Structural characterization of arachidonyl radicals formed by aspirin-treated prostaglandin H synthase-2

Cyclooxygenase-2 Inhibitors as a Therapeutic Target in Inflammatory Diseases

Inflammation plays a crucial role in the development of many complex diseases and disorders including autoimmune diseases, metabolic syndrome, neurodegenerative diseases, and cardiovascular pathologies. Prostaglandins play a regulatory role in inflammation. Cyclooxygenases are the main mediators of inflammation by catalyzing the initial step of arachidonic acid metabolism and prostaglandin synthesis. The differential expression of the constitutive isoform COX-1 and the inducible isoform COX-2, and the finding that COX-1 is the major form expressed in the gastrointestinal tract, lead to the search for COX-2-selective inhibitors as anti-inflammatory agents that might diminish the gastrointestinal side effects of traditional non-steroidal anti-inflammatory drugs (NSAIDs). COX-2 isoform is expressed predominantly in inflammatory cells and decidedly upregulated in chronic and acute inflammations, becoming a critical target for many pharmacological inhibitors. COX-2 selective inhibitors happen to show equivalent efficacy with that of conventional NSAIDs, but they have reduced gastrointestinal side effects. This review would elucidate the most recent findings on selective COX-2 inhibition and their relevance to human pathology, concretely in inflammatory pathologies characterized by a prolonged pro-inflammatory status, including autoimmune diseases, metabolic syndrome, obesity, atherosclerosis, neurodegenerative diseases, chronic obstructive pulmonary disease, arthritis, chronic inflammatory bowel disease and cardiovascular pathologies.

Residual cyclooxygenase activity of aspirin-acetylated COX-2 forms 15 R-prostaglandins that inhibit platelet aggregation

Aspirin (acetylsalicylic acid) inhibits prostaglandin (PG) synthesis by transfer of its acetyl group to a serine residue in the cyclooxygenase (COX) active site. Acetylation of Ser530 inhibits catalysis by preventing access of arachidonic acid substrate in the COX-1 isoenzyme. Acetylated COX-2, in contrast, gains a new catalytic activity and forms 15 R hydroxy-eicosatetraenoic acid (15 R-HETE) as alternate product. Here we show that acetylated COX-2 also retains COX activity, forming predominantly 15 R-configuration PGs (70 or 62% 15 R, respectively, determined using radiolabeled substrate or LC-MS analysis). Although the K_m of arachidonic acid for acetylated COX-2 was ∼3-fold lower than for uninhibited COX-2, the catalytic efficiency for PG formation by the acetylated enzyme was reduced 10-fold due to a concomitant decrease in V_max. Aspirin increased 15 R-PGD₂ but not 15 R-PGE₂ in isolated human leukocytes activated with LPS to induce COX-2. 15 R-PGD₂ inhibited human platelet aggregation induced by the thromboxane receptor agonist U46,619, and this effect was abrogated by an antagonist of the DP1 prostanoid receptor. We conclude that acetylation of Ser530 in COX-2 not only triggers formation of 15 R-HETE but also allows oxygenation and cyclization of arachidonic acid to a 15 R-PG endoperoxide. 15 R-PGs are novel products of aspirin therapy via acetylation of COX-2 and may contribute to its antiplatelet and other pharmacologic effects.-Giménez-Bastida, J. A., Boeglin, W. E., Boutaud, O., Malkowski, M. G., Schneider, C. Residual cyclooxygenase activity of aspirin-acetylated COX-2 forms 15 R-prostaglandins that inhibit platelet aggregation.

