Carbocations in the synthesis of prostaglandins by the cyclooxygenase of PGH synthase? A radical departure!
Evidence already available is used to demonstrate that although prostaglandin G/H synthase hydroxylates arachidonic acid through radical intermediates, it effects cyclizations through a carbocation center at C-10. This is produced following migration of H to the initial radical at C-13 and a 1epsilon oxidation. Under orbital symmetry control, the cyclizations can give only the ring size and trans stereochemistry actually observed. After cyclization, the H-shift reverses to take the sequence back into current radical theory for hydroxylation at C-15. Thus 10,10-difluoroarachidonic acid cannot be cyclized, although it can be hydroxylated. Acetylation of Ser516 in the isoform synthase-2 is considered to oppose carbocation formation and/or H-migration and so prevent cyclizations while permitting hydroxylations; the associated inversion of chirality at C-15 can then readily be accommodated without the change in conformation required by other schemes. Suicide inhibition occurs when carbocations form stable bonds upon (thermal) contact with adjacent heteroatoms, etc. Because the cyclooxygenase and peroxidase functions operate simultaneously through the same heme, phenol acts as reducing cosubstrate for the cyclooxygenase, thus enabling it to promote PGG2 production and protect the enzyme from oxidative destruction.