Lecture - 6 (1st DDS program)|
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Metabolic adaptations of parasites to parasitism
Biochemical differences between the metabolism of host and parasite are interesting since they are a molecular basis of antiparasitic chemotherapy.
Parasites have different stages in their life cycles and variations in the external conditions force those stages to adapt their energy metabolism to the environment of habitat. The availability of food and oxygen varies widely in the different habitats of parasites. For example, oxygen is virtually absent in bile ducts but in small amount present in the intestine External food supplies can range from abundance in the blood or intestine of a final host to absence for most free-living stages.
Generally, in contrary to mammmals, there are as much as 4 different following ways of ATP generation:
1) anaerobic fermentation (substrate phosphorylation);
2) aerobic respiration, as in mammals, when ATP is generated in a process of oxidative phosphorylation coupled with electron transport;
3) aerobic fermentation as in glycolysis;
4) anaerobic respiration.
In aerobic system, known in mammals, energy is made available to the cell in series of four phases summarized below:
Phase 1- macromolecules from food are broken up to law molecular weight components (this phase provides no energy);
Phase 2 - glucose or other sugars oxidized by glycolytic pathway to the level of CoA-
small amount of energy becomes available during glycolysis (2 ATP);
Phase 3 - the TCA cycle, by which the acetyl moiety of acetyl coenzyme A is oxidized to CO2 and H20 - reducing equivalents (in form of NADH) are generated, which are transferred to the inner membrane of mitochondria;
Phase 4 - the electron transport chain brings about the oxidation of those equivalents In this process oxygen is reduced to water and 32 particles of ATP is synthesized using the energy of the electrochemical gradient.
Classical mammalian electron transport system involves 4 complexes:
• l - a flavin-based NADH dehydrogenase;
• II - flavin-based succinate dehydrogenase;
• III-cyt. b+c1 complex and
• IV- cyt. a+a3 complex.
Electron transfer between complexes I or II and III is mediated by a carrier called ubiquinone. As each of the electron carriers is successively reduced and oxidized, protons are selectively transferred to outer side of the membrane. Proton transfer lowers the pH gradient. The "proton motive force" is released by another protein: F1F0ATP synthase, known as a complex V. It provides a channel for protons to flow back across the membrane to the inner surface.
Considering the metabolism of parasites, one has to remember that it differs between anaerobic parasites (Entamoeba, Giardia, epimastigote forms of Trypanosoma and adult stages of helminths ) and aerobic parasites (trypomastigote forms from a genus Trypanosoma or Leishmania, free-living stages of helminths, migratory larvae of Nematodes).
It is characteristic of parasites metabolism that a portion of their substrates is not fully oxidized. When the succinate is an end product the parasite produces 2 particles of ATP, but when propionate or valerate, it sums up to 5 ATP due to the action of succinate decarboxylase system.
In helminths studied so far glucose is a main respiratory substrate. Glucose is metabolized directly via a glycolytic sequence of reactions, like in mammalian cells, as far as phosphoenolopyruvate (PEP).
In those adult parasitic helminths who use to excreet fatty acids at the level of the enzyme - phosphoenolopyruvate carboxykinase (PEPCK) - CO2 fixation step yields oxaloacetate (OAA) which is reduced to malate (MA). The left part of classical TCA cycle is acting in a reverse direction than in mammalian TCA cycle.
Malate undergoes a dismutation, which means that 2 particles of malate have a different routes: one oxidative and the other one reductive. The oxidative one drives the reductive step.
The reaction of the conversion of PEP into OM by PEPCK is usually encountered in gluconeogenesis in mammals (but in reverse direction).
There are the other important differences between parasitic helminths and mammalian bioenergetic system:
• instead of UQ they possess rhodoquinone (RQ) with considerable more negative than UQ reductive potential (this property favours electron transport in direction of fumarate);
• the presence of enzyme NADH-dependent fumarate reductase (FR) complex that catalyses the reverse reaction than in aerobic organism the TCA cycle enzyme: succinate dehydrogenase (SDH).
