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J. Biol. Chem., Vol. 275, Issue 51, 40407-40415, December 22, 2000

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Arachidonic Acid and Nonsteroidal Anti-inflammatory Drugs Induce Conformational Changes in the Human Prostaglandin Endoperoxide H₂ Synthase-2 (Cyclooxygenase-2)^*

Timothy Smith, John McCracken^§, Yeon-Kyun Shin^¶, and David DeWitt

From the  Department of Biochemistry and the ^§ Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 and the ^¶ Department of Biochemistry and Biophysics, Iowa State University, Ames, Iowa 50011

By using the technique of site-directed spin labeling combined with EPR spectroscopy, we have observed that binding of arachidonic acid and nonsteroidal anti-inflammatory drugs induces conformational changes in the human prostaglandin endoperoxide H₂ synthase enzyme (PGHS-2). Line shape broadening resulting from spin-spin coupling of nitroxide pairs introduced into the membrane-binding helices of PGHS-2 was used to calculate the inter-helical distances and changes in these distances that occur in response to binding various ligands. The inter-residue distances determined for the PGHS-2 holoenzyme using EPR were 1-7.9 Å shorter than those of the crystal structure of the PGHS-2 holoenzyme. However, inter-helical distances calculated and determined by EPR for PGHS-2 complexed with arachidonic acid, flurbiprofen, and SC-58125 were in close agreement with those obtained from the cognate crystal structures. These results indicate that the structure of the solubilized PGHS-2 holoenzyme measured in solution differs from the crystal structure of PGHS-2 holoenzyme obtained by x-ray analysis. Furthermore, binding of ligands induces a conformational change in the holo-PGHS-2, converting it to a structure similar to those obtained by x-ray analysis. Proteolysis protection assays had previously provided circumstantial evidence that binding of heme and non-steroidal anti-inflammatory drugs alters the conformation of PGHS, but the present experiments are the first to directly measure such changes. The finding that arachidonate can also induce a conformational change in PGHS-2 was unexpected, and the magnitude of changes suggests this structural flexibility may be integral to the cyclooxygenase catalytic mechanism.

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