (Matesanz et al, 1999).
|The Apicoplast--A Vestigial Chloroplast-like Organelle|
|A non-photosynthetic plastid has been described in the Apicomplexa (Wilson, 2002). This plastid is most likely of red alga origin and has a long evolutionary history within the apicomplexa. Possible functions associated with the apicoplast are biosynthesis of 1) fatty acids, 2) isoprenoid precursors, and 3) heme. Plasmodium homologs of enzymes involved in type II fatty acid synthesis have apicoplast-targeting sequences and are sensitive to known inhibitors of type II fatty acid synthesis. Similarly, the synthesis of isoprenoids in Plasmodium also appears to involve enzymatic pathways that are found in bacteria and plastids and is distinct from the synthetic pathway found in eukaryotes. Both of these pathways are particularly attractive drug targets since the human host synthesizes fatty acids and isoprenoids via different pathways utilizing different enyzmes. Heme biosynthesis is not as clear. Some of the enzymes in heme biosynthesis appear to be targeted to the apicoplast, whereas others appear to be targeted to the mitochondrion.|
Proteins are composed of linear chains of amino acids which fold into 3-dimensional structures (see ppt figure). Through their roles as enzymes or structural proteins (see ppt figure) proteins are responsible for cellular structure and function. The blood-stage parasite obtains amino acids for protein synthesis from three sources: 1) degradation of ingested hemoglobin, 2) uptake of free amino acids from the host plasma (or cells), and 3) de novo synthesis. The most abundant source of amino acids is the ordered degradation of hemoglobin (see Hemoglobin Degradation and the Food Vacuole). The parasite digests up to 65% of the total host hemoglobin into amino acids. However, most of these amino acids are effluxed from the infected erythrocyte and only 16% of the digested hemoglobin is incorporated into parasite proteins (Krugliak, 2002).
Several amino acids are taken up by infected erythrocytes at accelerated rates (Ginsburg, 1994) and in vitro culture studies indicate that P. falciparum requires seven exogenously supplied amino acids: isoleucine, methionine, cysteine, glutamate, glutamine, proline, tyrosine (Divo et al, 1985). The parasite is also able to fix carbon dioxide and thereby synthesize alanine, aspartate and glutamate (Sherman, 1979?). However, the amino acids formed via carbon dioxide fixation and some of the exogenously added amino acids are not readily incorporated into proteins. Many of these amino acids (through transamination reactions) can interact with pathways involved in energy production and possibly serve as fuel sources. In addition, some amino acids serve as precursors or components of biosynthetic or other metabolic pathways (eg., see glutamate metabolism [HG] or methionine metabolism [HG]). Of particular note is the proposal that glutamate dehydrogenase provides the reduced NADPH needed for glutathione reductase (Krauth-Siegel et al, 1996) which presumably functions in redox metabolism.
Ribosomes are supramolecular complexes composed of ribosomal RNA and proteins. Their function is to translate mRNA into protein (see ppt figure). The mechanism of protein synthesis is presumably typical of other eukaryotes. Interestingly, different rRNA molecules are expressed during the vertebrate and invertebrate stages of the parasite's life cycle (McCutchan et al, 1995). The functional significance of stage specific ribosomes is not known.
DNA and RNA are polymers of nucleotides. Nucleotides consist of a ribose sugar group linked to either a purine (adenine and guanine) or a pyrimidine (cytosine, uracil, and thymine) base. These bases can either be obtained via de novo synthesis or from the environment by the 'salvage' pathway. The malarial parasite obtains preformed purines by the salvage pathway and synthesizes pyrimidines de novo. Since the host can obtain both types of bases by either pathway, it may be possible to exploit the parasite's limited capability in nucleotide metabolism.
The primary purine salvaged by the parasite is hypoxanthine which can be obtained from the host plasma. In addition, adenosine in the host plasma can be converted to hypoxanthine following deamination and dephosphorylation (see purine metabolism [HG]). Through a series of reactions the hypoxanthine is converted into ATP and GTP (or deoxy- ATP and GTP) and incorported in RNA (or DNA).
The parasite cannot utilize preformed pyrimidines and must synthesize them from bicarbonate and glutamine (see pyrimidine metabolism [HG]). One step of pyrimidine synthesis involves an electron transport in which dihydroorotate dehydrogenase transfers electrons to an electron transport chain involving ubiquinone, cytochrome and molecular oxygen (Gutteridge et al, 1979). This activity is probably located in the mitochondria and accounts for the microaerophillic requirements of the parasite. Pyrimidine synthesis also requires folates as co-factors (see below).
