Substrate access channel topology in membrane-bound prostacyclin synthase

A novel single-chain enzyme complex with chain reaction properties rapidly producing thromboxane A2 and exhibiting powerful anti-bleeding functions

Uncontrollable bleeding is still a worldwide killer. In this study, we aimed to investigate a novel approach to exhibit effective haemostatic properties, which could possibly save lives in various bleeding emergencies. According to the structure-based enzymatic design, we have engineered a novel single-chain hybrid enzyme complex (SCHEC), COX-1-10aa-TXAS. We linked the C-terminus of cyclooxygenase-1 (COX-1) to the N-terminus of the thromboxane A2 (TXA2 ) synthase (TXAS), through a 10-amino acid residue linker. This recombinant COX-1-10aa-TXAS can effectively pass COX-1-derived intermediate prostaglandin (PG) H2 (PGH2 ) to the active site of TXAS, resulting in an effective chain reaction property to produce the haemostatic prostanoid, TXA2 , rapidly. Advantageously, COX-1-10aa-TXAS constrains the production of other pro-bleeding prostanoids, such as prostacyclin (PGI2 ) and prostaglandin E2 (PGE2 ), through reducing the common substrate, PGH2 being passed to synthases which produce aforementioned prostanoids. Therefore, based on these multiple properties, this novel COX-1-10aa-TXAS indicated a powerful anti-bleeding ability, which could be used to treat a variety of bleeding situations and could even be useful for bleeding prone situations, including nonsteroidal anti-inflammatory drugs (NSAIDs)-resulted TXA2 -deficient and PGI2 -mediated bleeding disorders. This novel SCHEC has a great potential to be developed into a biological haemostatic agent to treat severe haemorrhage emergencies, which will prevent the complications of blood loss and save lives.

Structural and dynamic insights into substrate binding and catalysis of human lipocalin prostaglandin D synthase

Lipocalin prostaglandin D synthase (L-PGDS) regulates synthesis of an important inflammatory and signaling mediator, prostaglandin D2 (PGD2). Here, we used structural, biophysical, and biochemical approaches to address the mechanistic aspects of substrate entry, catalysis, and product exit of this enzyme. Structure of human L-PGDS was solved in a complex with a substrate analog (SA) and in ligand-free form. Its catalytic Cys 65 thiol group was found in two different conformations, each making a distinct hydrogen bond network to neighboring residues. These help in elucidating the mechanism of the cysteine nucleophile activation. Electron density for ligand observed in the active site defined the substrate binding regions, but did not allow unambiguous fitting of the SA. To further understand ligand binding, we used NMR spectroscopy to map the binding sites and to show the dynamics of protein-substrate and protein-product interactions. A model for ligand binding at the catalytic site is proposed, showing a second binding site involved in ligand exit and entry. NMR chemical shift perturbations and NMR resonance line-width alterations (observed as changes of intensity in two-dimensional cross-peaks in [¹H,¹⁵N]-transfer relaxation optimization spectroscopy) for residues at the Ω loop (A-B loop), E-F loop, and G-H loop besides the catalytic sites indicate involvement of these residues in ligand entry/egress.

Inducible COX-2 dominates over COX-1 in prostacyclin biosynthesis: mechanisms of COX-2 inhibitor risk to heart disease

AIM

Our aim is to understand the molecular mechanisms of the selective nonsteroidal anti-inflammatory drugs (NSAID), cyclooxygenase-2 (COX-2) inhibitors', higher "priority" to reduce synthesis of the vascular protector, prostacyclin (PGI2), compared to that of nonselective NSAIDs.

MAIN METHODS

COX-1 or COX-2 was co-expressed with PGI2 synthase (PGIS) in COS-7 cells. The Km and initial velocity (½t Vmax) of the coupling reaction between COX-1 and COX-2 to PGIS were established. The experiment was further confirmed by a kinetics study using hybrid enzymes linking COX-1 or COX-2 to PGIS. Finally, COX-1 or COX-2 and PGIS were respectively fused to red (RFP) and cyanic (CFP) fluorescence proteins, and co-expressed in cells. The distances between COXs and PGIS were compared by FRET.

