The N-terminal α-Helix of Pancreatic Phospholipase A2 Determines Productive-mode Orientation of the Enzyme at the Membrane Surface

Measuring Endocannabinoid System Interaction with Biomembranes

Understanding the correct interaction among the different components of the endocannabinoid (eCB) system is fundamental for a proper assessment of the function of eCBs as signaling molecules. The knowledge of how the membrane environment modulates the intracellular trafficking of the eCB system and its interacting proteins holds a huge potential in unraveling new mechanisms of its modulation. This chapter deals with the application of fluorescence resonance energy transfer technique to measure the binding affinity of eCB proteins to model membranes (i.e., large unilamellar vesicles, LUVs). In particular, we describe in detail the paradigmatic example of the interaction of rat recombinant fatty acid amide hydrolase with LUVs constituted of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine.

Role of Steroids on the Membrane Binding Ability of Fatty Acid Amide Hydrolase

Background: Fatty acid amide hydrolase (FAAH) is a membrane-bound homodimeric enzyme that gets in contact with a lipophilic substrate in the lipid bilayer, and then cleaves it into water soluble products. FAAH plays a critical role in modulating in vivo content and biological activity of endocannabinoids (eCBs), and its function is affected by membrane lipids. Increasing evidence suggests that also steroids can modulate endocannabinoid signaling, both in the central nervous system and at the periphery.

Methods: In this study, we interrogated the effect of six steroids with relevant biological activity (testosterone, hydrocortisone, estradiol, pregnenolone, progesterone, and cortisone) on the membrane binding ability of rat FAAH. The experimental data analysis obtained by Fluorescence Resonance Energy Transfer Spectroscopy was paralleled by computational docking analysis.

Results: Our data revealed distinct effects of the different steroids on the interaction of rat FAAH with model membranes. Among them, pregnenolone was found to be the most effective in raising rat FAAH affinity for model membranes. A possible binding pocket for steroid molecules was identified by docking analysis in the membrane-embedded region of the enzyme; such a pocket could account for the observed increase of the membrane affinity in the presence of the tested molecules.

Conclusions: Overall, the results point to steroids as new regulators of FAAH interaction with membranes, which may impact the biological activity of eCBs.

FTIR Analysis of Proteins and Protein-Membrane Interactions

Fourier transform infrared (FTIR) spectroscopy has become one of the major techniques of structural characterization of proteins, peptides, and protein-membrane interactions. While the method does not have the capability of providing the precise, atomic-resolution molecular structure, it is exquisitely sensitive to conformational changes occurring in proteins upon functional transitions or intermolecular interactions. The sensitivity of vibrational frequencies to atomic masses has led to development of "isotope-edited" FTIR spectroscopy, where structural effects in two proteins, one unlabeled and the other labeled with a heavier stable isotope, such as ¹³C, are resolved simultaneously based on spectral downshift (separation) of the amide I band of the labeled protein. The same isotope effect is used to identify site-specific conformational changes in proteins by site-directed or segmental isotope labeling. Negligible light scattering in the infrared region provides an opportunity to study intermolecular interactions between large protein complexes, interactions of proteins and peptides with lipid vesicles, or protein-nucleic acid interactions without light scattering problems often encountered in ultraviolet spectroscopy. Attenuated total reflection FTIR (ATR-FTIR) is a surface-sensitive version of infrared spectroscopy that has proved useful in studying membrane proteins and lipids, protein-membrane interactions, mechanisms of interfacial enzymes, the structural features of membrane pore forming proteins and peptides, and much more. The purpose of this chapter was to provide a practical guide to analyze protein structure and protein-membrane interactions by FTIR and ATR-FTIR techniques, including procedures of sample preparation, measurements, and data analysis. Basic background information on FTIR spectroscopy, as well as some relatively new developments in structural and functional characterization of proteins and peptides in lipid membranes, is also presented.

Studying the TRAF2 binding to model membranes: The role of subunits dissociation

The ability of a C-terminal truncated form of TRAF2 to bind synthetic vesicles has been quantitatively studied by steady-state fluorescence energy transfer from the protein to large unilamellar vesicles (LUVs) prepared with different lipid mixtures. The dissociation constants, the free energy of binding, and the average number of phospholipids interacting with truncated TRAF2 have been evaluated from the corresponding binding curves. The results indicate that the protein strongly interacts with the lipid bilayer, preferentially in the monomeric state. These findings have been discussed in terms of their possible role in the activity of TRAF2 in vivo.

