Starch and galactose metabolism
The pathway of starch and galactose metabolism is a metabolic capability which is absent in Plasmodium falciparum and present in Coccidia. This pathway is constructed with KEGG reference pathways of starch and sucrose metabolism and galactose metabolism as templates. Both KEGG pathways were combined as only small parts of both reference pathways are present in Toxoplasma gondii and there are few enzymes which are common to both pathways.
T. gondii can also store energy in the form of polysaccharides. Unlike Plasmodium, Coccidian species such as Toxoplasma and Eimeria can store carbohydrates in polysaccharide granules. This can be due to the differences in life cycle stages when compared to other apicomplexans. The bradyzoite and sporozoite forms of Coccidians T. gondii and Eimeria tenella store carbohydrates in the form of ‘floridean starch’, a variant of amylopectin which has only been found in red algae previously. It has also been shown that T. gondii and E. tenella prefers UDP-glucose than ADP-glucose as substrate [1, 2]. It has also been shown that the transcription of the Toxoplasma genes encoding the enzymes involved in floridean starch/amylopectin metabolism are stage dependent. The enzymes catalysing catabolic functions such as alpha-glucan phosphorylase, alpha-glucosidase and alpha-amylase are preferentially expressed at the bradyzoite stage and the anabolic enzymes involved in biosynthesis of floridean starch such as starch (amylopectin) synthase and 1,4-alpha-glucan branching enzyme are preferentially expressed in tachyzoites [1]. The gene for bifunctional trehalose phosphate synthase/trehalose phosphatase enzyme was predicted on all Coccidian species and its expression was experimentally confirmed in Cryptosporidium parvum oocysts. It is also suggested as a novel drug target as it is absent in animals [3]. Analysis of the Toxoplasma genome also shows that it can synthesise galactose from UDP-glucose via the action of UDP-glucose 4-epimerase (5.1.3.2) and beta-galactosidase (3.2.1.23).
Enzyme | EC Number | Gene id | Protein localisation | Localisation data source |
---|---|---|---|---|
Aldehyde reductase | 1.1.1.21 | TGME49_208040 | ||
Aldehyde reductase | 1.1.1.21 | TGME49_270100 | ||
Phosphorylase | 2.4.1.1 | TGME49_310670 | Cytosol | Previous publication |
Starch (amylopectin/floridean starch) synthase | 2.4.1.11 | TGME49_222800 | Nucleus | Previous publication |
Bifunctional trehalose-6-phosphate synthase/trehalose phosphatase | 2.4.1.15; 3.1.3.12 | TGME49_297720 | Nucleus | Previous publication |
1,4-alpha-glucan branching enzyme | 2.4.1.18 | TGME49_209960 | Nucleus | Previous publication |
1,4-alpha-glucan branching enzyme | 2.4.1.18 | TGME49_246690 | ||
1,4-alpha-glucan branching enzyme | 2.4.1.18 | TGME49_316520 | Cytosol | Previous publication |
Bifunctional 4-alpha-glucanotransferase/amylo-alpha-1,6-glucosidase | 2.4.1.25; 3.2.1.33 | TGME49_226910 | Cytosol | Previous publication |
4-alpha-glucanotransferase | 2.4.1.25 | TGME49_271210 | ||
1,3-beta-glucan synthase | 2.4.1.34 | TGME49_278110 | ||
Hexokinase | 2.7.1.1 | TGME49_265450 | Cytosol | Apiloc; Previous publication |
Galactokinase | 2.7.1.6 | TGME49_229780 | ||
UTP-glucose-1-phosphate uridylyltransferase | 2.7.7.9 | TGME49_218200 | ||
UTP-glucose-1-phosphate uridylyltransferase | 2.7.7.9 | TGME49_264780 | ||
alpha-amylase | 3.2.1.1 | TGME49_283490 | ||
alpha-galactosidase | 3.2.1.22 | TGME49_309930 | Plasma membrane | Previous publication |
beta-galactosidase | 3.2.1.23 | TGME49_320588 | ||
Glucan endo-1,3-beta-D-glucosidase | 3.2.1.39 | TGME49_270850 | ||
UDP-glucose 4-epimerase | 5.1.3.2 | TGME49_225880 | Mitochondrion | Previous publication |
Glucose-6-phosphate isomerase | 5.3.1.9 | TGME49_283780 | Cytosol | Apiloc; Previous publication |
Phosphoglucomutase | 5.4.2.2 | TGME49_285980 | Cytosol; Apical | Apiloc; Previous publication |
Phosphoglucomutase | 5.4.2.2 | TGME49_318580 | Cytosol; Mitochondrion | Apiloc; Previous publication |
Sources and fates of metabolites
Substrate | Source pathways | Product | Fate pathways |
---|---|---|---|
alpha-D-Glucose-6P | Glycolysis | alpha-D-Glucose | Glycolysis |
alpha-D-Glucose-1P | Glycolysis | beta-D-Fructose | Mannose and fructose metabolism |
beta-D-Fructose-6P | Glycolysis | Trehalose | ? |
Maltose | Host? | Dextrin | ? |
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