The formation of stable fatty acid substrate complexes in prostaglandin H(2) synthase-1

Defining the Conformational Ensembles Associated with Ligand Binding to Cyclooxygenase-2

Cyclooxygenases (COX) catalyze the committed step in the production of prostaglandins responsible for the maintenance of physiological homeostasis. While crystal structures of COX in complex with substrates and inhibitors have provided insight into the molecular interactions governing their binding, they have not uncovered specific details related to the protein conformational motions responsible for important aspects of the COX function. We created a cysteine-free COX-2 construct and introduced a free cysteine at position-122 to enable labeling with 3-bromo-1,1,1-trifluoroacetone (BTFA). Placement of the label adjacent to the cyclooxygenase channel entrance permitted the detection of alterations upon ligand binding. 19F-nuclear magnetic resonance spectroscopy (19F-NMR) was then used to probe the conformational ensembles arising from BTFA-labeled COX-2 constructs in the presence and absence of ligands known to allosterically activate or inhibit COX-2. 19F-NMR analyses performed in the presence of the time-dependent inhibitor flurbiprofen, as well as Arg-120, Tyr-355, and Glu-524 mutations, led to the classification of two ensembles as representing the relaxed and tightened states of the cyclooxygenase channel entrance. A third ensemble, generated in the presence of arachidonic acid and the Y355F mutant and modulated by the allosteric potentiators palmitic acid and oleic acid and the nonallosteric substrates 2-arachidonoyl glycerol ether and anandamide, was classified as being related to the allosteric regulation of COX activity. The ensemble-based insight into COX function demonstrated here complements the static information derived from crystal structure analyses, collectively providing a more detailed framework of the dynamics involved in the regulation of COX catalysis and inhibition.

Structural and Chemical Biology of the Interaction of Cyclooxygenase with Substrates and Non-Steroidal Anti-Inflammatory Drugs

Cyclooxgenases are key enzymes of lipid signaling. They carry out the first step in the production of prostaglandins, important mediators of inflammation, pain, cardiovascular disease, and cancer, and they are the molecular targets for nonsteroidal anti-inflammatory drugs, which are among the oldest and most chemically diverse set of drugs known. Homodimeric proteins that behave as allosterically modulated, functional heterodimers, the cyclooxygenases exhibit complex kinetic behavior, requiring peroxide-dependent activation and undergoing suicide inactivation. Due to their important physiological and pathophysiological roles and keen interest on the part of the pharmaceutical industry, the cyclooxygenases have been the focus of a vast array of structural studies, leading to the publication of over 80 crystal structures of the enzymes in complex with substrates or inhibitors supported by a wealth of functional data generated by site-directed mutation experiments. In this review, we explore the chemical biology of the cyclooxygenases through the lens of this wealth of structural and functional information. We identify key structural features of the cyclooxygenases, break down their active site into regional binding pockets to facilitate comparisons between structures, and explore similarities and differences in the binding modes of the wide variety of ligands (both substrates and inhibitors) that have been characterized in complex with the enzymes. Throughout, we correlate structure with function whenever possible. Finally, we summarize what can and cannot be learned from the currently available structural data and discuss the critical intriguing questions that remain despite the wealth of information that has been amassed in this field.

Fatty Acid Binding to the Allosteric Subunit of Cyclooxygenase-2 Relieves a Tonic Inhibition of the Catalytic Subunit

Prostaglandin endoperoxide H synthase-2 (PGHS-2), also called cyclooxygenase-2 (COX-2), converts arachidonic acid to PGH₂ PGHS-2 is a conformational heterodimer composed of allosteric (E_allo) and catalytic (E_cat) subunits. Fatty acids (FAs) bind to Arg-120 of E_allo increasing to different degrees, depending on the FA, the V_max of its E_cat partner. We report here that movement of helical residues 120-122 and loop residues 123-129 of E_allo underlies the allosteric effects of FAs and allosteric COX-2 inhibitors, including naproxen and flurbiprofen. An S121P substitution in both PGHS-2 monomers yields a variant (S121P/S121P PGHS-2) that has 1.7-1.8 times the V_max of native PGHS-2 and is relatively insensitive to activation by FAs or inhibition by allosteric inhibitors. The S121P substitution in E_allo is primarily responsible for these effects. In X-ray crystal structures, the Cα atoms of helical residues 119-122 of S121P/S121P PGHS-2 are displaced from their normal positions. Additionally, the S121P/S121P PGHS-2 variants in which Pro-127 and Ser-541 are replaced by cysteines spontaneously forms Cys-127 to Cys-541 cross-links between monomers. This is unlike the corresponding native PGHS-2 variant and suggests that S121P substitutions also unhinge the loop involving residues 123-129. We conclude the following: (a) the region involving residues 120-129 of unoccupied E_allo tonically inhibits E_cat; (b) binding of an activating FA (e.g. arachidonic, palmitic, or oleic acid) to E_allo or an S121P substitution in E_allo repositions this region to increase E_cat activity; and (c) allosteric COX inhibitors act by preventing FA binding to E_allo and additionally by relocating E_allo residues to inhibit E_cat.

