Schematic representation of the pathways of fatty acid β-oxidation (A), ether phospholipid synthesis (B) and purine catabolism (C) in peroxisomes. (A) Peroxisomes degrade fatty acids in four consecutive steps: (1) oxidation, (2) hydration, (3) dehydrogenation, and (4) thiolytic cleavage. Step 1 is performed by multiple acyl-CoA oxidases (1-n) with different substrate specificities. Steps 2 and 3 involve two bifunctional proteins (L-BP, D-BP) harbouring both enoyl-CoA hydratase and 3-hydroxy-acyl-CoA dehydrogenase activity. Step 4 requires different thiolases (e.g., ACAA1, SCPx). Peroxisomes have acquired a set of accessory enzymes (e.g., for fatty acid α-oxidation) to transform the acyl-CoA esters of those fatty acids (FA), which cannot directly enter the β-oxidation pathway (e.g., phytanic acid) (see text for details). (B) The first three steps of ether phospholipid synthesis take place in peroxisomes; synthesis continues in the endoplasmic reticulum (ER). (C) Enzymes involved in purine catabolism in different vertebrate groups and their excretion products. Note that the cellular localisation (cytosolic, peroxisomal) of these enzymes varies amongst vertebrate species. Xanthine oxidase is cytosolic (Cyt) in H. sapiens (Hs) and D. rerio (Dr). Urate oxidase, HIUase, OHCU decarboxylase, and allantoicase are supposed to localise to peroxisomes (PO) in D. rerio, but are absent in H. sapiens (asterisks) (see also Table 1). Allantoinase is cytosolic in D. rerio, but absent in H. sapiens (asterisks). AADHAPR, alkyl-dihydroxyacetone phosphate reductase; ADHAPS, alkyl-dihydroxyacetone phosphate synthase; DHAP, dihydroxyacetone phosphate; DHAPAT, dihydroxyacetone phosphate acyltransferase; FAR1/2, fatty acyl-CoA reductases 1/2; HIUase, 5-hydroxyisourate hydrolase; OHCU decarboxylase, 2-oxo-4-hydroxy-4-carboxy-5-ureidoimidazoline decarboxylase (adapted from Islinger et al., 2010).
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