There are two PGHS isoforms, PGHS-1 and PGHS-2.
PGHS-1, the constitutive isoform, has 599 AA.
PGHS-2, the inducible isoform, has 604 AA.
PGHS-2 has 61% amino acid sequence identity between PGHS-1.
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Click for PGHS-1 peptide sequence

PGHS is homodimeric , heme-containing , and membrane-binding.

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The structure can be subdivided into three folding unit:

• aa34-72: EGF-like module contains 3 intra-domain SS-Bond , and is linked to main body by another SS-Bond
  Function of this EGF-like domain in PGHS is still unclear.

• aa73-116: Membrane-binding domain , comprise 4 helices (A-D) and insert into only a single leaflet of the lipid bilayer
  Click for picture

• aa117-586: Catalytic domain

To see the catalytic site

The cyclooxygenase active site consists of a long, narrow channel (~8 x 25 A) extending from the outer surface of the membrane-binding motif to the centre of the PGHS monomer.
To see the channel (dots on)
, and channel only
Hydrophobic surface of the channel

Tyr 385 is found at the apex of the channel; it sits near the edge of the heme.
Ser 530 , which is known to be acetylated by aspirin, lies just below Tyr 385 at a point where its acetylation could easily block access to the upper part of the channel .

Reference

• Picot, D., Loll, P.J., Garavito, R.M. (1994) The X-ray crystal structure of the membrane protein prostaglandin H2 synthase-1 Nature 367, 243-249
  
• Smith, W.L., Garavito, R.M., DeWitt D.L. (1996) Prostaglandin endoperoxide H synthases (cyclooxygeases)-1 and -2 J. Biol. Chem. 271, 33157-33160