Measuring ECS Interaction with Biomembranes

Understanding the correct interaction among the different components of the endocannabinoid system (ECS) is fundamental for a proper assessment of the function of endocannabinoids (eCBs) as signaling molecules. The knowledge of how membrane environment is able to modulate intracellular trafficking of eCBs and their interacting proteins holds a huge potential in unraveling new mechanisms of ECS modulation.Here, fluorescence resonance energy transfer (FRET) technique is applied to measure the binding affinity of ECS proteins to model membranes (i.e., large unilamellar vesicles, LUVs). In particular, we describe in details the paradigmatic example of the interaction of recombinant rat FAAH-ΔTM with LUVs constituted by 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC).

Membrane Binding and Pore Formation by a Cytotoxic Fragment of Amyloid β Peptide

Amyloid β (Aβ) peptide contributes to Alzheimer's disease by a yet unidentified mechanism. In the brain tissue, Aβ occurs in various forms, including an undecapeptide Aβ_25-35, which exerts a neurotoxic effect through the mitochondrial dysfunction and/or Ca²⁺-permeable pore formation in cell membranes. This work was aimed at the biophysical characterization of membrane binding and pore formation by Aβ_25-35. Interaction of Aβ_25-35 with anionic and zwitterionic membranes was analyzed by microelectrophoresis. In pore formation experiments, Aβ_25-35 was incubated in aqueous buffer to form oligomers and added to Quin-2-loaded vesicles. Gradual increase in Quin-2 fluorescence was interpreted in terms of membrane pore formation by the peptide, Ca²⁺ influx, and binding to intravesicular Quin-2. The kinetics and magnitude of this process were used to evaluate the rate constant of pore formation, peptide-peptide association constants, and the oligomeric state of the pores. Decrease in membrane anionic charge and high ionic strength conditions significantly suppressed membrane binding and pore formation, indicating the importance of electrostatic interactions in these events. Circular dichroism spectroscopy showed that Aβ_25-35 forms the most efficient pores in β-sheet conformation. The data are consistent with an oligo-oligomeric pore model composed of up to eight peptide units, each containing 6-8 monomers.

Active Site Hydrophobicity and the Convergent Evolution of Paraoxonase Activity in Structurally Divergent Enzymes: The Case of Serum Paraoxonase 1

Serum paraoxonase 1 (PON1) is a native lactonase capable of promiscuously hydrolyzing a broad range of substrates, including organophosphates, esters, and carbonates. Structurally, PON1 is a six-bladed β-propeller with a flexible loop (residues 70–81) covering the active site. This loop contains a functionally critical Tyr at position 71. We have performed detailed experimental and computational analyses of the role of selected Y71 variants in the active site stability and catalytic activity in order to probe the role of Y71 in PON1’s lactonase and organophosphatase activities. We demonstrate that the impact of Y71 substitutions on PON1’s lactonase activity is minimal, whereas the k_cat for the paraoxonase activity is negatively perturbed by up to 100-fold, suggesting greater mutational robustness of the native activity. Additionally, while these substitutions modulate PON1’s active site shape, volume, and loop flexibility, their largest effect is in altering the solvent accessibility of the active site by expanding the active site volume, allowing additional water molecules to enter. This effect is markedly more pronounced in the organophosphatase activity than the lactonase activity. Finally, a detailed comparison of PON1 to other organophosphatases demonstrates that either a similar “gating loop” or a highly buried solvent-excluding active site is a common feature of these enzymes. We therefore posit that modulating the active site hydrophobicity is a key element in facilitating the evolution of organophosphatase activity. This provides a concrete feature that can be utilized in the rational design of next-generation organophosphate hydrolases that are capable of selecting a specific reaction from a pool of viable substrates.

