Prostaglandin D2 (PGD2) is a structural isomer of prostaglandin E2 (PGE2). There is a 9-keto and 11-hydroxy group on PGE2 with these substituents reversed on PGD2. PGD2 is formed by two evolutionarily distinct, but functionally convergent, prostaglandin D synthases: lipocalin-type prostaglandin-D synthase aka Prostaglandin-H2 D-isomerase (PTDGS) and hematopoietic prostaglandin D synthase (HPGDS). One of the main differences between these two proteins is that HPGDS requires glutathione (GSH) for catalysis while PTDGS can function without this cofactor. Here, PTDGS promotes the isomerisation of prostaglandin H2 (PGH2) to prostaglandin D2 (PGD2) (Zhou et al. 2010).