KEY FINDINGS

The Km for converting arachidonic acid (AA) to PGI2 by COX-2 coupled to PGIS is ~2.0μM; however, it was 3-fold more (~6.0μM) for COX-1 coupled to PGIS. The Km and ½t Vmax for COX-2 linked to PGIS were ~2.0μM and 20s, respectively, which were 2-5 folds faster than that of COX-1 linked to PGIS. The FRET study found that the distance between COX-2-RFP and PGIS-CFP is shorter than that between COX-1-RFP and PGIS-CFP.

SIGNIFICANCE

The study provided strong evidence suggesting that the low Km, faster ½t Vmax, and closer distance are the basis for COX-2 dominance over COX-1 (coupled to PGIS) in PGI2 synthesis, and further demonstrated the mechanisms of selective COX-2 inhibitors with higher potential to reduce synthesis of the vascular protector, PGI2.

Engineering of a novel hybrid enzyme: an anti-inflammatory drug target with triple catalytic activities directly converting arachidonic acid into the inflammatory prostaglandin E2

Cyclooxygenase isoform-2 (COX-2) and microsomal prostaglandin E(2) synthase-1 (mPGES-1) are inducible enzymes that become up-regulated in inflammation and some cancers. It has been demonstrated that their coupling reaction of converting arachidonic acid (AA) into prostaglandin (PG) E(2) (PGE(2)) is responsible for inflammation and cancers. Understanding their coupling reactions at the molecular and cellular levels is a key step toward uncovering the pathological processes in inflammation. In this paper, we describe a structure-based enzyme engineering which produced a novel hybrid enzyme that mimics the coupling reactions of the inducible COX-2 and mPGES-1 in the native ER membrane. Based on the hypothesized membrane topologies and structures, the C-terminus of COX-2 was linked to the N-terminus of mPGES-1 through a transmembrane linker to form a hybrid enzyme, COX-2-10aa-mPGES-1. The engineered hybrid enzyme expressed in HEK293 cells exhibited strong triple-catalytic functions in the continuous conversion of AA into PGG(2) (catalytic-step 1), PGH(2) (catalytic-step 2) and PGE(2) (catalytic-step 3), a pro-inflammatory mediator. In addition, the hybrid enzyme was also able to directly convert dihomo-gamma-linolenic acid (DGLA) into PGG(1), PGH(1) and then PGE(1) (an anti-inflammatory mediator). The hybrid enzyme retained similar K(d) and V(max) values to that of the parent enzymes, suggesting that the configuration between COX-2 and mPGES-1 (through the transmembrane domain) could mimic the native conformation and membrane topologies of COX-2 and mPGES-1 in the cells. The results indicated that the quick coupling reaction between the native COX-2 and mPGES-1 (in converting AA into PGE(2)) occurred in a way so that both enzymes are localized near each other in a face-to-face orientation, where the COX-2 C-terminus faces the mPGES-1 N-terminus in the ER membrane. The COX-2-10aa-mPGES-1 hybrid enzyme engineering may be a novel approach in creating inflammation cell and animal models, which are particularly valuable targets for the next generation of NSAID screening.

Manufactured nanoparticles: their uptake and effects on fish--a mechanistic analysis

There is an emerging literature reporting toxic effects of manufactured nanomaterials (NMs) and nanoparticles (NPs) in fish, but the mechanistic basis of both exposure and effect are poorly understood. This paper critically evaluates some of the founding assumptions in fish toxicology, and likely mechanisms of absorption, distribution, metabolism and excretion (ADME) of NPs in fish compared to other chemicals. Then, using a case study approach, the paper compares these assumptions for two different NPs; TiO2 and C60 fullerenes. Adsorption of NPs onto the gill surface will involve similar processes in the gill microenvironment and mucus layer to other substances, but the uptake mechanisms for NPs by epithelial cells are more likely to occur via vesicular processes (e.g., endocytosis) than uptake on membrane transporters or by diffusion through the cell membranes. Target organs may include the gills, gut, liver and sometimes the brain. Information on metabolism and excretion of NPs in fish is limited; but hepatic excretion into the bile seems a more likely mechanism, rather than mainly by renal or branchial excretion. TiO2 and C60 share some common chemical properties that appear to be associated with some similar toxic effects, but there are also differences, that highlight the notion that chemical reactivity can inform toxic effect of NPs in a fundamentally similar way to other chemicals. In this paper we identify many knowledge gaps including the lack of field observations on fish and other wildlife species for exposure and effects of manufactured NMs. Systematic studies of the abiotic factors that influence bioavailability, and investigation of the cell biology that informs on the mechanisms of metabolism and excretion of NMs, will greatly advance our understanding of the potential for adverse effects. There are also opportunities to apply existing tools and techniques to fundamental studies of fish toxicology with NPs, such as perfused organs and fish cell culture systems.

