Co-oxidation by cyclooxygenases.

Cyclooxygenases (COXs; prostaglandin H(2) synthases) catalyze the bis-dioxygenation of arachidonic acid (AA) to generate prostaglandin (PG) G(2) followed by the peroxidative cleavage of PGG(2) to yield PGH(2), the precursor to all of the vasoactive PGs. These enzymes utilize a Fe-protoporhyrin IX (heme) co-factor to catalyze peroxide bond cleavage, which puts the Fe at a higher oxidation state (Fe(3+) → Fe(5+)). The heme Fe requires two electrons (e(-)) to return to its resting state (Fe(3+)) for the next round of catalysis. Peroxide bond cleavage thus occurs via compound I and compound II, observed for horseradish peroxidase. To return to Fe(3+), electrons come from "co-reductants" and their subsequent oxidation by the enzyme is known as "co-oxidation". The protocols in this unit are aimed at characterizing this side reaction of COXs.