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Figure 4 | Kinetoplastid Biology and Disease

Figure 4

From: Evolution of energy metabolism and its compartmentation in Kinetoplastida

Figure 4

Comparison of glycolysis in primitive anaerobic organisms and trypanosomatids. The consumption of one glucose molecule through glycolysis generates two molecules of pyruvate, ATP and NADH. To maintain the redox balance (NAD+/NADH ratio) of the cell, NADH produced by GAPDH has to be oxidized to NAD+. In the simplest fermentation scheme, the lactic fermentation (panel A), which probably occurred in the primitive anaerobic organisms, NAD+ is stochiometrically regenerated by converting pyruvate into lactate by lactate dehydrogenase [68]. A variation of this primitive scheme, the "succinic fermentation" (Panel B), is the replacement of the reduction step (lactate dehydrogenase) by one carboxylation step (pyruvate carboxylase) followed by two reduction steps (MDH and FRD). The consequence of this adaptation is that one half of the PEP is converted into succinate to maintain the redox balance, while the other half can be used for biosynthetic pathways. In 1980, Gest [68] proposed that the primitive FRD was soluble and NADH dependent, whereas none of the known present-day FRDs possess both characteristics. Interestingly, the trypanosome glycosomal FRD is soluble and NADH dependent [62]. Panel C shows the succinate product pathway recently characterized in the procyclic form of T. brucei, which ressembles primitive "succinic fermentation". The NAD+, NADH, ADP and ATP molecules produced by this pathway are bold faced and underlined, with a special attention to NAD+/NADH which are circled. For the boxed glycolytic metabolites (G-3-P, 1, 3BPGA, 3-PGA, PEP and/or pyruvate), two molecules are produced per glucose consumed. End products (lactate, succinate and acetate) are in white characters on a black background. The metabolic flux at each enzymic step, necessary to maintain stoichiometric relationships of all reactions in the cell (panels A and B) or compartment (panel C), is tentatively represented by arrows with different thickness. Abbreviations: 1, 3BPGA, 1,3-bisphosphoglycerate; DHAP, dihydroxyacetone phosphate; FBP, fructose 1,6-bisphosphate; F6P, fructose 6-phosphate; G-3-P, glyceraldehyde 3-phosphate; G6P, glucose 6-phosphate; OAA, oxaloacetate; PEP, phosphoenolpyruvate; 3-PGA, 3-phosphoglycerate. Enzymes are: 1, HXK; 2, glucose-6-phosphate isomerase; 3, PFK; 4, aldolase; 5, triosephosphate isomerase; 6, GAPDH; 7, PGK; 8, phosphoglycerate mutase; 9, enolase; 10, PYK; 11, lactate dehydrogenase; 12, pyruvate carboxylase; 13, MDH; 14, fumarase; 15, NADH-dependent FRD; 16, phosphoenolpyruvate carboxykinase.

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