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.

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Sources and fates of metabolites