Fig. 27.41The most probable binding mode of acetylsalicylic acid (ASA) 27.94 with COX-1. ASA binds in the middle of the reaction channel (gray surface) that is normally occupied by the natural substrate, arachidonic acid 27.86. The channel spans through the protein with a bent shape from the lower left. It forms a salt bridge with Arg120 and reacts with the OH group of Ser530 by transferring its acetyl group. This blocks the channel irreversibly. The additional volume that the acetyl group blocks is indicated with a violet surface (interior is yellow). The displayed geometry is based on a crystal structure that was determined with a bromine derivative of ASA.