Kinetic basis for interfacial catalysis by phospholipase A2

The phospholipase A2 activity of peroxiredoxin 6

Peroxiredoxin 6 (Prdx6) is a Ca²⁺-independent intracellular phospholipase A₂ (called aiPLA₂) that is localized to cytosol, lysosomes, and lysosomal-related organelles. Activity is minimal at cytosolic pH but is increased significantly with enzyme phosphorylation, at acidic pH, and in the presence of oxidized phospholipid substrate; maximal activity with phosphorylated aiPLA₂ is ∼2 µmol/min/mg protein. Prdx6 is a "moonlighting" protein that also expresses glutathione peroxidase and lysophosphatidylcholine acyl transferase activities. The catalytic site for aiPLA₂ activity is an S32-H26-D140 triad; S32-H26 is also the phospholipid binding site. Activity is inhibited by a serine "protease" inhibitor (diethyl p-nitrophenyl phosphate), an analog of the PLA₂ transition state [1-hexadecyl-3-(trifluoroethyl)-sn-glycero-2-phosphomethanol (MJ33)], and by two naturally occurring proteins (surfactant protein A and p67^phox), but not by bromoenol lactone. aiPLA₂ activity has important physiological roles in the turnover (synthesis and degradation) of lung surfactant phospholipids, in the repair of peroxidized cell membranes, and in the activation of NADPH oxidase type 2 (NOX2). The enzyme has been implicated in acute lung injury, carcinogenesis, neurodegenerative diseases, diabetes, male infertility, and sundry other conditions, although its specific roles have not been well defined. Protein mutations and animal models are now available to further investigate the roles of Prdx6-aiPLA₂ activity in normal and pathological physiology.

IR spectroscopy analysis of pancreatic lipase-related protein 2 interaction with phospholipids: 2. Discriminative recognition of various micellar systems and characterization of PLRP2-DPPC-bile salt complexes

The interaction of pancreatic lipase-related protein 2 (PLRP2) with various micelles containing phospholipids was investigated using pHstat enzyme activity measurements, differential light scattering, size exclusion chromatography (SEC) and transmission IR spectroscopy. Various micelles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and lysophosphatidylcholine were prepared with either bile salts (sodium taurodeoxycholate or glycodeoxycholate) or Triton X-100, which are substrate-dispersing agents commonly used for measuring phospholipase activities. PLRP2 displayed a high activity on all phospholipid-bile salt micelles, but was totally inactive on phospholipid-Triton X-100 micelles. These findings clearly differentiate PLRP2 from secreted pancreatic phospholipase A2 which is highly active on both types of micelles. Using an inactive variant of PLRP2, SEC experiments allowed identifying two populations of PLRP2-DPPC-bile salt complexes corresponding to a high molecular weight 1:1 PLRP2-micelle association and to a low molecular weight association of PLRP2 with few monomers of DPPC/bile salts. IR spectroscopy analysis showed how DPPC-bile salt micelles differ from DPPC-Triton X-100 micelles by a higher fluidity of acyl chains and higher hydration/H-bonding of the interfacial carbonyl region. The presence of bile salts allowed observing changes in the IR spectrum of DPPC upon addition of PLRP2 (higher rigidity of acyl chains, dehydration of the interfacial carbonyl region), while no change was observed with Triton X-100. The differences between these surfactants and their impact on substrate recognition by PLRP2 are discussed, as well as the mechanism by which high and low molecular weight PLRP2-DPPC-bile salt complexes may be involved in the overall process of DPPC hydrolysis.

Phosphorothiolate analogues of phosphatidylinositols as assay substrates for phospholipase C

Accurate measurement of phosphatidylinositol-specific phospholipase C (PI-PLC) activity is important in view of the key role of this enzyme in signal-transduction pathways. In this work we synthesized enantiomerically pure phosphorothiolate analogues of all natural PI-PLC substrates, including those of phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2), 4-phosphate (PI-4-P), 5-phosphate (PI-5-P) and unphosphorylated PI, in both long- and short-chain versions. The enzymatic cleavage of these substrates produces thiol analogues of diacyl glycerol, which can be quantified by UV absorbance after treatment with dipyridyl disulfide. The monodisperse dihexanoyl derivatives are suitable substrates for PI-PLC assay: they give rise to high enzyme activity, and provide excellent linear kinetic responses. For all substrates, we found a good linear correlation between the reaction rate and the amount of enzyme; this indicated the suitability of this assay for enzyme quantification. The short-chain substrates enable the enzyme specificity with variously phosphorylated inositol head groups to be established--unobstructed by substrate aggregation, "scooting" kinetics on micelles, or surface dilution effects. The kinetic results indicated allosteric behavior of PLC for all substrates tested. We found that substrates phosphorylated at the inositol 4-position (phosphorothiolate analogues of PI-4,5-P2 and PI-4-P) displayed very similar kinetic properties, and were cleaved with approximately 20- to 30-fold higher activity than the 4-nonphosphorylated substrates (analogues of PI-5-P and PI). Hence it appears that interactions between the enzyme and the 4-phosphate group of the substrate, but not its 5-phosphate group, is important for PI-PLC catalysis. In addition, the binding affinities of all four substrate types were found to be quite similar; this indicates that the energy of enzyme interaction with the 4-phosphate group is directed almost entirely to catalysis.

