Title : Substrate-selective Inhibition of Cyclooxygeanse-2 by Fenamic Acid Derivatives Is Dependent on Peroxide Tone
Abstract :
- Cyclooxygenase-2 ( COX-2 ) catalyzes the oxygenation of arachidonic acid (AA) and endocannabinoid substrates, placing the enzyme at a unique junction between the eicosanoid and endocannabinoid signaling pathways
- COX-2 is a sequence homodimer , but the enzyme displays half-of-site reactivity, such that only one monomer of the dimer is active at a given time
- Certain rapid reversible, competitive nonsteroidal anti-inflammatory drugs (NSAIDs) have been shown to inhibit COX-2 in a substrate-selective manner, with the binding of inhibitor to a single monomer sufficient to inhibit the oxygenation of endocannabinoids but not arachidonic acid
- The underlying mechanism responsible for substrate-selective inhibition has remained elusive
- We utilized structural and biophysical methods to evaluate flufenamic acid, meclofenamic acid, mefenamic acid, and tolfenamic acid for their ability to act as substrate-selective inhibitors
- Crystal structures of each drug in complex with human COX-2 revealed that the inhibitor binds within the cyclooxygenase channel in an inverted orientation, with the carboxylate group interacting with Tyr-385 and Ser-530 at the top of the channel
- Tryptophan fluorescence quenching, continuous-wave electron spin resonance, and UV-visible spectroscopy demonstrate that flufenamic acid, mefenamic acid, and tolfenamic acid are substrate-selective inhibitors that bind rapidly to COX-2 , quench tyrosyl radicals, and reduce higher oxidation states of the heme moiety
- Substrate-selective inhibition was attenuated by the addition of the lipid peroxide 15-hydroperoxyeicosatertaenoic acid
- Collectively, these studies implicate peroxide tone as an important mechanistic component of substrate-selective inhibition by flufenamic acid, mefenamic acid, and tolfenamic acid
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