Vitamin and cofactor biosynthesis pathways in Plasmodium and other apicomplexan parasites

Abstract

Vitamins are essential components of the human diet. By contrast, the malaria parasite Plasmodium falciparum and related apicomplexan parasites synthesize certain vitamins de novo, either completely or in parts. The various biosynthesis pathways are specific to different apicomplexan parasites and emphasize the distinct requirements of these parasites for nutrients and growth factors. The absence of vitamin biosynthesis in humans implies that inhibition of the parasite pathways might be a way to interfere specifically with parasite development. However, the roles of biosynthesis and uptake of vitamins in the regulation of vitamin homeostasis in parasites needs to be established first. In this article, the procurement of vitamins B(1), B(5) and B(6) by Plasmodium and other apicomplexan parasites is discussed.

Figure 1. Vitamin B1 biosynthesis

The left panel shows the biosynthesis of 4-methyl-5-(2-phosphoethyl)-thiazole (THZ-P). Plants and E. coli use 1-deoxyxylulose-5-phosphate (DOXP), tyrosine (Tyr) and cysteine (Cys) as precursors whereas Bacillus subtilis and yeast use a pentulose, glycine (Gly) and cysteine (Cys). In order to generate THZ-P, a number of enzymatic reactions are required and the best investigated systems are the bacterial pathways whereas the eukaryotic pathways are still poorly understood. In bacteria it is established that the ubiquitin-like protein ThiS-CoSH acts as sulphur carrier; ThiF catalyses the adenylation of ThiS; NifS or IscS transfer sulphur from cysteine to ThiS; in B. subtilis ThiO oxidises glycine and ThiG catalyses the formation of the thiazole phosphate ring , . The only enzyme shown to be involved in THZ-P biosynthesis in eukaryotes is Thi4 , . On the right, the biosynthesis of 4-amino-2-methyl-5-hydroxymethyl pyrimidine diposphate (HMP–PP) is shown. Again different substrates are used in prokaryotes and eukaryotes. Bacteria generate HMP-PP from an intermediate of purine biosynthesis (AIR; 5-amino-imidazole ribonucleotide) whereas yeast uses pyridoxine and histidine (His) as precursors , . The enzymes that catalyse the reactions that lead to the formation of HMP-PP are ThiD (4-amino-2-methyl-5-hydroxymethyl pyrimidine kinase) and possibly pyridoxal kinase (PdxK). Red letters indicate that a gene has been identified in P. falciparum whereas black letters mean that orthologous genes have not been found in the Plasmodium genome databases. Abbreviations used: AIR, 5-amino-imidazole ribonucleotide; Cys, cysteine; Gly, glycine; DOXP, 1-deoxy-D-xylulose-5-phosphate; His, histidine; HMP, 4-amino-2-methyl-5-hydroxymethyl pyrimidine; HMP-P, 4-amino-2-methyl-5-hydroxymethyl pyrimidine phosphate; HMP-PP, 4-amino-2-methyl-5-hydroxymethyl pyrimidine diphosphate; NMT1, no message in thiamine; THZ, thiazole; THZ-P, 4-methyl-5-(2-phosphoethyl)-thiazole; THI, thiamine; THI-P, thiamine phosphate; THI-PP, thiamine pyrophosphate; ThiC, thiC gene product; ThiD, 4-amino-2-methyl-5-hydroxymethyl pyrimidine kinase; ThiE, thiamine phosphate synthase; ThiM, 4-methyl-5-β-hydroxyethylthiazole kinase TPK, thiamine diphosphokinase. This figure was generated following Bodzech and Ginsburg and http://sites.huji.ac.il/malaria/.

Figure 2. Vitamin B5/CoA biosynthesis

Genes encoding proteins potentially synthesising pantothenate from valine and aspartate were identified in T. gondii when searching the ApiDB database (www.ApiDB.org). P. falciparum and C. parvum/hominis do not possess the ability to generate vitamin B₅. All three apicomplexans contain genes encoding for the enzymes necessary for CoA biosynthesis. In T. gondii PPAT and DPCK form a bifunctional protein whereas in Plasmodium and Cryptosporidium these reactions are catalysed by distinct enzymes. Abbreviations used: BCAT, branched-chain amino acid transaminase; PanB, 3-methyl-2-oxobutanoate hydroxymethyltransferase; PanE, 2-dehydropantoate 2-reductase; PanC, pantothenate synthase; PanK, pantothenate kinase; PPCS, phosphopantothenoyl synthase; PPCDC, phosphopantothenoyl cysteine decarboxylase; PPAT, pantetheine-phosphate adenylyltransferase; DPCK, de-phospho-CoA kinase.

Figure 3. Vitamin B₆ metabolism

The expression vitamin B₆ comprises pyridoxal, pyridoxine and pyridoxamine and their phosphate esters. In eukaryotes such as fungi and plants and most bacteria (except the γ-subdivision of proteobacteria including E. coli) pyridoxal-phosphate (PLP) is synthesised by the enzyme complex Pdx1/Pdx2 from ribose or ribulose-5-phosphate, glyceraldehyde-3-phosphate and glutamine. Crystal structures of both proteins have been solved and they reveal that Pdx1 forms a doughnut-like hexameric structure, which binds the glutamine amidotransferase Pdx2 to form the PLP-synthase complex , , , , . Regulation of PLP levels is crucial and can be achieved in several ways. PLP is supplied by biosynthesis, oxidation from pyridoxamine phosphate or pyridoxine phosphate as well as from pyridoxal by phosphorylation. Whether apicomplexan parasites can convert pyridoxamine phosphate and pyridoxine phosphate into PLP is not clear, because a gene encoding an ortholog of pyridoxmaine/pyridoxine phosphate oxidase (PdxH) was not unambiguously identified in the Plasmodium genome (question mark in Figure). Therefore this figure deviates from that published on http://sites.huji.ac.il/malaria/. Thus it is possible that PLP levels in apicomplexans may solely depend on PLP biosynthesis, pyridoxal uptake and phosphorylation as well as on the degree of PLP catabolism. Abbreviations used: PdxH, pyridoxamine/pyridoxine phosphate oxidase; PdxK, pyridoxal kinase.