Fig 1: Prostanoid biosynthesis and signalling pathways. Arachidonic acid (AA) is converted by PTGS1 and PTGS2 to PGG2 and PGH2. PGH2 is then converted into one of the active prostanoids by specific terminal synthases such as PGE synthases (PTGES, PTGES2 and PTGES3), PGF synthases (AKR1B1 and AKR1C3), PGD synthases (PTGDS, HPGDS), PGI synthase (PTGIS) and thromboxane synthase (TBXAS1). PGE2 and PGF2α may also be synthesized by interconversion by PGF2α isomerases AKR1C1 and AKR1C2. PGF2α may also be synthesized by conversion of PGD2 by AKR1C3 or PGE2 by CBR1. Synthesized prostanoids cross the plasma membrane via a carrier mediated process and bind to their respective cell surface receptors. Prostanoids may also be actively transported into the cytosol for degradation to inactive metabolites (PGEM and PGFM) by HPGD and CBR1. For abbreviations see Table I.