## 3 Biosynthesis of Aromatic and Isoprenic Precursors of Prenylquinones in Parasitic Protozoa
### Isoprenic Side Chain Biosynthesis
#### 3.2.2. The Methylerythritol 4-Phosphate Pathway  
Generalities about the Methylerythritol 4-Phosphate Pathway  
The MEP pathway begins with 1-deoxy-d-xylulose 5-phosphate synthase (DXS), a thiamine-dependent enzyme that condenses a glyceraldehyde 3-phosphate molecule with a pyruvate molecule to form 1-deoxy-p-xylulose-5-phosphate (DOXP). Then, the enzyme 1-deoxy-p-xylulose 5-phosphate reductase isomerase (DXR) forms 2-C-methyl-p-erythrol-4-phosphate (MEP). Fosmidomycin was initially identified as a specific inhibitor of the DXR enzyme but is currently known to also have inhibitory activity against other MEP enzymes [110]. In the next step, the enzyme 2-C-methyl-p-erythro-4-phosphate cytidine transferase (MCT) condenses MEP with cytidine triphosphate (CTP). The obtained product is 4-(cytidine-5-diphosphate)-2C-methyl-p-erythritol (CDP-ME). The enzyme 4-(cytidine-5-diphosphate)-2C-methyl-p-erythro-kinase phosphorylase, using an ATP molecule, catalyzes the conversion of CDP-ME to 2-phospho-4 (cytidine 5'-diphosphate)-2-methyl-p-erythritol (CDP-MEP). Subsequently, the enzyme 2C-methyl-p-erythritol-2,4-cyclodextrin diphosphate synthase (MCS) transforms CDP-MEP to 2C-methyl-p-erythritol-2,4-cyclodextrin diphosphate (MEcPP). Then, the enzyme 4-hydroxy-3-methylbut-2-enyl-diphosphate synthase (GcpE) reduces MEcPP to 1-hydroxy-2-methyl-2-(_E_)-butenyl 4-diphosphate (HMBPP). Finally, HMBPP is transformed to IPP/DMAPP by the enzyme hydroxymethylbutenyl diphosphate reductase (LytIB) [96; 111]. The Methylerythritol 4-Phosphate Pathway in Parasitic Protozoa  
The growth inhibitory effect of fosmidomycin on _Plasmodium_ spp., _Babesia_ spp. and _Theileria_ spp. and the detection of DOXP reductoisomerase activity in _P.__falciparum_ protein extracts were the first indicators of the existence of the MEP pathway in apicomplexan parasites [79; 80; 85; 112; 113]. MEP enzymes and intermediates were identified in _P. falciparum_[80; 85], in addition to the inhibitory effects of fosmidomycin in some intermediates and final products of isoprenoid biosynthesis [85; 86]. In other apicomplexan parasites such as _T. gondii_ and _Eimeria tenella_ the MEP pathway has been identified as an isoprenoid source [10; 79; 80; 85; 112; 113; 114; 115]. However, fosmidomycin seems to have little effect on the viability of _Toxoplasma_, _Theileria_ and _Eimeria_[10; 86] and fosmidomycin recrudescence phenomena appear to be frequent in malaria [112]. Therefore, MEP pathway role and fosmidomycin resistance phenomena had been extensively studied in _Plasmodium_ and _Toxoplasma_ rather than other apicomplexan parasites. In _P. falciparum_ it was demonstrated that the antiparasitic activity of fosmidomycin as well as of some antibiotics that cause loss of the apicoplast can be grown indefinitely in asexual blood stage culture if exogenous IPP is proportioned. The authors suggested that isoprenoid precursor biosynthesis is the only essential function of the organelle at least during intraerythrocytic stages [79]. Also, in _Plasmodium_, Fosmidomycin-resistant _P. falciparum_ parasites showed loss of PF3D7_1033400. PF3D7_1033400 encodes a homologue of haloacid dehalogenase-like sugar phosphatases (PfHAD1) which dephosphorylates a variety of sugar phosphates, including glycolytic intermediates [112]. Parasites lacking PfHAD1 have increased MEP pathway metabolites and therefore it was suggested to control substrate availability to the MEP pathway. Focussing in _T. gondii_, knockout of both DOXP reductoisomerase and LytB demonstrated to be lethal for the parasite. However, it was previously commented that fosmidomycin seems to have little effect on the viability of _Toxoplasma_[115]. Furthermore, intracellular stages of _T. gondii_ mutants that lack the enzyme farnesyl diphosphate/geranyldignadher synthase (TgFPP/TgGGPPS) have only a mild growth phenotype and an isoprenoid composition similar to wild type parasites However, extracellular parasites variabilities were clearly affected by the loss of the enzyme.