Trypanosomes are unicellular protozoa that are ubiquitous parasites of higher eukaryotes, including insects, plants, and mammals. Among the numerous species belonging to the trypanosomatid family, Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. are responsible for Human diseases. Most of these parasites live in more than one host over their life cycle and encounter very different environments, such as insect vectors' gut and vertebrate bloodstream. Consequently, the different parasitic forms have developed distinct morphologies and metabolisms.
We will consider here T. brucei
, which belongs to the group of parasites responsible for sleeping sickness in Africa. T. brucei
belongs to the only group of organisms that performs glycolysis in a peroxisome-like organelle, called glycosome [1
]. It is widely considered that this compartmentalized glycolysis requires impermeability of glycosomal membrane to cofactors, such as NAD(P)+
and NAD(P)H, and nucleotides (ATP, ADP, etc.) [2
]. As a consequence, the intraglycosomal NAD+
/NADH and ATP/ADP balances need to be maintained, which implies that each NAD+
or ATP molecules consumed during the first glycolytic steps have to be regenerated inside the organelle (see ).
Metabolic network of glucose degradation for the bloodstream and procyclic forms of T. brucei.
Panels (a) and 1(b) correspond to the metabolic model of the bloodstream forms of T. brucei (BSF) in the aerobic and anaerobic conditions, respectively. Panel (c) represents the metabolic model for the procyclic form grown in glucose-rich medium. For both forms, the major part of the glycolytic pathway is compartmentalized in glycosomes (peroxisome-like organelles). Excreted end-products from glucose metabolism are in red, green, or purple characters on a grey rectangle as background. In Panels (a) and (b), metabolic branches consuming and regenerating NAD+ are in blue and red, respectively, while the color code in Panel (c) is blue, red, and purple for the acetate, glycosomal succinate, and mitochondrial succinate branches, respectively. NAD+ and ATP molecules are underlined, when consumed in the glycosomes, and boxed, when produced in the glycosomes. In aerobic conditions, BSF converts one molecule of glucose into two molecules of pyruvate with consumption of one molecule of dioxygen (Panel (a)) and net production of two molecules of ATP, while in anaerobic conditions one molecule of pyruvate, glycerol, and ATP is produced per molecule of glucose consumed (Panel (b)). Abbreviations: 1,3BPGA, 1,3-bisphosphoglycerate; DHAP, dihydroxyacetone phosphate; FBP, fructose 1,6-bisphosphate; FUM, fumarate; Gly3P, glycerol 3-phosphate; G3P, glyceraldehyde 3-phosphate; MAL, malate; OAA, oxaloacetate; PEP, phosphoenolpyruvate; PYR, pyruvate; SUC, succinate. Individual enzymes included in the model are 1, hexokinase: 2, glucose-6-phosphate isomerase; 3, phosphofructokinase; 4, aldolase; 5, triose-phosphate isomerase; 6, glyceraldehyde-3-phosphate dehydrogenase; 7, phosphoglycerate kinase; 8, phosphoglycerate mutase; 9, enolase; 10, pyruvate kinase; 11, glycosomal glyceraldehyde-3-phosphate dehydrogenase; 12, FAD-dependent glycerol-3-phosphate dehydrogenase; 13, ubiquinone; 14, SHAM-sensitive alternative oxidase; 15, glycerol kinase; 16, pyruvate phosphate dikinase; 17, pyruvate dehydrogenase complex; 18, acetate:succinate CoA-transferase and acetyl-CoA thioesterase; 19, phosphoenolpyruvate carboxykinase; 20, glycosomal malate dehydrogenase; 21, cytosolic (and glycosomal) fumarase; 22, glycosomal NADH-dependent fumarate reductase; 23, mitochondrial fumarase; 24, mitochondrial NADH-dependent fumarate reductase; 25, cytosolic malic enzyme; 26, mitochondrial malic enzyme.
In the mammalian host, the bloodstream forms of T. brucei (BSF) develop a very simple and well-known glucose-based energy metabolism, with glucose being converted into the pyruvate, which is the only end product excreted in the presence of oxygen (). In aerobiosis, equimolar amounts of pyruvate and glycerol are excreted from glucose metabolism (). In both conditions, all ATP required for the parasite development is produced in by the cytosolic pyruvate kinase (step 10 in ).
In contrast, the procyclic form of T. brucei
(PF), which evolves in the midgut of the insect vector (tsetse fly), develops a more complex branched energy metabolism. When grown in standard rich medium, PF primarily uses glucose to provide the cell with carbon and ATP. In the course of glycolysis, phosphoenol
pyruvate (PEP) is produced in the cytosol, where it is located at a branching point (). It can be converted into pyruvate, which enters the mitochondrion to produce acetate [3
]. PEP can also reenter the glycosomes to be converted to succinate in either the glycosomes or the mitochondrion [5
]. Although the topology of the glucose metabolism network is known for the procyclic form, the flux distribution between the different branches of the network has not been addressed so far.
The main objective of this paper is to propose a bioinformatics analysis, integrating multipurposed experimental data, to investigate the flux distribution in the main branches of glucose metabolism of the PF trypanosomes. To address this question, we developed a model based on (i) the published topology of the metabolic network [7
], (ii) the maintenance of the glycosomal redox (NAD+
/NADH) and (ATP/ADP) balances, with no exchange of these cofactors with other subcellular compartments [7
], and (iii) experimental data.