Crystal Structure of Aspirin-Acetylated Human Cyclooxygenase-2: Insight into the Formation of Products with Reversed Stereochemistry

Aspirin and other nonsteroidal anti-inflammatory drugs target the cyclooxygenase enzymes (COX-1 and COX-2) to block the formation of prostaglandins. Aspirin is unique in that it covalently modifies each enzyme by acetylating Ser-530 within the cyclooxygenase active site. Acetylation of COX-1 leads to complete loss of activity, while acetylation of COX-2 results in the generation of the monooxygenated product 15(R)-hydroxyeicosatetraenoic acid (15R-HETE). Ser-530 has also been shown to influence the stereochemistry for the addition of oxygen to the prostaglandin product. We determined the crystal structures of S530T murine (mu) COX-2, aspirin-acetylated human (hu) COX-2, and huCOX-2 in complex with salicylate to 1.9, 2.0, and 2.4 Å, respectively. The structures reveal that (1) the acetylated Ser-530 completely blocks access to the hydrophobic groove, (2) the observed binding pose of salicylate is reflective of the enzyme-inhibitor complex prior to acetylation, and (3) the observed Thr-530 rotamer in the S530T muCOX-2 crystal structure does not impede access to the hydrophobic groove. On the basis of these structural observations, along with functional analysis of the S530T/G533V double mutant, we propose a working hypothesis for the generation of 15R-HETE by aspirin-acetylated COX-2. We also observe differential acetylation of COX-2 purified in various detergent systems and nanodiscs, indicating that detergent and lipid binding within the membrane-binding domain of the enzyme alters the rate of the acetylation reaction in vitro.

Cyclooxygenase reaction mechanism of PGHS--evidence for a reversible transition between a pentadienyl radical and a new tyrosyl radical by nitric oxide trapping

Incubation of prostaglandin H synthase-1 (PGHS-1) under anaerobic conditions with peroxide and arachidonic acid leads to two major radical species: a pentadienyl radical and a radical with a narrow EPR spectrum. The proportions of the two radicals are sensitive to temperature, favoring the narrow radical species at 22 °C. The EPR characteristics of this latter radical are somewhat similar to the previously reported narrow-singlet tyrosine radical NS1a and are insensitive to deuterium labeling of AA. To probe the origin and structure of this radical, we combined EPR analysis with nitric oxide (NO) trapping of tyrosine and substrate derived radicals for both PGHS-1 and -2. Formation of 3-nitrotyrosine in the proteins was analyzed by immunoblotting, whereas NO adducts to AA and AA metabolites were analyzed by mass spectrometry and by chromatography of (14)C-labeled products. The results indicate that both nitrated tyrosine residues and NO-AA adducts formed upon NO trapping. The NO-AA adduct was predominantly an oxime at C11 of AA with three conjugated double bonds, as indicated by absorption at 275 nm and by mass spectral analysis. This adduct amounted to 10% and 20% of the heme concentration of PGHS-1 and -2, respectively. For PGHS-1, the yield of NO-AA adduct matched the yield of the narrow radical signal obtained in parallel EPR experiments. High frequency EPR characterization of this narrow radical, reported in an accompanying paper, supports assignment to a new tyrosyl radical, NS1c, rather than an AA-based radical. To reconcile the results from EPR and NO-trapping studies, we propose that the NS1c is in equilibrium with an AA pentadienyl radical, and that the latter reacts preferentially with NO.

Human cyclooxygenase-2 is a sequence homodimer that functions as a conformational heterodimer

Prostaglandin endoperoxide H synthases 1 and 2, also known as cyclooxygenases (COXs) 1 and 2, convert arachidonic acid (AA) to prostaglandin endoperoxide H(2). Prostaglandin endoperoxide H synthases are targets of nonspecific nonsteroidal anti-inflammatory drugs and COX-2-specific inhibitors called coxibs. PGHS-2 is a sequence homodimer. Each monomer has a peroxidase and a COX active site. We find that human PGHS-2 functions as a conformational heterodimer having a catalytic monomer (E(cat)) and an allosteric monomer (E(allo)). Heme binds tightly only to the peroxidase site of E(cat), whereas substrates, as well as certain inhibitors (e.g. celecoxib), bind the COX site of E(cat). E(cat) is regulated by E(allo) in a manner dependent on what ligand is bound to E(allo). Substrate and nonsubstrate fatty acids (FAs) and some COX inhibitors (e.g. naproxen) preferentially bind to the COX site of E(allo). AA can bind to E(cat) and E(allo), but the affinity of AA for E(allo) is 25 times that for E(cat). Palmitic acid, an efficacious stimulator of human PGHS-2, binds only E(allo) in palmitic acid/murine PGHS-2 co-crystals. Nonsubstrate FAs can potentiate or attenuate actions of COX inhibitors depending on the FA and whether the inhibitor binds E(cat) or E(allo). Our studies suggest that the concentration and composition of the free FA pool in the environment in which PGHS-2 functions in cells, the FA tone, is a key factor regulating PGHS-2 activity and its responses to COX inhibitors. We suggest that differences in FA tone occurring with different diets will likely affect both base-line prostanoid synthesis and responses to COX inhibitors.