Many biochemical processes require co-factors which do not directly participate in growth processes as do the bulk nutrients. Instead, vitamins are usually required in smaller amounts and are usually recycled. Pantothenate appears to be the only vitamin not supplied by the erythrocyte (Divo et al, 1985) and is probably needed for the formation of acyl-Coenzyme A which is needed in lipid biosynthesis.
Folate and its derivatives are important co-factors in the synthesis of nucleotides and amino acids and especially for the transfer of methyl (one carbon) groups (see folate metabolism [HG]). Especially important is the role of the dihydrofolate cycle in de novo pyrimidine synthesis. Dihydrofolate is reduced to tetrahydrofolate by dihydrofolate reductase (DHFR). Several anti-malarials, such as pyrimethamine and cycloguanil, preferentially inhibit parasite DHFR. The tetrahydrofolate is methylated by serine hydroxymethyl transferase and the resulting methylene tetrahydrofolate functions as a methyl donor. For example, thymidylate synthase catalyzes the formation of dTMP from dUMP by transferring the methyl group from methylene tetrahydrofolate. During this reaction the methylene tetrahydrofolate is converted back to dihydrofolate, which is then recycled.
Increased folates are needed to accomodate the demand for pyrimidines which are associated with DNA replication. The parasite cannot utilized preformed folate and must synthesize dihydrofolate from GTP, para-aminobenzoic acid and glutamate. Sulfadoxine and other sulfa drugs inhibit the de novo synthesis of dihydrofolate. Fansidar, a combination of sulfadoxine and pyrimethamine, inhibits folate metabolism at two distinct places in the pathway. [See more detail discussion on action of anti-folates.]
Heme is an important component of many enzymes. The parasite cannot utilize the heme that is released as a result of hemoglobin degradation and must synthesize heme de novo. All of the enzymes necessary for the synthesis of heme are found within the parasite's genome (see detailed pathways of heme biosynthesis [HG]). There is some uncertainty about whether heme is synthesized in the mitochondria or the apicoplast (see box).
A bi-product of metabolism and respiration are reactive oxygen intermediates (ROI) such as superoxide, hydroxyl radical and hydrogen peroxide. In particular, the digestion of oxy-hemoglobin results in the production of ROI (see Food Vacuole Figure). These ROI can damage lipids, proteins and nucleic acids and therefore need to be oxidized to oxygen and water. Parasite enyzmes involved in redox metabolism have been identified. Superoxide dismutase (SOD), catalase, and glutathione peroxidase are involved in the detoxification of ROI (see detailed pathways of redox metabolism [HG]). Oxidized glutathione is recycled by glutathione reductase and the reducing equivalents of NADPH are probably generated through the pentose phosphate cycle. Glutamate dehydrogenase is another potential source of NADPH. It has also been proposed that the parasite uses host catalase and SOD within the food vacuole. Interestingly, the malaria parasite may supply the host erythrocyte with glutathione which could participate in protecting the host cell from oxidative damage (Atamna and Ginsburg, 1997). [See more discussion of drugs that affect redox metabolism.]
The malarial parasite is a rapidly growing organism that exhibits a high metabolic rate and has a large demand for small molecular metabolites that will serve as precursors for the synthesis of nucleic acids, proteins and lipids. Thus the host erythrocyte with its rather sluggish metabolism and limited transport capabilities poses some potential problems for the actively growing parasite. The infected erythrocyte exhibits a substantial increase in its permeability to low molecular weight solutes as compared to the uninfected erythrocyte (Ginsburg, 1994). (See figure on alteration of host erythrocyte membrane permeability [HG].) Some of this increase represents higher rates of the endogenous erythrocyte transporters and reflects the high anabolic metabolism of the parasite. (See figure of possible mechanisms [HG].) In addition, new permeation pathways (NPP), which are not found in the uninfected erythrocyte, are observed on the erythrocyte membrane after infection. Much of this increase in permeability can be attributed to single type of permeation pathway with characteristics quite distinct from those of the host erythrocyte (Kirk et al, 1999; Kirk 2004). This new permeation pathway has a broad range of specificity for small molecular weight solutes but demonstrates a preference for neutral compounds and anions. (see Figure, click here for larger image)
Metabolites also need to cross the PVM and the parasite plasma membrane. A channel on the PVM has been described (Desai et al, 1993) and projections from the PVM, called the tubulo-vesicular membrane network (TVM), has been implicated in the acquistion of nutrients (Lauer et al, 1997). Others have proposed a direct connection to the host plasma via a 'parasitophorous duct' (Taraschi and Nicolas, 1994). Presumably the parasite plasma membrane has transporters which are typical of other eukaryotes, as has been described for the hexose (i.e., glucose) transporter (Woodrow et al, 1999; Kirk 2004). (See also figure of various transporters located on the parasite plasma membrane [HG].)