Lipid News

A new hotspot
for cyclooxygenase inhibition

Michael Malkowski
By Michael Malkowski
Jan. 21, 2020

Prostaglandins, or PGs, are critical players in such physiological housekeeping functions as the regulation of renal water and sodium metabolism, blood clotting, parturition, and stomach acid secretion. These potent lipid signaling molecules are derived from the oxygenation of arachidonic acid by prostaglandin endoperoxide H synthases 1 and 2, commonly called cyclooxygenase-1 and -2, or COX-1 and COX-2.

Abnormal changes in PG production contribute to disease pathologies including inflammation and cancer, so researchers view COX-1 and COX-2 as pharmacological targets. Indeed, nonselective nonsteroidal anti-inflammatory drugs, or NSAIDs, such as ibuprofen, inhibit both isoforms, while coxibs such as Celebrex selectively inhibit COX-2.

feb-lipid-news-Primary-900x506-1.jpg
Michael Malkowski
Cyclooxygenase structure. Each COX monomer comprises three domains and is bound by heme (red). Arachidonic acid (yellow) binds within the cyclooxygenase active site in an L-shaped conformation. The side chain of Leu-531 rotates from a closed conformation (blue) to an open conformation (gold) and in conjunction with Arg-513, located at the base of the side pocket, serves as a critical determinant for the inhibition of COX-2 by aspirin and Celebrex.

COX enzymes are sequence homodimers; each subunit consists of physically distinct cyclooxygenase and peroxidase active sites linked by a bridging heme moiety. In solution, the enzymes function as heterodimers, such that only one subunit is catalytically active at a time.

Structurally, COX-1 and COX-2 are virtually identical. However, subtle differences drive isoform-specific substrate specificity and inhibition. Among these are the substitutions of cyclooxygenase channel residues Ile-434, His-513 and Ile-523 in COX-1 for Val-434, Arg-513 and Val-523 in COX-2, which result in the formation of a unique side pocket and increased volume of the cyclooxygenase active site in COX-2. This side pocket, with Arg-513 at its base, has been exploited in the design of coxibs aimed at reducing the gastrointestinal side effects caused by conventional NSAIDs.

Many researchers focus on time-dependent inhibition of COX-2, working to identify the conformational changes that drive the transitions between the kinetically observable inhibitor binding states. These studies originally focused on the COX-2 side pocket, but recent molecular dynamics simulations identified a different region of the active site critical for aspirin and Celebrex to inhibit COX-2. These simulations identified rotation of the side chain of the residue Leu-531, opposite the side pocket, as a way to expand further the volume of the cyclooxygenase channel. Indeed, crystal structures of COX-2 have provided evidence for the existence of alternate rotamer conformations for this side chain when different ligands are bound.

To evaluate the importance of the conformational flexibility of the side chain of Leu-531, our research group generated L531F and L531N mutations and showed that these substitutions reduced aspirin and Celebrex’s inhibition of COX-2. Thus, expansion of the active site volume via the flexibility associated with the side chain of Leu-531 allows Celebrex to achieve a high-affinity binding state and facilitates formation of the initial noncovalent complex in the case of aspirin acetylation.

Deciphering the protein conformational motions associated with COX catalysis and inhibition is the next major frontier in studying this enzyme. If we understand how the binding of substrates, allosteric activators and inhibitors induce conformational motions to modulate COX activity, we will be closer to developing and repurposing drugs for maximum efficacy with minimal risks.

Michael Malkowski
Michael Malkowski

Michael G. Malkowski is a professor and chair of the department of structural biology in the Jacobs School of Medicine and Biomedical Sciences at the State University of New York at Buffalo.
 

Join the ASBMB Today mailing list

Sign up to get updates on articles, interviews and events.

Latest in Science

Science highlights or most popular articles

Meet Stephanie Moon
Observance

Meet Stephanie Moon

Aug. 2, 2021

She studies messenger RNA regulation at the University of Michigan in Ann Arbor.

Researchers find a cell surface decorated with sugar-coated RNAs
News

Researchers find a cell surface decorated with sugar-coated RNAs

Aug. 1, 2021

Finding not just glycoproteins and glycolipids but also glycoRNA means “now there are three hands, and we don’t know what that third hand is doing.”

Unraveling the mysterious mutations that make delta the most transmissible COVID virus yet
News

Unraveling the mysterious mutations that make delta the most transmissible COVID virus yet

July 31, 2021

As of this week, the delta variant had caused at least 92% of the new infections in the United States, according to a research firm in Switzerland.

Cats communicate with the help of bacteria living in their butts
News

Cats communicate with the help of bacteria living in their butts

July 31, 2021

KittyBiome researchers want to study the cat microbiome to improve health and understand scent-based communication.

From the journals: JLR
Journal News

From the journals: JLR

July 29, 2021

Reversing alcoholism’s effects on lipid droplets. How HDL cholesterol might reduce COVID-19 risk. Shining light on the cholesterol–GPCR relationship. Read about recent papers on these topics in the Journal of Lipid Research.

Starved to death: Can dietary methionine combat cancer?
Journal News

Starved to death: Can dietary methionine combat cancer?

July 27, 2021

Scientists draw a connection between this essential amino acid and cancer lipid metabolism.