Structural comparisons of arachidonic acid-induced radicals formed by prostaglandin H synthase-1 and -2

Cyclooxygenase catalysis by prostaglandin H synthase (PGHS)-1 and -2 involves reaction of a peroxide-induced Tyr385 radical with arachidonic acid (AA) to form an AA radical that reacts with O(2). The potential for isomeric AA radicals and formation of an alternate tyrosyl radical at Tyr504 complicate analysis of radical intermediates. We compared the EPR spectra of PGHS-1 and -2 reacted with peroxide and AA or specifically deuterated AA in anaerobic, single-turnover experiments. With peroxide-treated PGHS-2, the carbon-centered radical observed after AA addition was consistently a pentadienyl radical; a variable wide-singlet (WS) contribution from mixture of Tyr385 and Tyr504 radicals was also present. Analogous reactions with PGHS-1 produced EPR signals consistent with varying proportions of pentadienyl and tyrosyl radicals, and two additional EPR signals. One, insensitive to oxygen exposure, is the narrow singlet tyrosyl radical with clear hyperfine features found previously in inhibitor-pretreated PGHS-1. The second type of EPR signal is a narrow singlet lacking detailed hyperfine features that disappeared upon oxygen exposure. This signal was previously ascribed to an allyl radical, but high field EPR analysis indicated that ~90% of the signal originates from a novel tyrosyl radical, with a small contribution from a carbon-centered species. The radical kinetics could be resolved by global analysis of EPR spectra of samples trapped at various times during anaerobic reaction of PGHS-1 with a mixture of peroxide and AA. The improved understanding of the dynamics of AA and tyrosyl radicals in PGHS-1 and -2 will be useful for elucidating details of the cyclooxygenase mechanism, particularly the H-transfer between tyrosyl radical and AA.

Identification and absolute configuration of dihydroxy-arachidonic acids formed by oxygenation of 5S-HETE by native and aspirin-acetylated COX-2

Biosynthesis of the prostaglandin endoperoxide by the cyclooxygenase (COX) enzymes is accompanied by formation of a small amount of 11R-hydroxyeicosatetraenoic acid (HETE), 15R-HETE, and 15S-HETE as by-products. Acetylation of COX-2 by aspirin abrogates prostaglandin synthesis and triggers formation of 15R-HETE as the sole product of oxygenation of arachidonic acid. Here, we investigated the formation of by-products of the transformation of 5S-HETE by native COX-2 and by aspirin-acetylated COX-2 using HPLC-ultraviolet, GC-MS, and LC-MS analysis. 5S,15S- dihydroxy (di)HETE, 5S,15R-diHETE, and 5S,11R-diHETE were identified as by-products of native COX-2, in addition to the previously described di-endoperoxide (5S,15S-dihydroxy-9S,11R,8S,12S-diperoxy-6E,13E-eicosadienoic acid) as the major oxygenation product. 5S,15R-diHETE was the only product formed by aspirin-acetylated COX-2. Both 5,15-diHETE and 5,11-diHETE were detected in CT26 mouse colon carcinoma cells as well as in lipopolysaccharide-activated RAW264.7 cells incubated with 5S-HETE, and their formation was attenuated in the presence of the COX-2 specific inhibitor, NS-398. Aspirin-treated CT26 cells gave 5,15-diHETE as the most prominent product formed from 5S-HETE. 5S,15S-diHETE has been described as a product of the cross-over of 5-lipoxygenase (5-LOX) and 15-LOX activities in elicited rat mononuclear cells and human leukocytes, and our studies implicate cross-over of the 5-LOX and COX-2 pathways as an additional biosynthetic route.

