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. 2016 May 13;10(5):e0004714. doi: 10.1371/journal.pntd.0004714

Fig 1. Folate metabolism and the thymidylate cycle in T. brucei.

Fig 1

Enzymes catalysing key metabolic steps are: 1, thymidylate synthase (EC 2.1.1.45); 2, dihydrofolate reductase (EC 1.5.1.3); 3, glycine cleavage system (EC 1.4.4.2, 1.8.1.4 and 2.1.2.10; 4 pteridine reductase (EC 1.5.1.33); 5, methionine synthase (EC 2.1.1.13); 6, thymidine kinase (EC 2.7.1.21). These parasites are folate and purine auxotrophs and obtain these essential metabolites from the extracellular medium. Although they have retained the capacity for de novo pyrimidine biosynthesis, they can also salvage extracellular thymidine, thereby by-passing inhibition of the thymidylate cycle. As reported in this manuscript, folic acid is not a substrate for DHFR, but TbPTR1 is reported to display low activity in reducing folate and dihydrofolate (DHF) [5].