Features of SDH-FR complexes:
• SDH (complex II) is expressed for succinate oxidation under aerobic conditions
• FR is expressed for fumarate reduction under anaerobic conditions. The FR and SDH complexes are structurally very similar and each is usually composed of four non-identical subunits. During evolution of anaerobic mitochondria their SDH has been rebuilt to run backwards and to use Q with lower redox potential.
In large parasitic helminths (as for example Ascaris) ETS possess altemative terminal oxidases; there appears to be two branches: one leads to cytochrome oxidase (as complex IV in mammals) but a branch at the level of cyt.b leads to cyt.o (b-type). The product of its reaction with oxygen is H202. The major constituent in Ascaris mitochondria is cyt.b658 and this ETS chain shows also a single phosphorylation site (1 ATP) if the anaerobic route is followed.
Evolutionary reactions between distinct energy generating organells:
• in parasitic helminths the later adaptations to anaerobic functioning must have been accompanied by minor modifications of SDH enzyme to an enzyme that is structurally more related to SDH, but functionally more related to the FRD of prokaryotes;
• such adaptations require also the biosynthesis of RQ; the respective enzymes involved in biosynthesis are still unknown.
Trypanosoma specific pathways as a targets for chemotherapy:
Cyanide-insensitive trypanosome alternative oxidase (TAO) is the terminal oxidase of the respiratory chain of long slender bloodstream forms of African trypanosomes. It does not exist in host, so it seemed to be a good target for antiparasitic drugs. Ubiquinone is an essential component of the G-3-P–oxidase system used in this parasites to re-oxidize of NADH in glycosomes. Group of Kita and co. from Japan looking for the new antitrypanosomal drug discovered unexpectedly the strong inhibitory effect of ASCOFURANONE on G-3-P-oxidase system (Ki in nmoles in comparison to well known drug SURAMIN which Ki is expressed in μM).
The main modifications of classis aerobic mitochondria for energy generation in muscles of body wall in nematode Ascaris suum are as follows:
1) deletation or reduction of nonessential activities (UQ, reduced cytochrome oxidase, citrate synthase or isocitrate deh.);
2) overexpression of key enzymes ( complex II succinate: ubiquinone oxidoreductase);
3) expression of novel components (RQ);
4) altered kinetics of some enzymes.
The metabolism of helminths parasites is not coupled to that of the host as it is in viruses. Parasitic helminths show considerable reduction in both catabolic and anabolic pathways. Adult parasitic helminths appear to have an absolute dependency on carbohydrates for their energy metabolism. Energy metabolism would appear to be essentially anaerobic, although all helminths have cytochromee chains and appear to be capable of oxidative phosphorylation. Although adult parasites lack a number of the anabolic and catabolic pathways found in free-living organisms, these pathways are often present in free-living and intermediate stages of their life cycles. So the genetic information necessary to synthesize, for example the beta-oxidation sequence, is still present in the adults, but not expressed.
Recent reports proved the interesting observation - immune status of the host have a determining effect on parasite metabolism.
1) The metabolism of helminths parasites is not coupled to that of the host as it is in viruses. However, compared with free-living animals, parasitic helminths show considerable reduction in both catabolic and anabolic pathways.
2) Adult parasitic helminths appear to have an absolute dependency on carbohydrate for their energy metabolism and produce a bewildering array of end-products. Energy metabolism would appear to be essentially anaerobic, although all helminths have cytochrome chains and appear to be capable of oxidative phosphorylation.
3) Adult helminths lack what would appear to be a number of basic synthetic pathways, such as the ability to synthesize unsaturated fatty acids, sterols, porphyrins and in some cases purines This has been compensated for by the development of extreemly efficient uptake mechanisms and helminths are capable of high rate of growth and gamete production.
4) Although adult parasites lack a number of the anabolic and catabolic pathways found in free-living organisms, these pathways are often present in the free-living and intermediate stages of the life cycle. So the genetic information necessary to synthesize, for example the beta-oxidation sequence, is still present in the adults, but not expressed.
Fig. 1. Malate dismutation in mitochondria of parasitic helminths
Fig. 2. Respiratory chain in helminths (ETS)