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 (126.96.36.199) as opposed to 3 isoenzymes in T. gondii (two with 188.8.131.52 and other with 184.108.40.206 annotations).
|Enzyme||EC Number||Gene id|
|Oxoglutarate dehydrogenase E1 subunit||220.127.116.11||CMU_035820|
|Dihydrolipoamide S-succinyl transferase||18.104.22.168||CMU_020490|
|Mitochondrial carrier protein||none||CMU_025080|
Sources and fates of metabolites
|Substrate||Source pathways||Product||Fate pathways|
|Acetyl-CoA||Pyruvate metabolism, Fatty acid elongation in the cytosol, Fatty acid elongation in the ER|