Cyclooxygenases: structural and functional insights

Abstract

Cyclooxygenase (COX; prostaglandin G/H synthase, EC 1.14.99.1) catalyzes the first two steps in the biosynthesis of prostaglandins (PGs). The two COX isoforms COX-1 and COX-2 are the targets of the widely used nonsteroidal anti-inflammatory drugs, indicating a role for these enzymes in pain, fever, inflammation, and tumorigenesis. The ubiquitous constitutive expression of COX-1 and inducible expression of COX-2 have led to the widely held belief that COX-1 produces homeostatic PGs, while PGs produced by COX-2 are primarily pathophysiological. However, recent discoveries call this paradigm into question and reveal as yet underappreciated functions for both enzymes. This review focuses on some of these new insights.

Fig. 1.

The cyclooxygenase reaction. A: The peroxidase cycle leads to abstraction of a hydrogen atom from Tyr-385 forming a tyrosyl radical and activating the cyclooxygenase active site. The tyrosyl radical abstracts the pro-S hydrogen atom from carbon-13 of AA, initiating the cyclooxygenase reaction, the final step of which regenerates the tyrosyl radical. B: Conversion of PGH₂ to the biologically active PGs.

Fig. 2.

Selective COX-2 substrates. The structures of endocannabinoids and free fatty acids that are metabolized more efficiently by COX-2 than by COX-1 are shown.