Interfacial Enzymes: Membrane Binding, Orientation, Membrane Insertion, and Activity

Most interfacial enzymes undergo activation upon membrane binding. Interfacial activation is determined not only by the binding strength but also by the specific mode of protein-membrane interactions, including the angular orientation and membrane insertion of the enzymes. This chapter describes biophysical techniques to quantitatively evaluate membrane binding, orientation, membrane insertion, and activity of secreted phospholipase A2 (PLA2) and lipoxygenase (LO) enzymes. Procedures for recombinant production and purification of human pancreatic PLA2 and human 5-lipoxygenase (5-LO) are also presented. Several methods for measurements of membrane binding of peripheral proteins are described, i.e., fluorescence resonance energy transfer (FRET) from tryptophan or tyrosine residues of the protein to a fluorescent lipid in vesicles, changes in fluorescence of an environment-sensitive fluorescent lipid upon binding of proteins to membranes, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. These methods produce the apparent binding constant, the protein-to-lipid binding stoichiometry, and the Hill cooperativity coefficient. Experimental procedures for segmental isotope labeling of proteins and determination of the orientation of membrane-bound proteins by polarized ATR-FTIR spectroscopy are described. Furthermore, evaluation of membrane insertion of peripheral proteins by a fluorescence quenching technique is outlined. Combination of the orientation and membrane insertion provides a unique configuration of the protein-membrane complex and hence elucidates certain details of the enzyme function, such as the modes of acquisition of a membrane-residing substrate and product release. Finally, assays for determination of the activities of secreted PLA2, soybean LO, and human 5-LO are described.

Catalytic stimulation by restrained active-site floppiness--the case of high density lipoprotein-bound serum paraoxonase-1

Despite the abundance of membrane-associated enzymes, the mechanism by which membrane binding stabilizes these enzymes and stimulates their catalysis remains largely unknown. Serum paraoxonase-1 (PON1) is a lipophilic lactonase whose stability and enzymatic activity are dramatically stimulated when associated with high-density lipoprotein (HDL) particles. Our mutational and structural analyses, combined with empirical valence bond simulations, reveal a network of hydrogen bonds that connect HDL binding residues with Asn168--a key catalytic residue residing >15Å from the HDL contacting interface. This network ensures precise alignment of N168, which, in turn, ligates PON1's catalytic calcium and aligns the lactone substrate for catalysis. HDL binding restrains the overall motion of the active site and particularly of N168, thus reducing the catalytic activation energy barrier. We demonstrate herein that disturbance of this network, even at its most far-reaching periphery, undermines PON1's activity. Membrane binding thus immobilizes long-range interactions via second- and third-shell residues that reduce the active site's floppiness and pre-organize the catalytic residues. Although this network is critical for efficient catalysis, as demonstrated here, unraveling these long-rage interaction networks is challenging, let alone their implementation in artificial enzyme design.

Structural basis for the recruitment and activation of the Legionella phospholipase VipD by the host GTPase Rab5

A challenge for microbial pathogens is to assure that their translocated effector proteins target only the correct host cell compartment during infection. The Legionella pneumophila effector vacuolar protein sorting inhibitor protein D (VipD) localizes to early endosomal membranes and alters their lipid and protein composition, thereby protecting the pathogen from endosomal fusion. This process requires the phospholipase A1 (PLA1) activity of VipD that is triggered specifically on VipD binding to the host cell GTPase Rab5, a key regulator of endosomes. Here, we present the crystal structure of VipD in complex with constitutively active Rab5 and reveal the molecular mechanism underlying PLA1 activation. An active site-obstructing loop that originates from the C-terminal domain of VipD is repositioned on Rab5 binding, thereby exposing the catalytic pocket within the N-terminal PLA1 domain. Substitution of amino acid residues located within the VipD-Rab5 interface prevented Rab5 binding and PLA1 activation and caused a failure of VipD mutant proteins to target to Rab5-enriched endosomal structures within cells. Experimental and computational analyses confirmed an extended VipD-binding interface on Rab5, explaining why this L. pneumophila effector can compete with cellular ligands for Rab5 binding. Together, our data explain how the catalytic activity of a microbial effector can be precisely linked to its subcellular localization.

Ligand- and drug-binding studies of membrane proteins revealed through circular dichroism spectroscopy

A great number of membrane proteins have proven difficult to crystallise for use in X-ray crystallographic structural determination or too complex for NMR structural studies. Circular dichroism (CD) is a fast and relatively easy spectroscopic technique to study protein conformational behaviour. In this review examples of the applications of CD and synchrotron radiation CD (SRCD) to membrane protein ligand binding interaction studies are discussed. The availability of SRCD has been an important advancement in recent progress, most particularly because it can be used to extend the spectral region in the far-UV region (important for increasing the accuracy of secondary structure estimations) and for working with membrane proteins available in only small quantities for which SRCD has facilitated molecular recognition studies. Such studies have been accomplished by probing in the near-UV region the local tertiary structure of aromatic amino acid residues upon addition of chiral or non-chiral ligands using long pathlength cells of small volume capacity. In particular, this review describes the most recent use of the technique in the following areas: to obtain quantitative data on ligand binding (exemplified by the FsrC membrane sensor kinase receptor); to distinguish between functionally similar drugs that exhibit different mechanisms of action towards membrane proteins (exemplified by secretory phospholipase A2); and to identify suitable detergent conditions to observe membrane protein-ligand interactions using stabilised proteins (exemplified by the antiseptic transporter SugE). Finally, the importance of characterising in solution the conformational behaviour and ligand binding properties of proteins in both far- and near-UV regions is discussed. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

