THE STRUCTURE OF PROSTAGLANDIN SYNTHASE: A MEMBRANE-BOUND ENZYME. R. Michael Garavito, Department of Biochemistry, Michigan State University, East Lansing, MI 48824-1319.
Prostaglandin H synthase (PGHS) is an integral membrane enzyme which converts arachidonic acid, an essential fatty acid, into the prostaglandin precursors PGG2 and PGH2 by means of a free radical mechanism. Nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit prostanoid biosynthesis by targeting the cyclooxygenase activity of PGHS, are used to treat certain symptoms of inflammatory and cardiovascular diseases as well as cancer; in the latter, aspirin is now a proven anti-cancer prophylaxis. Two isoforms of PGHS have been discovered: PGHS-1 is involved in homeostatic or "house-keeping" prostaglandin biosynthesis while PGHS-2 induced by cytokines during inflammatory events.
We have refined the structure of ovine PGHS-1 to 3.1 Å resolution and have characterized PGHS-1 complexed with four NSAIDs: bromoaspirin, flurbiprofen, iodosuprofen and iodoindomethacin. I will also discuss the nature of enzyme-drug interactions and structure-function relationships, with particular focus on the mechanisms of substrate and NSAID interactions as well as the functional differences in NSAID binding between PGHS-1 and PGHS-2.
An unexpected conclusion from the crystal structure of PGHS-1 is that it is a monotopic membrane protein: a symmetric dimer of PGHS apparently integrates only into one leaflet of the lipid bilayer. Moreover, there is increasing evidence that specific sites on the PGHS molecule interact with other proteins in the lumen of the endoplasmic reticulum (ER), particularly those involved with membrane protein targeting. The nature of integration of PGHS into the membrane bilayer and the mode of targeting the enzyme to the ER membrane and nuclear envelope will be major points of discussion.
Support from the American Cancer Society is gratefully acknowledged