Prostaglandin H synthase: resolved and unresolved mechanistic issues

The cyclooxygenase and peroxidase activities of prostaglandin H synthase (PGHS)-1 and -2 have complex kinetics, with the cyclooxygenase exhibiting feedback activation by product peroxide and irreversible self-inactivation, and the peroxidase undergoing an independent self-inactivation process. The mechanistic bases for these complex, non-linear steady-state kinetics have been gradually elucidated by a combination of structure/function, spectroscopic and transient kinetic analyses. It is now apparent that most aspects of PGHS-1 and -2 catalysis can be accounted for by a branched chain radical mechanism involving a classic heme-based peroxidase cycle and a radical-based cyclooxygenase cycle. The two cycles are linked by the Tyr385 radical, which originates from an oxidized peroxidase intermediate and begins the cyclooxygenase cycle by abstracting a hydrogen atom from the fatty acid substrate. Peroxidase cycle intermediates have been well characterized, and peroxidase self-inactivation has been kinetically linked to a damaging side reaction involving the oxyferryl heme oxidant in an intermediate that also contains the Tyr385 radical. The cyclooxygenase cycle intermediates are poorly characterized, with the exception of the Tyr385 radical and the initial arachidonate radical, which has a pentadiene structure involving C11-C15 of the fatty acid. Oxygen isotope effect studies suggest that formation of the arachidonate radical is reversible, a conclusion consistent with electron paramagnetic resonance spectroscopic observations, radical trapping by NO, and thermodynamic calculations, although moderate isotope selectivity was found for the H-abstraction step as well. Reaction with peroxide also produces an alternate radical at Tyr504 that is linked to cyclooxygenase activation efficiency and may serve as a reservoir of oxidizing equivalent. The interconversions among radicals on Tyr385, on Tyr504, and on arachidonate, and their relationships to regulation and inactivation of the cyclooxygenase, are still under active investigation for both PGHS isozymes.

Control of oxygenation in lipoxygenase and cyclooxygenase catalysis

Lipoxygenases (LOX) and cyclooxygenases (COX) react an achiral polyunsaturated fatty acid with oxygen to form a chiral peroxide product of high regio- and stereochemical purity. Both enzymes employ free radical chemistry reminiscent of hydrocarbon autoxidation but execute efficient control during catalysis to form a specific product over the multitude of isomers found in the nonenzymatic reaction. Exactly how both dioxygenases achieve this positional and stereo control is far from clear. We present four mechanistic models, not mutually exclusive, that could account for the specific reactions of molecular oxygen with a fatty acid in the LOX or COX active site.

Synthesis of 7-thiaarachidonic acid as a mechanistic probe of prostaglandin H synthase-2

The mechanism by which prostaglandin synthase converts arachidonic acid to prostaglandin G(2), creating five new chiral centers in the process, is still incompletely understood. The first radical intermediate has been characterized by EPR spectroscopy but subsequent proposed intermediates have not succumbed to detection. We report the synthesis of 7-thiaarachidonic acid designed to stabilize one of the proposed radical intermediates, which may allow its detection.