Bacterial and algal orthologs of prostaglandin H₂synthase: novel insights into the evolution of an integral membrane protein

Prostaglandin H₂synthase (PGHS; EC 1.14.99.1), a bi-functional heme enzyme that contains cyclooxygenase and peroxidase activities, plays a central role in the inflammatory response, pain, and blood clotting in higher eukaryotes. In this review, we discuss the progenitors of the mammalian enzyme by using modern bioinformatics and homology modeling to draw comparisons between this well-studied system and its orthologs from algae and bacterial sources. A clade of bacterial and algal orthologs is described that have salient structural features distinct from eukaryotic counterparts, including the lack of a dimerization and EGF-like domains, the absence of gene duplicates, and minimal membrane-binding domains. The functional implications of shared and variant features are discussed.

The crystal structure of α-Dioxygenase provides insight into diversity in the cyclooxygenase-peroxidase superfamily

α-Dioxygenases (α-DOX) oxygenate fatty acids into 2(R)-hydroperoxides. Despite the low level of sequence identity, α-DOX share common catalytic features with cyclooxygenases (COX), including the use of a tyrosyl radical during catalysis. We determined the X-ray crystal structure of Arabidopsis thaliana α-DOX to 1.5 Å resolution. The α-DOX structure is monomeric, predominantly α-helical, and comprised of two domains. The base domain exhibits a low degree of structural homology with the membrane-binding domain of COX but lies in a similar position with respect to the catalytic domain. The catalytic domain shows the highest degree of similarity with the COX catalytic domain, where 21 of the 22 α-helical elements are conserved. Helices H2, H6, H8, and H17 form the heme binding cleft and walls of the active site channel. His-318, Thr-323, and Arg-566 are located near the catalytic tyrosine, Tyr-386, at the apex of the channel, where they interact with a chloride ion. Substitutions at these positions coupled with kinetic analyses confirm previous hypotheses that implicate these residues as being involved in binding and orienting the carboxylate group of the fatty acid for optimal catalysis. Unique to α-DOX is the presence of two extended inserts on the surface of the enzyme that restrict access to the distal face of the heme, providing an explanation for the observed reduced peroxidase activity of the enzyme. The α-DOX structure represents the first member of the α-DOX subfamily to be structurally characterized within the cyclooxygenase-peroxidase family of heme-containing proteins.

The structural basis of endocannabinoid oxygenation by cyclooxygenase-2

The cyclooxygenases (COX-1 and COX-2) oxygenate arachidonic acid (AA) in the committed step of prostaglandin biogenesis. Substitutions of I434V, H513R, and I523V constitute the only differences in residues lining the cyclooxygenase channel between COX-1 and COX-2. These changes create a hydrophobic pocket in COX-2, with Arg-513 located at the base of the pocket, which has been exploited in the design of COX-2-selective inhibitors. Previous studies have shown that COX-2, but not COX-1, can oxygenate endocannabinoid substrates, including 2-arachidonoyl glycerol (2-AG). To investigate the isoform-specific structural basis of endocannabinoid binding to COX-2, we determined the crystal structure of the 2-AG isomer 1-arachidonoyl glycerol (1-AG) in complex with wild type and R513H murine (mu) COX-2 to 2.2 and 2.35 Å, respectively, and R513H muCOX-2 in complex with AA to 2.45 Å resolution. The 2,3-dihydroxypropyl moiety of 1-AG binds near the opening of the cyclooxygenase channel in the space vacated by the movement of the Leu-531 side chain, validating our previous hypothesis implicating the flexibility of the Leu-531 side chain as a determinant for the ability of COX-2 to oxygenate endocannabinoid substrates. Functional analyses carried out to compliment our structural findings indicated that Y355F and R513H muCOX-2 constructs had no effect on the oxygenation of 1-AG and 2-AG, whereas substitutions that resulted in a shortened side chain for Leu-531 had only modest effects. Both AA and 1-AG bind to R513H muCOX-2 in conformations similar to those observed in the co-crystal structures of these substrates with wild type enzyme.