Molecular determinants of bacterial sensitivity and resistance to mammalian Group IIA phospholipase A2

Group IIA secretory phospholipase A2 (sPLA(2)-IIA) of mammalian species is unique among the many structurally and functionally related mammalian sPLA(2) in their high net positive charge and potent (nM) antibacterial activity. Toward the Gram-positive bacteria tested thus far, the global cationic properties of sPLA(2)-IIA are necessary for optimal binding to intact bacteria and penetration of the multi-layered thick cell wall, but not for the degradation of membrane phospholipids that is essential for bacterial killing. Various Gram-positive bacterial species can differ as much as 1000-fold in sPLA(2)-IIA sensitivity despite similar intrinsic enzymatic activity of sPLA(2)-IIA toward the membrane phospholipids of various bacteria. d-alanylation of wall- and lipo-teichoic acids in Staphylococcus aureus and sortase function in Streptococcus pyogenes increase bacterial resistance to sPLA(2)-IIA by up to 100-fold apparently by affecting translocation of bound sPLA(2)-IIA to the cell membrane. Action of the sPLA(2)-IIA and other related sPLA(2) against Gram-negative bacteria is more dependent on cationic properties of the enzyme near the amino-terminus of the protein and collaboration with other host defense proteins that produce alterations of the unique Gram-negative bacterial outer membrane that normally represents a barrier to sPLA(2)-IIA action. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

A model for the interfacial kinetics of phospholipase D activity on long-chain lipids

The membrane-active enzyme phospholipase D (PLD) catalyzes the hydrolysis of the phosphodiester bond in phospholipids and plays a critical role in cell signaling. This catalytic reaction proceeds on lipid-water interfaces and is an example of heterogeneous catalysis in biology. Recently we showed that planar lipid bilayers, a previously unexplored model membrane for these kinetic studies, can be used for monitoring interfacial catalytic reactions under well-defined experimental conditions with chemical and electrical access to both sides of the lipid membrane. Employing an assay that relies on the conductance of the pore-forming peptide gramicidin A to monitor PLD activity, the work presented here reveals the kinetics of hydrolysis of long-chain phosphatidylcholine lipids in situ. We have developed an extension of a basic kinetic model for interfacial catalysis that includes product activation and substrate depletion. This model describes the kinetic behavior very well and reveals two kinetic parameters, the specificity constant and the interfacial quality constant. This approach results in a simple and general model to account for product accumulation in interfacial enzyme kinetics.

Antioxidant protection by PECAM-targeted delivery of a novel NADPH-oxidase inhibitor to the endothelium in vitro and in vivo

Oxidant stress caused by pathological elevation of reactive oxygen species (ROS) production in the endothelial cells lining the vascular lumen is an important component of many vascular and pulmonary disease conditions. NADPH oxidase (NOX) activated by pathological mediators including angiotensin and cytokines is a major source of endothelial ROS. In order to intercept this pathological pathway, we have encapsulated an indirect NOX inhibitor, MJ33, into immunoliposomes (Ab-MJ33/IL) targeted to endothelial marker platelet endothelial cell adhesion molecule (PECAM-1). Ab-MJ33/IL, but not control IgG-MJ33/IL are specifically bound to endothelium and attenuated angiotensin-induced ROS production in vitro and in vivo. Additionally, Ab-MJ33/IL inhibited endothelial expression of the inflammatory marker vascular cell adhesion molecule (VCAM) in cells and animals challenged with the cytokine TNF. Furthermore, Ab-MJ33/IL alleviated pathological disruption of endothelial permeability barrier function in cells exposed to vascular endothelial growth factor (VEGF) and in the lungs of mice challenged with lipopolysaccharide (LPS). Of note, the latter beneficial effect has been achieved both by prophylactic and therapeutic injection of Ab-MJ33/IL in animals. Therefore, specific suppression of ROS production by NOX in endothelium, attainable by Ab-MJ33/IL targeting, may help deciphering mechanisms of vascular oxidative stress and inflammation, and potentially improve treatment of these conditions.