Structural characterization of membrane proteins and peptides by FTIR and ATR-FTIR spectroscopy

Fourier transform infrared (FTIR) spectroscopy is widely used in structural characterization of proteins or peptides. While the method does not have the capability of providing the precise, atomic-resolution molecular structure, it is exquisitely sensitive to conformational changes occurring in proteins upon functional transitions or upon intermolecular interactions. Sensitivity of vibrational frequencies to atomic masses has led to development of "isotope-edited" FTIR spectroscopy, where structural effects in two proteins, one unlabeled and the other labeled with a heavier stable isotope, such as (13)C, are resolved simultaneously based on spectral downshift (separation) of the amide I band of the labeled protein. The same isotope effect is used to identify site-specific conformational changes in proteins by site-directed or segmental isotope labeling. Negligible light scattering in the infrared region provides an opportunity to study intermolecular interactions between large protein complexes, interactions of proteins and peptides with lipid vesicles, or protein-nucleic acid interactions without light scattering problems often encountered in ultraviolet spectroscopy. Attenuated total reflection FTIR (ATR-FTIR) is a surface-sensitive version of infrared spectroscopy that has proved useful in studying membrane proteins and lipids, protein-membrane interactions, mechanisms of interfacial enzymes, and molecular architecture of membrane pore or channel forming proteins and peptides. The purpose of this article was to provide a practical guide to analyze protein structure and protein-membrane interactions by FTIR and ATR-FTIR techniques, including procedures of sample preparation, measurements, and data analysis. Basic background information on FTIR spectroscopy, as well as some relatively new developments in structural and functional characterization of proteins and peptides in lipid membranes, are also presented.

Lipid rafts alter the stability and activity of the cholera toxin A1 subunit

Cholera toxin (CT) travels from the cell surface to the endoplasmic reticulum (ER) as an AB holotoxin. ER-specific conditions then promote the dissociation of the catalytic CTA1 subunit from the rest of the toxin. CTA1 is held in a stable conformation by its assembly in the CT holotoxin, but the dissociated CTA1 subunit is an unstable protein that spontaneously assumes a disordered state at physiological temperature. This unfolding event triggers the ER-to-cytosol translocation of CTA1 through the quality control mechanism of ER-associated degradation. The translocated pool of CTA1 must regain a folded, active structure to modify its G protein target which is located in lipid rafts at the cytoplasmic face of the plasma membrane. Here, we report that lipid rafts place disordered CTA1 in a functional conformation. The hydrophobic C-terminal domain of CTA1 is essential for binding to the plasma membrane and lipid rafts. These interactions inhibit the temperature-induced unfolding of CTA1. Moreover, lipid rafts could promote a gain of structure in the disordered, 37 °C conformation of CTA1. This gain of structure corresponded to a gain of function: whereas CTA1 by itself exhibited minimal in vitro activity at 37 °C, exposure to lipid rafts resulted in substantial toxin activity at 37 °C. In vivo, the disruption of lipid rafts with filipin substantially reduced the activity of cytosolic CTA1. Lipid rafts thus exhibit a chaperone-like function that returns disordered CTA1 to an active state and is required for the optimal in vivo activity of CTA1.