Pleiotropic mechanisms facilitated by resveratrol and its metabolites

Resveratrol has demonstrated cancer chemopreventive activity in animal models and some clinical trials are underway. In addition, resveratrol was shown to promote cell survival, increase lifespan and mimic caloric restriction, thereby improving health and survival of mice on high-calorie diet. All of these effects are potentially mediated by the pleiotropic interactions of resveratrol with different enzyme targets including COX-1 (cyclo-oxygenase-1) and COX-2, NAD+-dependent histone deacetylase SIRT1 (sirtuin 1) and QR2 (quinone reductase 2). Nonetheless, the health benefits elicited by resveratrol as a direct result of these interactions with molecular targets have been questioned, since it is rapidly and extensively metabolized to sulfate and glucuronide conjugates, resulting in low plasma concentrations. To help resolve these issues, we tested the ability of resveratrol and its metabolites to modulate the function of some known targets in vitro. In the present study, we have shown that COX-1, COX-2 and QR2 are potently inhibited by resveratrol, and that COX-1 and COX-2 are also inhibited by the resveratrol 4'-O-sulfate metabolite. We determined the X-ray structure of resveratrol bound to COX-1 and demonstrate that it occupies the COX active site similar to other NSAIDs (non-steroidal anti-inflammatory drugs). Finally, we have observed that resveratrol 3- and 4'-O-sulfate metabolites activate SIRT1 equipotently to resveratrol, but that activation is probably a substrate-dependent phenomenon with little in vivo relevance. Overall, the results of this study suggest that in vivo an interplay between resveratrol and its metabolites with different molecular targets may be responsible for the overall beneficial health effects previously attributed only to resveratrol itself.

Physical evidence for substrate binding in preventing cyclooxygenase inactivation under nitrative stress

Prostaglandin biosynthesis is catalyzed by two spatially and functionally distinct active sites in cyclooxygenase (COX) enzymes. Despite the crucial role of COXs in biology, molecular details regarding the function and regulation of these enzymes are incompletely defined. Reactive nitrogen species, formed during oxidative stress, produce modifications that alter COX functionalities and prostaglandin biosynthesis. We previously established that COX-1 undergoes selective nitration on Tyr385 via a mechanism that requires the presence of bound heme cofactor. As this is a critical residue for COX-1 catalysis, nitration at this site results in enzyme inactivation. We now show that occupancy of the COX-1 active site with substrate protects against Tyr385 nitration and redirects nitration to alternative Tyr residues on COX-1, preserving catalytic activity. This study reveals a novel role for the substrate in protecting COX-1 from inactivation by nitration in pathophysiological settings.

Structural basis of fatty acid substrate binding to cyclooxygenase-2

The cyclooxygenases (COX-1 and COX-2) are membrane-associated heme-containing homodimers that generate prostaglandin H(2) from arachidonic acid (AA). Although AA is the preferred substrate, other fatty acids are oxygenated by these enzymes with varying efficiencies. We determined the crystal structures of AA, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) bound to Co(3+)-protoporphyrin IX-reconstituted murine COX-2 to 2.1, 2.4, and 2.65 A, respectively. AA, EPA, and docosahexaenoic acid bind in different conformations in each monomer constituting the homodimer in their respective structures such that one monomer exhibits nonproductive binding and the other productive binding of the substrate in the cyclooxygenase channel. The interactions identified between protein and substrate when bound to COX-1 are conserved in our COX-2 structures, with the only notable difference being the lack of interaction of the carboxylate of AA and EPA with the side chain of Arg-120. Leu-531 exhibits a different side chain conformation when the nonproductive and productive binding modes of AA are compared. Unlike COX-1, mutating this residue to Ala, Phe, Pro, or Thr did not result in a significant loss of activity or substrate binding affinity. Determination of the L531F:AA crystal structure resulted in AA binding in the same global conformation in each monomer. We speculate that the mobility of the Leu-531 side chain increases the volume available at the opening of the cyclooxygenase channel and contributes to the observed ability of COX-2 to oxygenate a broad spectrum of fatty acid and fatty ester substrates.

