Starch metabolism

The pathway of starch metabolism is a metabolic capability which is present in Coccidia including Cryptosporidia and absent in Plasmodium and Piroplasma species. The Coccidian parasites such as Toxoplasma gondii and Neospora caninum also possess a branch of galactose metabolism, which is absent in Cryptosporidia. Therefore, the starch and galactose metabolism pathway of T. gondii is shortened to starch metabolism for Cryptosporidium species.


Coccidia parasites including Cryptosporidium species can store energy in the form of polysaccharides. Unlike Plasmodium, these parasites can store carbohydrates in polysaccharide granules. This can be due to the differences in life cycle stages when compared to other apicomplexans and the stress they undergo during sporozoite stage in the external environment. 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]. The gene for bifunctional trehalose phosphate synthase/trehalose phosphatase enzyme was predicted on all Coccidia 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 Cryptosporidium genomes also shows that they possess the genes for the enzymes UDP-glucose 6-dehydrogenase and UDP-glucuronate decarboxylase and it is suggestive of their ability to produce UDP-xylose.


Enzyme EC Number Gene id
UDP-glucose 6-dehydrogenase cgd8_920
Phosphorylase cgd6_2450
Starch synthase cgd5_3140
Bifunctional trehalose-6-phosphate synthase/trehalose phosphatase; cgd8_4940
1,4-alpha-glucan branching enzyme cgd4_2100
1,4-alpha-glucan branching enzyme cgd6_3280
1,4-alpha-glucan branching enzyme cgd6_3750
Bifunctional 4-alpha-glucanotransferase/amylo-alpha-1,6-glucosidase; cgd6_880
Hexokinase cgd6_3800
UTP-glucose-1-phosphate uridylyltransferase cgd4_810
UTP-glucose-1-phosphate uridylyltransferase cgd7_1830
UDP-glucuronate decarboxylase cgd2_1900
UDP-glucose 4-epimerase cgd4_2600
Glucose-6-phosphate isomerase cgd2_3200
Phosphoglucomutase cgd2_3260
Phosphoglucomutase cgd2_3270


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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? UDP-D-Xylose ?