Fig. 27.39 Superposition of arachidonic acid 27.86 (violet) and PGH2 27.88 (gray) in the reaction channel of COX. The heme center to which oxygen is bound is at the top of the right side. Tyr385 (yellow) is responsible for the hydrogen abstraction from C13 of arachidonic acid. The atoms of the protein are largely removed, and the reaction channel is indicated with a transparent surface. The displayed geometry is based on the crystallographically determined complexes of COX with arachidonic acid and PGH2.