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 Cryptosporidium muris genome. The TCA cycle is absent in other Cryptosporidium species such as Cryptosporidium parvum and Cryptosporidium hominis. Comparison of C. muris pathway to the TCA cycle of Toxoplasma gondii shows that C. muris possess one isoform of citrate synthase (2.3.3.1) as opposed to 3 isoenzymes in T. gondii (two with 2.3.3.1 and other with 2.3.3.8 annotations).

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Sources and fates of metabolites