Acetylation of prostaglandin H2 synthases by aspirin is inhibited by redox cycling of the peroxidase

Aspirin exerts its unique pharmacological effects by irreversibly acetylating a serine residue in the cyclooxygenase site of prostaglandin-H2-synthases (PGHSs). Despite the irreversibility of the inhibition, the potency of aspirin varies remarkably between cell types, suggesting that molecular determinants could contribute to cellular selectivity. Using purified enzymes, we found no evidence that aspirin is selective for either of the two PGHS isoforms, and we showed that hydroperoxide substrates of the PGHS peroxidase inhibited the rate of acetylation of PGHS-1 by 68%. Using PGHS-1 reconstituted with cobalt protoporphyrin, a heme devoid of peroxidase activity, we demonstrated that reversal by hydroperoxides of the aspirin-mediated acetylation depends upon the catalytic activity of the PGHS peroxidase. We demonstrated that inhibition of PGHS-2 by aspirin in cells in culture is reversed by 12-hydroperoxyeicosatetraenoic acid dose-dependently (ED50=0.58+/-0.15 microM) and that in cells with high levels of hydroperoxy-fatty acids (RAW264.7) the efficacy of aspirin is markedly decreased as compared to cells with low levels of hydroperoxides (A549; IC50s=256+/-22 microM and 11.0+/-0.9 microM, respectively). Together, these findings indicate that acetylation of the PGHSs by aspirin is regulated by the catalytic activity of the peroxidase, which yields a higher oxidative state of the enzyme.

Structural basis of enantioselective inhibition of cyclooxygenase-1 by S-alpha-substituted indomethacin ethanolamides

The modification of the nonselective nonsteroidal anti-inflammatory drug, indomethacin, by amidation presents a promising strategy for designing novel cyclooxygenase (COX)-2-selective inhibitors. A series of alpha-substituted indomethacin ethanolamides, which exist as R/S-enantiomeric pairs, provides a means to study the impact of stereochemistry on COX inhibition. Comparative studies revealed that the R- and S-enantiomers of the alpha-substituted analogs inhibit COX-2 with almost equal efficacy, whereas COX-1 is selectively inhibited by the S-enantiomers. Mutagenesis studies have not been able to identify residues that manifest the enantioselectivity in COX-1. In an effort to understand the structural impact of chirality on COX-1 selectivity, the crystal structures of ovine COX-1 in complexes with an enantiomeric pair of these indomethacin ethanolamides were determined at resolutions between 2.75 and 2.85 A. These structures reveal unique, enantiomer-selective interactions within the COX-1 side pocket region that stabilize drug binding and account for the chiral selectivity observed with the (S)-alpha-substituted indomethacin ethanolamides. Kinetic analysis of binding demonstrates that both inhibitors bind quickly utilizing a two-step mechanism. However, the second binding step is readily reversible for the R-enantiomer, whereas for the S-enantiomer, it is not. These studies establish for the first time the structural and kinetic basis of high affinity binding of a neutral inhibitor to COX-1 and demonstrate that the side pocket of COX-1, previously thought to be sterically inaccessible, can serve as a binding pocket for inhibitor association.

Inducible nitric oxide synthase mediates prostaglandin h2 synthase nitration and suppresses eicosanoid production

Nitric oxide (NO) modulates the biological levels of arachidonate-derived cell signaling molecules by either enhancing or suppressing the activity of prostaglandin H(2) isoforms (PGHS-1 and PGHS-2). Whether NO activates or suppresses PGHS activity is determined by alternative protein modifications mediated by NO and NO-derived species. Here, we show that inducible NO synthase (iNOS) and PGHS-1 co-localize in atherosclerotic lesions of ApoE(-/-) mouse aortae. Immunoblotting and immunohistochemistry revealed Tyr nitration in PGHS-1 in aortic lesions but markedly less in adjacent nonlesion tissue. PGHS-2 was also found in lesions, but 3-nitrotyrosine incorporation was not detected. 3-Nitrotyrosine formation in proteins is considered a hallmark reaction of peroxynitrite, which can form via NO-superoxide reactions in an inflammatory setting. That iNOS-derived NO is essential for 3-nitrotyrosine modification of PGHS-1 was confirmed by the absence of 3-nitrotyrosine in lesions from ApoE(-/-)iNOS(-/-) mice. Mass spectrometric studies specifically identified the active site residue Tyr385 as a 3-nitrotyrosine modification site in purified PGHS-1 exposed to peroxynitrite. PGHS-mediated eicosanoid (PGE(2)) synthesis was more than fivefold accelerated in cultured iNOS(-/-) versus iNOS-expressing mouse aortic smooth muscle cells, suggesting that iNOS-derived NO markedly suppresses PGHS activity in vascular cells. These results further suggest a regulatory role of iNOS in eicosanoid biosynthesis in human atherosclerotic lesions.