Oxidized phospholipid content destabilizes the structure of reconstituted high density lipoprotein particles and changes their function

High density lipoprotein (HDL) particles are made up of lipid and protein constituents and apolipoprotein A-I (apoA-I) is a principal protein component that facilitates various biological activities of HDL particles. Increase in Ox-PL content of HDL particles makes them 'dysfunctional' and such modified HDL particles not only lose their athero-protective properties but also acquire pro-atherogenic and pro-inflammatory functions. The details of Ox-PL-induced alteration in the molecular properties of HDL particles are not clear. Paraoxonase 1 (PON1) is an HDL-associated enzyme that possesses anti-inflammatory and anti-atherogenic properties; and many of the athero-protective functions of HDL are attributed to the associated PON1. In this study we have characterized the physicochemical properties of reconstituted HDL (rHDL) particles containing varying amounts of Ox-PL and have compared their PON1 stimulation capacity. Our results show that increased Ox-PL content (a) modifies the physicochemical properties of the lipid domain of the rHDL particles, (b) decreases the stability and alters the conformation as well as orientation of apoA-I molecules on the rHDL particles, and (c) decreases the PON1 stimulation capacity of the rHDL particles. Our data indicate that the presence of Ox-PLs destabilizes the structure of the HDL particles and modifies their function.

C2 domain-containing phosphoprotein CDP138 regulates GLUT4 insertion into the plasma membrane

The protein kinase B(β) (Akt2) pathway is known to mediate insulin-stimulated glucose transport through increasing glucose transporter GLUT4 translocation from intracellular stores to the plasma membrane (PM). Combining quantitative phosphoproteomics with RNAi-based functional analyses, we show that a previously uncharacterized 138 kDa C2 domain-containing phosphoprotein (CDP138) is a substrate for Akt2, and is required for optimal insulin-stimulated glucose transport, GLUT4 translocation, and fusion of GLUT4 vesicles with the PM in live adipocytes. The purified C2 domain is capable of binding Ca(2+) and lipid membranes. CDP138 mutants lacking the Ca(2+)-binding sites in the C2 domain or Akt2 phosphorylation site S197 inhibit insulin-stimulated GLUT4 insertion into the PM, a rate-limiting step of GLUT4 translocation. Interestingly, CDP138 is dynamically associated with the PM and GLUT4-containing vesicles in response to insulin stimulation. Together, these results suggest that CDP138 is a key molecule linking the Akt2 pathway to the regulation of GLUT4 vesicle-PM fusion.

Biochemical and pharmacological characterization of PhTX-I a new myotoxic phospholipase A2 isolated from Porthidium hyoprora snake venom

This paper reports the biochemical and pharmacological characterization of a new myotoxic PLA(2) (EC 3.1.1.4) called PhTX-I, purified from Porthidium hyoprora venom by one step analytical chromatography reverse phase HPLC. The homogeneity of the PhTX-I fraction and its molecular mass were initially evaluated by SDS-PAGE and confirmed by MALDI-TOF spectrometry, indicating a molecular mass of 14.249Da and constituted of a single polipeptidic chain. Amino acid sequence was determined by "de novo sequencing," in tandem mass spectrometry, belonging to D49-PLA(2) enzyme class and exhibiting high identity (44-90%) with other myotoxics PLA(2) from snake venoms. The enzymatic investigation showed maximal activity at pH 8 and 35-45°C. This activity was dependent on Ca(2+), other cations (Mg(2+), Mn(2+), Cd(2+) and Zn(2+)) reduced notably the enzymatic activity, suggesting that the arrangement of the catalytic site presents an exclusive structure for Ca(2+). Ex vivo, whole venom and PhTX-I PLA(2) caused blockade of the neuromuscular transmission in young chick biventer cervicis preparations similar to other isolated snake venom toxins from the Bothrops genus. In vivo, both induced local myotoxicity and systemic interleukin-6 response upon intramuscular injection, additionally, induced moderate footpad edema. In vitro, both induced low cytotoxicity in skeletal muscle myoblasts, however PhTX-I PLA(2) was able to lyse myotubes.

Proteolytic cleavage of stingray phospholipase A2: isolation and biochemical characterization of an active N-terminal form

Background

Mammalian GIB-PLA2 are well characterized. In contrast, much less is known about aquatic ones. The aquatic world contains a wide variety of living species and, hence represents a great potential for discovering new lipolytic enzymes. The aim of this study was to check some biochemical and structural properties of a marine stingray phospholipase A2 (SPLA2).

