Tricarboxylic acid (TCA) cycle

Tricarboxylic acid cycle also called citrate cycle and Krebs cycle is the third step in aerobic respiration. In eukaryotes, this process takes place in the mitochondrial matrix. It utilises acetyl-CoA from pyruvate oxidation or fatty acid/protein degradation as substrate. In addition, each cycle utilises 2 H2O molecules and generates CO2 and coenzyme-A. This is a cycle of eight enzymes catalysing nine reactions where oxaloacetate, which reacts with acetyl-CoA in the first step, is replenished in the last step. The reverse of this cycle called reverse or reductive Krebs cycle also occurs in some bacteria which utilises CO2 and H2O to synthesise carbon compounds.


Genes for all the enzymes of this pathway are present in the Piroplasma genomes. It has been demonstrated that the TCA cycle is not involved in the energy generation at least in the intracellular schizont stage of Theileria parva. The enzyme assays carried out by Kiama et al showed that the activities of most of the enzymes of TCA cycle are extremely low (0.3 – 1.6 nmol/min/mg protein). The higher activities were seen with malate dehydrogenase (both malate oxidation (55.7 nmol/min/mg protein) and oxaloacetate reduction (15.5 nmol/min/mg protein)), fumarase (23.6 nmol/min/mg protein) and succinate dehydrogenase (6.1 nmol/min/mg protein). This suggests that only a branch of TCA cycle is active in the schizont stage [1]. The anaplerotic reactions in the pyruvate metabolism pathway lead to increased synthesis of oxaloacetate. Therefore, the enzymes malate dehydrogenase, fumerase and succinate dehydrogenase will operate in the reverse direction of TCA cycle, synthesising succinate. This may be due to the accumulation of oxaloacetate. It was also speculated that succinate might not be the end product and the formation of succinate is coupled to electron transport chain and pyrimidine synthesis as in Plasmodium falciparum. The investigations with radio-labelled substrates and enzyme assays in uninfected erythrocytes, erythrocytes infected with Babesia rodhaini and freed parasites also demonstrated the absence of complete TCA cycle. Incorporation of 14C-labelled fumarate and malate and high levels of malate dehydrogenase were also observed in B. rodhaini. There was no evidence of either succinate synthesis or metabolism observed with radio-labelling. In addition, the activity of succinate dehydrogenase enzyme was not detectable. This study also suggested that the pathway from oxaloacetate to 2-oxoglutarate generation is functional [2]. Despite the presence of all enzymes of TCA cycle in P. falciparum, functional pathway is absent at least in the asexual stages. Therefore, Plasmodium does not require TCA cycle for energy generation [3] and depends on glycolysis and fermentation alone for energy needs [4]. The evidence above suggests the same for asexual stages of Piroplasms. The need for functional TCA cycle in the vector stages of Piroplasma and Plamodia is not yet known.


Enzyme EC Number Gene id
Isocitrate dehydrogenase TA10850
Malate:quinone oxidoreductase TA18100
Oxoglutarate dehydrogenase E1 subunit (part of oxoglutarate dehydrogenase complex) TA05275
Succinate dehydrogenase TA03455
Succinate dehydrogenase TA19430
Dihydrolipoamide dehydrogenase (part of oxoglutarate dehydrogenase complex) TA03445
Dihydrolipoamide S-succinyl transferase (part of oxoglutarate dehydrogenase complex) TA19690
Citrate (si)-synthase TA14450
Fumarate hydratase TA03430
Aconitate hydratase TA17020
Succinate-CoA ligase TA10625
Succinate-CoA ligase TA02815
Acetyl-CoA transporter none TA03590
Mitochondrial carrier protein none TA04120
Oxoglutarate/malate translocator none TA13260


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Sources and fates of metabolites


Substrate Source pathways Product Fate pathways
CoA CoA biosynthesis Malate Host
Glutamate Glutamate metabolism Acetyl-CoA Pyruvate metabolism
2-oxoglutarate Pyruvate metabolism