Heme catalyzes tyrosine 385 nitration and inactivation of prostaglandin H2 synthase-1 by peroxynitrite

The mechanism by which the inflammatory enzyme prostaglandin H(2) synthase-1 (PGHS-1) deactivates remains undefined. This study aimed to determine the stabilizing parameters of PGHS-1 and identify factors leading to deactivation by nitric oxide species (NO(x)). Purified PGHS-1 was stabilized when solubilized in beta-octylglucoside (rather than Tween-20 or CHAPS) and when reconstituted with hemin chloride (rather than hematin). Peroxynitrite (ONOO(-)) activated the peroxidase site of PGHS-1 independently of the cyclooxygenase site. After ONOO(-) exposure, holoPGHS-1 could not metabolize arachidonic acid and was structurally compromised, whereas apoPGHS-1 retained full activity once reconstituted with heme. After incubation of holoPGHS-1 with ONOO(-), heme absorbance was diminished but to a lesser extent than the loss in enzymatic function, suggesting the contribution of more than one process to enzyme inactivation. Hydroperoxide scavengers improved enzyme activity, whereas hydroxyl radical scavengers provided no protection from the effects of ONOO(-). Mass spectral analyses revealed that tyrosine 385 (Tyr 385) is a target for nitration by ONOO(-) only when heme is present. Multimer formation was also observed and required heme but could be attenuated by arachidonic acid substrate. We conclude that the heme plays a role in catalyzing Tyr 385 nitration by ONOO(-) and the demise of PGHS-1.

The structure of mammalian cyclooxygenases

Cyclooxygenases-1 and -2 (COX-1 and COX-2, also known as prostaglandin H2 synthases-1 and -2) catalyze the committed step in prostaglandin synthesis. COX-1 and -2 are of particular interest because they are the major targets of nonsteroidal antiinflammatory drugs (NSAIDs) including aspirin, ibuprofen, and the new COX-2-selective inhibitors. Inhibition of the COXs with NSAIDs acutely reduces inflammation, pain, and fever, and long-term use of these drugs reduces the incidence of fatal thrombotic events, as well as the development of colon cancer and Alzheimer's disease. In this review, we examine how the structures of COXs relate mechanistically to cyclooxygenase and peroxidase catalysis and how alternative fatty acid substrates bind within the COX active site. We further examine how NSAIDs interact with COXs and how differences in the structure of COX-2 result in enhanced selectivity toward COX-2 inhibitors.

The structures of prostaglandin endoperoxide H synthases-1 and -2

Despite the marked differences in their physiological roles, the structures and catalytic functions of the prostaglandin H2 endoperoxide synthases-1 and -2 (PGHS-1 and -2) are almost completely identical. These integral membrane proteins catalyze the conversion of arachidonic acid to PGG2 and finally to PGH2. The crystal structures of PGHS-1 and -2 provide new insights into the catalytic mechanism for fatty acid oxygenation. Moreover, a clearer picture emerges to explain how a handful of amino acid substitutions can give rise to subtle differences in ligand binding between the two isoforms. These "small" alterations of isozyme structure are sufficient to allow the design of new, isoform-selective drugs.