Results

The effect of some proteolytic enzymes on SPLA2 was checked. Chymotrypsin and trypsin were able to hydrolyze SPLA2 in different ways. In both cases, only N-terminal fragments were accumulated during the hydrolysis, whereas no C-terminal fragment was obtained in either case. Tryptic and chymotryptic attack generated 13 kDa and 12 kDa forms of SPLA2, respectively. Interestingly, the SPLA2 13 kDa form was inactive, whereas the SPLA2 12 kDa form conserved almost its full phospholipase activity. In the absence of bile slats both native and 12kDa SPLA2 failed to catalyse the hydrolysis of PC emulsion. When bile salts were pre-incubated with the substrate, the native kinetic protein remained linear for more than 25 min, whereas the 12 kDa form activity was found to decrease rapidly. Furthermore, The SPLA2 activity was dependent on Ca²⁺; other cations (Mg²⁺, Mn²⁺, Cd²⁺ and Zn²⁺) reduced the enzymatic activity notably, suggesting that the arrangement of the catalytic site presents an exclusive structure for Ca²⁺.

Conclusions

Although marine and mammal pancreatic PLA2 share a high amino acid sequence homology, polyclonal antibodies directed against SPLA2 failed to recognize mammal PLA2 like the dromedary pancreatic one. Further investigations are needed to identify key residues involved in substrate recognition responsible for biochemical differences between the 2 classes of phospholipases.

Surface pressure-dependent interactions of secretory phospholipase A2 with zwitterionic phospholipid membranes

The hydrolytic activity of secretory phospholipase A(2) (PLA(2)) is regulated by many factors, including the physical state of substrate aggregates and the chemical nature of phospholipid molecules. In order to achieve strong binding of PLA(2) on its substrates, many previous works have used anionic lipid dispersion to characterize the orientation and penetration depth of PLA(2) molecules on membrane surfaces. In this study, we applied monolayer technique with controllable surface area to investigate the PLA(2)s of Taiwan cobra venom and bee venom on zwitterionic phophatidylcholine monolayers and demonstrated an optimum hydrolytic activity at a surface pressure of 18 and 24 mN/m, respectively. By combining polarized attenuated total reflection Fourier-transform infrared spectroscopy and monolayer-binding experiments, we found that the amount of membrane-bound PLA(2) decreased markedly as the surface pressure of the monolayer was increased. Interestingly, the insertion area of the PLA(2)s decreased to near zero as the surface pressure increased to the optimum pressure for hydrolytic activity. On the basis of the measured infrared dichroic ratio, the orientation of the PLA(2)s bound to zwitterionic membranes was similar to that observed on a negatively charged membrane and was independent of the surface pressure. Our findings suggest that both PLA(2)s were located on the membrane surface rather than penetrating the membrane bilayer and that the deeply inserted mode is not a favorable condition for the hydrolysis of phospholipids in zwitterionic phospholipid membranes. The results are discussed in terms of the easy access of catalytic water for the PLA(2) activity and the mobilization of its substrate and product to facilitate the catalytic process.

A novel role for iron in modulating the activity and membrane-binding ability of a trimmed soybean lipoxygenase-1

Lipoxygenases (LOXs) are iron-containing enzymes that play critical roles in plants and animals. As yet, metal atom extraction, reconstitution, and substitution have not been successfully applied to soybean LOX-1 [Glycine max (L.) Merrill], a prototype member of the LOX family that is widely used in structural and kinetic studies. Here, tryptic digestion of native LOX-1, used as a control, allowed us to isolate the 60-kDa C-terminal region (termed miniLOX), that retains the catalytically active iron in a more accessible position. Then, iron was removed to obtain an unprecedented apo-miniLOX, which was reconstituted and substituted with different metal ions. These forms of miniLOX were characterized vs. native LOX-1 by kinetic analysis, near UV circular dichroism, steady-state fluorescence, and fluorescence resonance energy transfer. MiniLOX showed a 2-fold increase in the membrane-binding affinity compared with native LOX-1 and a remarkable 4-fold increase compared with apo-miniLOX (K(d)=9.2+/-1.0, 17.9+/-2.0, and 45.4+/-4.3 microM, respectively). Furthermore, miniLOX reconstituted with Fe(II) or Fe(III) partially recovered its membrane-binding ability (K(d)=21.4+/-2.4 and 18.9+/-5.5 microM, respectively), overall supporting a novel noncatalytic role for iron in the LOX family.