Molecular dynamics simulations of arachidonic acid-derived pentadienyl radical intermediate complexes with COX-1 and COX-2: insights into oxygenation regio- and stereoselectivity

The two cyclooxygenase enzymes, COX-1 and COX-2, are responsible for the committed step in prostaglandin biosynthesis and are the targets of the nonsteroidal antiinflammatory drugs aspirin and ibuprofen and the COX-2 selective inhibitors, Celebrex, Vioxx, and Bextra. The enzymes are remarkable in that they catalyze two dioxygenations and two cyclizations of the native substrate, arachidonic acid, with near absolute regio- and stereoselectivity. Several theories have been advanced to explain the nature of enzymatic control over this series of reactions, including suggestions of steric shielding and oxygen channeling. As proposed here, selective radical trapping and spin localization in the substrate-derived pentadienyl radical intermediate can also be envisioned. Herein we describe the results of explicit, 10 ns molecular dynamics simulations of both COX-1 and COX-2 with the substrate-derived pentadienyl radical intermediate bound in the active site. The enzymes' influence on the conformation of the pentadienyl radical was investigated, along with the accessible space above and below the radical plane and the width of several channels to the active site that could function as access routes for molecular oxygen. Additional simulations demonstrated the extent of molecular oxygen mobility within the active site. The results suggest that spin localization is unlikely to play a role in enzymatic control of this reaction. Instead, a combination of oxygen channeling, steric shielding, and selective radical trapping appears to be responsible. This work adds a dynamic perspective to the strong foundation of static structural data available for these enzymes.

Peroxidases: a role in the metabolism and side effects of drugs

Current safety screening of drug candidates or new chemical entities for reactive metabolite formation focuses on the role of cytochrome P450. However, peroxidases also have a major role in drug metabolism, and peroxidase-catalyzed drug oxidation could lead to reactive metabolite formation, resulting in oxidative stress and cytotoxicity. Here, the different classes of human peroxidases are summarized and the molecular mechanisms of peroxidase-catalyzed drug metabolism are discussed. In addition, evidence is presented that indicates a role of these enzymes in drug toxicity.

Role of Asn-382 and Thr-383 in Activation and Inactivation of Human Prostaglandin H Synthase Cyclooxygenase Catalysis

Cyclooxygenase catalysis by prostaglandin H synthase-1 and -2 (PGHS-1 and -2) requires activation of the normally latent enzyme by peroxide-dependent generation of a free radical at Tyr-385 (PGHS-1 numbering) in the cyclooxygenase active site; the Tyr-385 radical has also been linked to self-inactivation processes that impose an ultimate limit on cyclooxygenase catalysis. Cyclooxygenase activation is more resistant to suppression by cytosolic glutathione peroxidase in PGHS-2 than in PGHS-1. This differential response to peroxide scavenging enzymes provides a basis for the differential catalytic regulation of the two PGHS isoforms observed in vivo. We sought to identify structural differences between the isoforms, which could account for the differential cyclooxygenase activation, and used site-directed mutagenesis of recombinant human PGHS-2 to focus on one heme-vicinity residue that diverges between the two isoforms, Thr-383, and an adjacent residue that is conserved between the isoforms, Asn-382. Substitutions of Thr-383 (histidine in most PGHS-1) with histidine or aspartate decreased cyclooxygenase activation efficiency by about 40%, with little effect on cyclooxygenase specific activity or self-inactivation. Substitutions of Asn-382 with alanine, aspartate, or leucine had little effect on the cyclooxygenase specific activity or activation efficiency but almost doubled the cyclooxygenase catalytic output before self-inactivation. Asn-382 and Thr-383 mutations did not appreciably alter the Km value for arachidonate, the cyclooxygenase product profile, or the Tyr-385 radical spectroscopic characteristics, confirming the structural integrity of the cyclooxygenase site. The side chain structures of Asn-382 and Thr-383 in PGHS-2 thus selectively influence two important aspects of cyclooxygenase catalytic regulation: activation by peroxide and self-inactivation.

