CYCLOOXYGENASE-1 ACTIVE SITE

The cyclooxygenase active site is a channel approximately 8 x 25 angstroms (3). For ease of describing the active site, the molecule shown is a monomer of COX-1. The mouth of the active site is formed by the four amphipathic helices bound to the lipid membrane. Defining the entrance to the channel makes it much easier to orient oneself to the cyclooxygenase active site. The apex of the channel, near the heme , is considered the upper portion of the channel.

The stereochemically reasonable model for cyclooxygenase is as follows: arachadonic acid enters the active site in a bent conformation with C-13 in the vicinity of Tyr 385 . Arg 120, being the only polar residue in the channel besides Glu 524, is the ligand for the carboxyl group of the substrate. This model allows the production of a radical on C-13 by the stereospecific removal of the pro-S hydrogen, allowing molecular oxygen to be inserted at C-11. This radical initiates the production of 9,11-endoperoxide into which another molecule of oxygen is added. PGG2 results from the donation of hydrogen back onto the molecule. Click here to review the structure the PGG2.

Aspirin acetylates Ser 530 in COX-1 (Ser 516 of Cox-2) (5). This covalent modification occurs just below Tyr 385 blocking access of arachodonic acid into the upper portion of the channel.

Comparisons between COX-1 and COX-2 show that the two isoforms are 63% identical and 77% similar at the amino acid level (6). The residues involved in heme binding and catalysis are highly conserved. The majority of differences occur in the mouth of the channel, but seem to have little effect on the selectivity of NSAIDs.

Many NSAIDs are competitive inhibitors, while some cause a time-dependent inhibition. The possibility of many different binding sites within the channel suggests that many subsites may exist for drug binding (3). Identifying the subsite differences between the two isoforms will be essential in creating NSAIDs that are COX-2 specific.

HoHomeme