Structural insight into the activation mechanism of human pancreatic prophospholipase A2

Pancreatic phospholipase A2 (phospholipase A2 group 1B, G1B) belongs to the superfamily of secreted phospholipase A2 (PLA2) enzymes. G1B has been proposed to be a potential target for diseases such as hypertension, obesity, and diabetes. Human pancreatic prophospholipase A2 (pro-hG1B) is activated by cleavage of the first seven-residue propeptide (phospholipase A2 propeptide, PROP). However, questions still remain on the mode of action for pro-hG1B. In this work, we expressed pro-hG1B in Pichia pastoris and determined the crystal structure at 1.55-A resolution. The x-ray structure demonstrates that pro-hG1B forms a trimer. In addition, PROP occupies the catalytic cavity and can be self-cleaved at 37 degrees C. A new membrane-bound surface and activation mechanism are proposed based on the trimeric model of pro-hG1B. We also propose a new autoproteolytic mechanism for pro-hG1B by the reaction triad Asp49-Arg0-Ser(-2) that is similar to the serine protease catalytic triad.

Effect of guggulsterone and cembranoids of Commiphora mukul on pancreatic phospholipase A(2): role in hypocholesterolemia

Guggulsterone (7) and cembranoids (8-12) from Commiphora mukul stem bark resin guggul were shown to be specific modulators of two independent sites that are also modulated by bile salts (1-6) to control cholesterol absorption and catabolism. Guggulsterone (7) antagonized the chenodeoxycholic acid (3)-activated nuclear farnesoid X receptor (FXR), which regulates cholesterol metabolism in the liver. The cembranoids did not show a noticeable effect on FXR, but lowered the cholate (1)-activated rate of human pancreatic IB phospholipase A2 (hPLA2), which controls gastrointestinal absorption of fat and cholesterol. Analysis of the data using a kinetic model has suggested an allosteric mechanism for the rate increase of hPLA2 by cholate and also for the rate-lowering effect by certain bile salts or cembranoids on the cholate-activated hPLA2 hydrolysis of phosphatidylcholine vesicles. The allosteric inhibition of PLA2 by certain bile salts and cembranoids showed some structural specificity. Biophysical studies also showed specific interaction of the bile salts with the interface-bound cholate-activated PLA2. Since cholesterol homeostasis in mammals is regulated by FXR in the liver for metabolism and by PLA2 in the intestine for absorption, modulation of PLA2 and FXR by bile acids and selected guggul components suggests novel possibilities for hypolipidemic and hypocholesterolemic therapies.

Sensor of phospholipids in Streptomyces phospholipase D

Recently, we identified Ala426 and Lys438 of phospholipase D from Streptomyces septatus TH-2 (TH-2PLD) as important residues for activity, stability and selectivity in transphosphatidylation. These residues are located in a C-terminal flexible loop separate from two catalytic HxKxxxxD motifs. To study the role of these residues in substrate recognition, we evaluated the affinities of inactive mutants, in which these residues were substituted with Phe and His, toward several phospholipids by SPR analysis. By substituting Ala426 and Lys438 with Phe and His, respectively, the inactive mutant showed a much stronger interaction with phosphatidylcholine and a weaker interaction with phosphatidylglycerol than the inactive TH-2PLD mutant. We demonstrated that Ala426 and Lys438 of TH-2PLD play a role in sensing the head group of phospholipids.

Desolvation map of the i-face of phospholipase A2

The changes in the microenvironment of the Trp-3 on the i-face of pig pancreatic IB phospholipase A2 (PLA2) provide a measure of the tight contact (Ramirez and Jain, Protein Sci. 9, 229-239, 1991) with the substrate interface during the processive interfacial turnover. Spectral changes from the single Trp-substituent at position 1, 2, 6, 10, 19, 20, 31, 53, 56 or 87 on the surface of W3F PLA2 are used to probe the Trp-environment. Based on our current understanding only the residue 87 is away from i-face, therefore all other mutants are well suited to report modest differences along the i-face. All Trp-mutants bind tightly to anionic vesicles. Only those with Trp at 1, 2 or 3 near the rim of the active site on the i-face cause significant perturbation of the catalytic functions. Most other Trp-mutants showed < 3-fold change in the interfacial processive turnover rate and the competitive inhibition by MJ33. Binding of calcium to the enzyme in the aqueous phase had modest effect on the Trp-emission intensity. However, on the binding of the enzyme to the interface the fluorescence change is large, and the rate of oxidation of the Trp-substituent with N-bromosuccinimide depends on the location of the Trp-substituent. These results show that the solvation environment of the Trp-substituents on the i-face is shielded in the enzyme bound to the interface. Additional changes are noticeable if the active site of the bound enzyme is also occupied, however, the catalytically inert zymogen of PLA2 (proPLA2) does not show such changes. Significance of these results in relation to the changes in the solvent accessibility and desolvation of the i-face of PLA2 at the interface is discussed.

