Figure 2.

Biosynthesis of PGE₂ in algae and in mammals. The canonical pathway in mammals follows the thick red arrows. Arachidonic acid is cyclized by COX‐1 or COX‐2 to PGG₂, which is reduced by the peroxidase activity of the COX to PGH₂ and subsequently converted by prostaglandin E synthase to PGE₂. An alternative pathway was discovered in the red alga Gracilaria vermiculophylla (thick green arrows). There PGG₂ is transformed into the novel intermediate 15‐hydroperoxy‐PGE₂, which is enzymatically or nonenzymatically transformed to PGE₂. Upon re‐investigation of the biosynthesis in human macrophages, the alternative route via 15‐hydroperoxy‐PGE₂ was established in mammals as well (thin red arrows).32