Histidine 386 and its role in cyclooxygenase and peroxidase catalysis by prostaglandin-endoperoxide H synthases

Prostaglandin-endoperoxide H synthases (PGHSs) have a cyclooxygenase that forms prostaglandin (PG) G2 from arachidonic acid (AA) plus oxygen and a peroxidase that reduces the PGG2 to PGH2. The peroxidase activates the cyclooxygenase. This involves an initial oxidation of the peroxidase heme group by hydroperoxide, followed by oxidation of Tyr385 to a tyrosyl radical within the cyclooxygenase site. His386 of PGHS-1 is not formally part of either active site, but lies in an extended helix between Tyr385, which protrudes into the cyclooxygenase site, and His388, the proximal ligand of the peroxidase heme. When His386 was substituted with alanine in PGHS-1, the mutant retained <2.5% of the native peroxidase activity, but >20% of the native cyclooxygenase activity. However, peroxidase activity could be restored (10-30%) by treating H386A PGHS-1 with cyclooxygenase inhibitors or AA, but not with linoleic acid; in contrast, mere occupancy of the cyclooxygenase site of native PGHS-1 had no effect on peroxidase activity. Heme titrations indicated that H386A PGHS-1 binds heme less tightly than does native PGHS-1. The low peroxidase activity and decreased affinity for heme of H386A PGHS-1 imply that His386 helps optimize heme binding. Molecular dynamic simulations suggest that this is accomplished in part by a hydrogen bond between the heme D-ring propionate and the N-delta of Asn382 of the extended helix. The structure of the extended helix is, in turn, strongly supported by stable hydrogen bonding between the N-delta of His386 and the backbone carbonyl oxygens of Asn382 and Gln383. We speculate that the binding of cyclooxygenase inhibitors or AA to the cyclooxygenase site of ovine H386A PGHS-1 reopens the constriction in the cyclooxygenase site between the extended helix and a helix containing Gly526 and Ser530 and restores native-like structure to the extended helix. Being less bulky than AA, linoleic acid is apparently unable to reopen this constriction.

Identification of two cyclooxygenase active site residues, Leucine 384 and Glycine 526, that control carbon ring cyclization in prostaglandin biosynthesis

The cyclooxygenase (COX) reaction of prostaglandin (PG) biosynthesis begins with the highly specific oxygenation of arachidonic acid in the 11R configuration and ends with a 15S oxygenation to form PGG2. To obtain new insights into the mechanisms of stereocontrol of oxygenation, we mutated active site residues of human COX-2 that have potential contacts with C-11 of the reacting substrate. Although the 11R oxygenation was not perturbed, changing Leu-384 (into Phe, Trp), Trp-387 (Phe, Tyr), Phe-518 (Ile, Trp, Tyr), and Gly-526 (Ala, Ser, Thr, Val) impaired or abrogated PGG2 synthesis, and typically 11R-HETE was the main product formed. The Gly-526 and Leu-384 mutants formed, in addition, three novel products identified by LC-MS, NMR, and circular dichroism as 8,9-11,12-diepoxy-13R-(or 15R)-hydro(pero)xy derivatives of arachidonic acid. Mechanistically, we propose these arise from a free radical intermediate in which a C-8 carbon radical displaces the 9,11-endoperoxide O-O bond to yield an 8,9-11,12-diepoxide that is finally oxygenated stereospecifically in the 13R or 15R configuration. Formation of these novel products signals an arrest in the normal course of prostaglandin synthesis just prior to closing of the 5-membered carbon ring, and points to a crucial role for Leu-384 and Gly-526 in the correct positioning of the reacting fatty acid intermediate. Some of the Gly-526 and Leu-384 mutants catalyzed both formation of PGG2 (with the normal 15S configuration) and the 13R- or 15R-oxygenated diepoxides. This result suggests that oxygenation specificity can be determined by the orientation of the reacting fatty acid radical and is not a predetermined outcome based solely on the structure of the cyclooxygenase active site.

Comparison of the properties of prostaglandin H synthase-1 and -2

Biosynthesis of prostanoid lipid signaling agents from arachidonic acid begins with prostaglandin H synthase (PGHS), a hemoprotein in the myeloperoxidase family. Vertebrates from humans to fish have two principal isoforms of PGHS, termed PGHS-1 and-2. These two isoforms are structurally quite similar, but they have very different pathophysiological roles and are regulated very differently at the level of catalysis. The focus of this review is on the structural and biochemical distinctions between PGHS-1 and-2, and how these differences relate to the functional divergence between the two isoforms.