Evidence for the Regulatory Role of the N-terminal Helix of Secretory Phospholipase A2 from Studies on Native and Chimeric Proteins

The phospholipase A(2) (PLA(2)) enzymes are activated by binding to phospholipid membranes. Although the N-terminal alpha-helix of group I/II PLA(2)s plays an important role in the productive mode membrane binding of the enzymes, its role in the structural aspects of membrane-induced activation of PLA(2)s is not well understood. In order to elucidate membrane-induced conformational changes in the N-terminal helix and in the rest of the PLA(2), we have created semisynthetic human group IB PLA(2) in which the N-terminal decapeptide is joined with the (13)C-labeled fragment, as well as a chimeric protein containing the N-terminal decapeptide from human group IIA PLA(2) joined with a (13)C-labeled fragment of group IB PLA(2). Infrared spectral resolution of the unlabeled and (13)C-labeled segments suggests that the N-terminal helix of membrane-bound IB PLA(2) has a more rigid structure than the other helices. On the other hand, the overall structure of the chimeric PLA(2) is more rigid than that of the IB PLA(2), but the N-terminal helix is more flexible. A combination of homology modeling and polarized infrared spectroscopy provides the structure of membrane-bound chimeric PLA(2), which demonstrates remarkable similarity but also distinct differences compared with that of IB PLA(2). Correlation is delineated between structural and membrane binding properties of PLA(2)s and their N-terminal helices. Altogether, the data provide evidence that the N-terminal helix of group I/II PLA(2)s acts as a regulatory domain that mediates interfacial activation of these enzymes.

Positioning Membrane Proteins by Novel Protein Engineering and Biophysical Approaches

Membrane proteins are unique, in that they can function properly only when they are bound to cellular membranes in a distinct manner. Therefore, positioning of membrane proteins with respect to the membrane is required in addition to the three-dimensional structures in order to understand their detailed molecular mechanisms. Atomic-resolution structures of membrane proteins that have been determined to date provide the atom coordinates in arbitrary coordinate systems with no relation to the membrane and therefore provide little or no information on how the protein would interact with the membrane. This is especially true for peripheral membrane proteins, because they, unlike integral proteins, are devoid of well-defined hydrophobic transmembrane domains. Here, we present a novel technique for determination of the configuration of a protein-membrane complex that involves protein ligation, segmental isotope labeling, polarized infrared spectroscopy, membrane depth-dependent fluorescence quenching, and analytical geometry algorithms. We have applied this approach to determine the structure of a membrane-bound phospholipase A2. Our results provide an unprecedented structure of a membrane-bound protein in which the z-coordinate of each atom is the distance from the membrane center and therefore allows precise location of each amino acid relative to the membrane. Given the functional significance of the orientation and location of membrane-bound proteins with respect to the membrane, we propose to specify this structural feature as the "quinary" structure of membrane proteins.

Membrane fluidity is a key modulator of membrane binding, insertion, and activity of 5-lipoxygenase

Mammalian 5-lipoxygenase (5-LO) catalyzes conversion of arachidonic acid to leukotrienes, potent mediators of inflammation and allergy. Upon cell stimulation, 5-LO selectively binds to nuclear membranes and becomes activated, yet the mechanism of recruitment of 5-LO to nuclear membranes and the mode of 5-LO-membrane interactions are poorly understood. Here we show that membrane fluidity is an important determinant of membrane binding strength of 5-LO, penetration into the membrane hydrophobic core, and activity of the enzyme. The membrane binding strength and activity of 5-LO increase with the degree of lipid acyl chain cis-unsaturation and reach a plateau with 1-palmitoyl-2-arachidonolyl-sn-glycero-3-phosphocholine (PAPC). A fraction of tryptophans of 5-LO penetrate into the hydrocarbon region of fluid PAPC membranes, but not into solid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine membranes. Our data lead to a novel concept of membrane binding and activation of 5-LO, suggesting that arachidonic-acid-containing lipids, which are present in nuclear membranes at higher fractions than in other cellular membranes, may facilitate preferential membrane binding and insertion of 5-LO through increased membrane fluidity and may thereby modulate the activity of the enzyme. The data presented in this article and earlier data allow construction of a model for membrane-bound 5-LO, including the angular orientation and membrane insertion of the protein.