Structure of eicosapentaenoic and linoleic acids in the cyclooxygenase site of prostaglandin endoperoxide H synthase-1

Prostaglandin endoperoxide H synthases-1 and -2 (PGHSs) can oxygenate 18-22 carbon polyunsaturated fatty acids, albeit with varying efficiencies. Here we report the crystal structures of eicosapentaenoic acid (EPA, 20:5 n-3) and linoleic acid (LA, 18:2 n-6) bound in the cyclooxygenase active site of Co(3+) protoporphyrin IX-reconstituted ovine PGHS-1 (Co(3+)-oPGHS-1) and compare the effects of active site substitutions on the rates of oxygenation of EPA, LA, and arachidonic acid (AA). Both EPA and LA bind in the active site with orientations similar to those seen previously with AA and dihomo-gamma-linolenic acid (DHLA). For EPA, the presence of an additional double bond (C-17/C-18) causes this substrate to bind in a "strained" conformation in which C-13 is misaligned with respect to Tyr-385, the residue that abstracts hydrogen from substrate fatty acids. Presumably, this misalignment is responsible for the low rate of EPA oxygenation. For LA, the carboxyl half binds in a more extended configuration than AA, which results in positioning C-11 next to Tyr-385. Val-349 and Ser-530, recently identified as important determinants for efficient oxygenation of DHLA by PGHS-1, play similar roles in the oxygenation of EPA and LA. Approximately 750- and 175-fold reductions in the oxygenation efficiency of EPA and LA were observed with V349A oPGHS-1, compared with a 2-fold change for AA. Val-349 contacts C-2 and C-3 of EPA and C-4 of LA orienting the carboxyl halves of these substrates so that the omega-ends are aligned properly for hydrogen abstraction. An S530T substitution decreases the V(max)/K(m) of EPA and LA by 375- and 140-fold. Ser-530 makes six contacts with EPA and four with LA involving C-8 through C-16; these interactions influence the alignment of the substrate for hydrogen abstraction. Interestingly, replacement of Phe-205 increases the volume of the cyclooxygenase site allowing EPA to be oxygenated more efficiently than with native oPGHS-1.

Mutational and X-ray crystallographic analysis of the interaction of dihomo-gamma -linolenic acid with prostaglandin endoperoxide H synthases

Prostaglandin endoperoxide H synthases-1 and -2 (PGHSs) catalyze the committed step in prostaglandin biosynthesis. Both isozymes can oxygenate a variety of related polyunsaturated fatty acids. We report here the x-ray crystal structure of dihomo-gamma-linolenic acid (DHLA) in the cyclooxygenase site of PGHS-1 and the effects of active site substitutions on the oxygenation of DHLA, and we compare these results to those obtained previously with arachidonic acid (AA). DHLA is bound within the cyclooxygenase site in the same overall L-shaped conformation as AA. C-1 and C-11 through C-20 are in the same positions for both substrates, but the positions of C-2 through C-10 differ by up to 1.74 A. In general, substitutions of active site residues caused parallel changes in the oxygenation of both AA and DHLA. Two significant exceptions were Val-349 and Ser-530. A V349A substitution caused an 800-fold decrease in the V(max)/K(m) for DHLA but less than a 2-fold change with AA; kinetic evidence indicates that C-13 of DHLA is improperly positioned with respect to Tyr-385 in the V349A mutant thereby preventing efficient hydrogen abstraction. Val-349 contacts C-5 of DHLA and appears to serve as a structural bumper positioning the carboxyl half of DHLA, which, in turn, positions properly the omega-half of this substrate. A V349A substitution in PGHS-2 has similar, minor effects on the rates of oxygenation of AA and DHLA. Thus, Val-349 is a major determinant of substrate specificity for PGHS-1 but not for PGHS-2. Ser-530 also influences the substrate specificity of PGHS-1; an S530T substitution causes 40- and 750-fold decreases in oxygenation efficiencies for AA and DHLA, respectively.

The productive conformation of arachidonic acid bound to prostaglandin synthase

Prostaglandin H synthase-1 and -2 (PGHS-1 and -2) catalyze the committed step in prostaglandin synthesis and are targets for nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin. We have determined the structure of PGHS-1 at 3 angstrom resolution with arachidonic acid (AA) bound in a chemically productive conformation. The fatty acid adopts an extended L-shaped conformation that positions the 13proS hydrogen of AA for abstraction by tyrosine-385, the likely radical donor. A space also exists for oxygen addition on the antarafacial surface of the carbon in the 11-position (C-11). While this conformation allows endoperoxide formation between C-11 and C-9, it also implies that a subsequent conformational rearrangement must occur to allow formation of the C-8/C-12 bond and to position C-15 for attack by a second molecule of oxygen.