Glycolysis

Glycolysis (glycose+lysis= breaking of glucose) is a sequence of 10 definite reactions which leads to breaking of glucose into pyruvate. This is the first of four steps of aerobic respiration. This cytosolic metabolic pathway produces a net gain of 2 ATPs. This step is also common to anaerobic respiration as it does not require O₂. In anaerobic respiration, pyruvate will then be converted to lactate (e.g. skeletal musles) or ethanol and carbon dioxide (e.g. yeast) to re-oxidise electron carrier NADH to provide NAD⁺ for glycolysis.

The apicomplexan Toxoplasma gondii possesses all the ten enzymes of glycolysis in the genome. It also possesses 2 lactate dehydrogenase isoforms (converts pyruvate to lactate). The lactate dehydrogenases are expressed in stage specific manner, LDH1 is tachyzoite specific and LDH2 is bradyzoite specific [1, 2, 3]. These LDH isoforms also showed differences in substrate specificities between them and in comparison to Plasmodium LDH. They also showed differences in sensitivities to LDH inhibitors such as gossypol [4, 5]. In addition, it has been mentioned that the comparison of X-ray crystal structure of LDH1 with human muscle and heart-specific isoforms shows significant differences in residues in active site. This suggests it will make a good drug target for encephalitis and other complications of Toxoplasmosis in immuno-compromised patients [5]. The absence of two subunits of ATP synthase and the presence of lactate dehydrogenase in P. falciparum suggests that it relies mainly on glycolysis for ATP generation. It has also been demonstrated in intra-erythrocytic stage P. falciparum that all the glucose consumed are converted to fermentation products [6]. Although electron transport chain is functional and essential for ATP production at least in tachyzoite [7], it is not exactly known of the role of different metabolic pathways in ATP generation in T. gondii. The glycolysis and anaerobic fermentation may also be the main source of ATP in other life cycle stages.

The enzymes NAD-dependent glycerol-3-phosphate dehydrogenase and FAD-dependent glycerol-3-phophate dehydrogenase which catalyse bidirectional conversion of glycerol-3-phosphate to glycerone phosphate were added to glycolysis pathway in MPMP. This is because the reaction catalysed by these  enzymes will lead to increased glycolytic intermediate glycerone phosphate, which will increase pyruvate and ATP production via the actions of triose phosphate isomerise and enzymes downstream of it. The above mentioned enzymes and other enzymes added in MPMP glycolysis pathway such as aldehyde reductase, phosphoglucomutase and acylphosphatase are also present in T. gondii and added to the pathway here. In addition, gluconeogenesis enzyme, fructose bisphosphatase (absent in P. falciparum) is present in T. gondii and added to the glycolysis pathway. The enzyme glycerol kinase, present in P. falciparum glycolysis pathway is absent in Toxoplasma genome. 

The three dimensional models and phylogeny analysis showed that the enzymes enolase and glucose-6-phosphate isomerise are related to plant homologues rather than animal enzymes suggesting that they can also be possible drug targets [8]. The enolase isoform ENO2 is tachyzoite specific and ENO1 is bradyzoite specific. The labelling experiments showed more intense staining of both isoforms in nucleus compared to cytosol in proliferating phases of all life cycle stages. When compared to cytosol, weaker nuclear staining was observed in mature phases suggesting the nuclear role of enolase in regulation of transcription/cell division in addition to its role in glycolysis [3]. The same has also been observed with P. falciparum enolase, which also localises to nucleus, vacuole and merozoite surface in addition to cytosol [9].

The pyrophosphate-dependent activity of phosphofructokinase enzyme was observed in three Apicomplexa parasites, T. gondii, Eimeria tenella and Cryptosporidium parvum in the early nineties. The Toxoplasma enzyme was identified and purified to near homogeneity from extracts and the enzymes assays were carried out identify substrate specificities and kinetic properties [10]. This enzyme catalyses the reaction in both forward and reverse directions with pyrophosphate as co-substrate. None of the nucleoside phosphates including ATP was able to act as either phosphate donor or effector for the enzyme. The enzyme activity is dependent on Mg(2+) ions. This pyrophosphate specific enzyme was also characterised in other apicomplexans Eimeria [11] and Cryptosporidium [12]. All showed activity with pyrophosphate and not with ATP. These protozoan enzymes are homodimers and are not allosterically regulated.

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Sources and fates of metabolites