On the mechanism of biosynthesis of 19-hydroxyprostaglandins of human seminal fluid and expression of cyclooxygenase-2, PGH 19-hydroxylase (CYP4F8) and microsomal PGE synthase-1 in seminal vesicles and vas deferens

The predominating prostaglandins of human seminal fluid are 19R-hydroxyprostaglandins E1 and E2, conceivably formed sequentially by prostaglandin H (PGH) synthase-2, PGH 19-hydroxylase (CYP4F8), and microsomal PGE synthase-1 of seminal vesicles. Our aim was to study this enzyme system. Quantification by real-time PCR suggested that the transcripts of PGH synthase-2, CYP4F8, and microsomal PGE synthase-1 were abundant and correlated in seminal vesicles of seven patients (p < 0.05). The three enzymes were detected in seminal vesicles by Western blot analysis, and immunohistological analysis confirmed the localization to the epithelia of seminal vesicles and distal vas deferens. Immunofluorescence analysis showed co-localization of the three enzymes in epithelial cells of seminal vesicles and vas deferens. 19-Hydroxy-PGE compounds were detected by mass spectrometry in the mucosa of distal vas deferens. Recombinant CYP4F8 catalyzes n-2 hydroxylation of PGH1 and PGH2 and n-3 hydroxylation of arachidonic acid. Arachidonic acid was oxidized to 18-hydroxyarachidonic acid and to PGE2 and by microsomes of seminal vesicles in the presence of NADPH and GSH, and to relatively small amounts of 19-hydroxy-PGE2. We conclude that PGH synthase-2, CYP4F8, and PGE synthase-1 likely forms 19-hydroxy-PGE compounds in seminal vesicles and vas deferens, but the catalytic properties of CYP4F8 suggest additional biological functions. Recombinant CYP4F8 was also found to catalyze n-2 hydroxylation of PGI2 and carbaprostacyclin (Km to approximately 40 microM), and n-2 and n-3 hydroxylation of carbocyclic TXA2.

Solution structure and topology of the N-terminal membrane anchor domain of a microsomal cytochrome P450: prostaglandin I2 synthase

The three-dimensional structure of a synthetic peptide corresponding to the N-terminal membrane anchor domain (residues 1-25) of prostaglandin I(2) synthase (also known as cytochrome P450 8A1), an eicosanoid-synthesizing microsomal cytochrome P450, has been determined by two-dimensional (1)H NMR spectroscopy in trifluoroethanol and dodecylphosphocholine which mimic the hydrophobic membrane environment. A combination of two-dimensional NMR experiments, including NOESY, TOCSY and double-quantum-filtered COSY, was used to obtain complete (1)H NMR assignments for the peptide. Using the NOE data obtained from the assignments and simulated annealing calculations, the N-terminal membrane domain reveals a bent-shaped structure comprised of an initial helix (residues 3-11), followed by a turn (residues 12-16) and a further atypical helix (residues 17-23). The hydrophobic side chains of the helix and turn segments (residues 1-20) are proposed to interact with the hydrocarbon interior of the phospholipid bilayer of the endoplasmic reticulum (ER) membrane. The hydrophilic side chains of residues 21-25 (Arg-Arg-Arg-Thr-Arg) point away from the hydrophobic residues 1-20 and are expected to be exposed to the aqueous environment on the cytoplasmic side of the ER membrane. The distance between residues 1 and 20 is approx. 20 A (1 A=0.1 nm), less than the thickness of a lipid bilayer. This indicates that the N-terminal membrane anchor domain of prostaglandin I(2) synthase does not penetrate the ER membrane.