Scheme 3. Hypothetical location and function of Y385 NS1c tyrosyl radical.

In AA-bound Intermediate II, Y385^· is H-bonded to Y348 (red dashed line) and exhibits a wide doublet EPR lineshape (WD1 radical). Y385^· abstracts hydrogen from AA C13 atom (blue arrow) to generate arachidonyl carbon-centered radical. The arachidonyl radical is then converted to PGG₂· radical through a series of COX reactions including insertion of two oxygen molecules, ring formations and radical shifts (horizontal black arrow). After generation of the arachidonyl radical, the H-bond between Y348 and Y385 is disrupted and Y385 adopts a new conformation ready for hydrogen transfer back from Y385 to C15-OO· of PGG₂· radical (blue arrow) to generate PGG₂ and regenerate Y385^· radical. After the release of PGG₂ and binding of the next AA molecule (tilted black arrows), H-bond between Y348 and Y385^· radical is re-established. The AA-bound PGHS-1 structure is based on crystal structure 1DIY (PDB code) for AA-bound PGHS-1 with Co protoporphyrin IV. The different conformers of Y385 and Y348 in the other two structures are arbitrarily plotted and the chemical structure of PGG₂ is based on the calculated structure 1DD0 (PDB code). All the structures are plotted using PyMOL. Green: carbon; red: oxygen; blue: nitrogen and brown ball: cobalt atom. (The color version of the figure is available in the electronic copy of the article).