Hypervalent nonbonded interactions of a divalent sulfur atom. Implications in protein architecture and the functions

In organic molecules a divalent sulfur atom sometimes adopts weak coordination to a proximate heteroatom (X). Such hypervalent nonbonded S···X interactions can control the molecular structure and chemical reactivity of organic molecules, as well as their assembly and packing in the solid state. In the last decade, similar hypervalent interactions have been demonstrated by statistical database analysis to be present in protein structures. In this review, weak interactions between a divalent sulfur atom and an oxygen or nitrogen atom in proteins are highlighted with several examples. S···O interactions in proteins showed obviously different structural features from those in organic molecules (i.e., π(o) → σ(s)* versus n(o) → σ(s)* directionality). The difference was ascribed to the HOMO of the amide group, which expands in the vertical direction (π(o)) rather than in the plane (n(o)). S···X interactions in four model proteins, phospholipase A₂ (PLA₂), ribonuclease A (RNase A), insulin, and lysozyme, have also been analyzed. The results suggested that S···X interactions would be important factors that control not only the three-dimensional structure of proteins but also their functions to some extent. Thus, S···X interactions will be useful tools for protein engineering and the ligand design.

Cholesterol oxidase: structure and function

Cholesterol oxidase is a bacterial-specific flavoenzyme that catalyzes the oxidation and isomerisation of steroids containing a 3beta hydroxyl group and a double bond at the Delta5-6 of the steroid ring system. The enzyme is a member of a large family of flavin-specific oxidoreductases and is found in two different forms: one where the flavin adenine dinucleotide (FAD) cofactor is covalently linked to the protein and one where the cofactor is non-covalently bound to the protein. These two enzyme forms have been extensively studied in order to gain insight into the mechanism of flavin-mediated oxidation and the relationship between protein structure and enzyme redox potential. More recently the enzyme has been found to play an important role in bacterial pathogenesis and hence further studies are focused on its potential use for future development of novel antibacterial therapeutic agents. In this review the biochemical, structural, kinetic and mechanistic features of the enzyme are discussed.

Structure and phospholipase function of peroxiredoxin 6: identification of the catalytic triad and its role in phospholipid substrate binding

Peroxiredoxin 6 (Prdx6) is a bifunctional protein with glutathione peroxidase and phospholipase A(2) (PLA(2)) activities, and it alone among mammalian peroxiredoxins can hydrolyze phospholipids. After identifying a potential catalytic triad (S32, H26, D140) from the crystal structure, site-specific mutations were used to evaluate the role of these residues in protein structure and function. The S32A mutation increased Prdx6 alpha-helical content, whereas secondary structure was unchanged by mutation to H26A and D140A. Lipid binding by wild-type Prdx6 to negatively charged unilamellar liposomes showed an apparent rate constant of 11.2 x 10(6) M(-1) s(-1) and a dissociation constant of 0.36 microM. Both binding and PLA(2) activity were abolished in S32A and H26A; in D140A, activity was abolished but binding was unaffected. Overoxidation of the peroxidatic C47 had no effect on lipid binding or PLA(2) activity. Fluorescence resonance energy transfer from endogenous tryptophanyls to lipid probes showed binding of the phospholipid polar head in close proximity to S32. Thus, H26 is a site for interfacial binding to the liposomal surface, S32 has a key role in maintaining Prdx6 structure and for phospholipid substrate binding, and D140 is involved in catalysis. This putative catalytic triad plays an essential role for interactions of Prdx6 with phospholipid substrate to optimize the protein-substrate complex for hydrolysis.

Development of a constant surface pressure penetration langmuir balance based on axisymmetric drop shape analysis

A new constant pressure pendant-drop penetration surface balance has been developed combining a pendant-drop surface balance, a rapid-subphase-exchange technique, and a fuzzy logic control algorithm. Beside the determination of insoluble monolayer compression-expansion isotherms, it allows performance of noninvasive kinetic studies of the adsorption of surfactants added to the new subphase onto the free surface and of the adsorption/penetration/reaction of the former onto/into/with surface layers, respectively. The interfacial pressure pi is a fundamental parameter in these studies: by working at constant pi one controls the height of the energy barrier to adsorption/penetration and can select different regimes and steps of the adsorption/penetration process. In our device a solution drop is formed at the tip of a coaxial double capillary, connected to a double microinjector. Drop profiles are extracted from digital drop micrographs and fitted to the equation of capillarity, yielding pi, the drop volume V, and the interfacial area A. pi is varied changing V (and hence A) with the microinjector. Control is based on a case-adaptable modulated fuzzy-logic PID algorithm able to maintain constant pi (or A) under a wide range of experimental conditions. The drop subphase liquid can be exchanged quantitatively by the coaxial capillaries. The adsorption/penetration/reaction kinetics at constant pi are then studied monitoring A(t), i.e., determining the relative area change necessary at each instant to compensate the pressure variation due to the interaction of the surfactant in the subsurface with the surface layer. A fully Windows-integrated program manages the whole setup. Examples of experimental protein adsorption and monolayer penetration kinetics are presented.

Exploring the specific features of interfacial enzymology based on lipase studies

Many enzymes are active at interfaces in the living world (such as in the signaling processes at the surface of cell membranes, digestion of dietary lipids, starch and cellulose degradation, etc.), but fundamental enzymology remains largely focused on the interactions between enzymes and soluble substrates. The biochemical and kinetic characterization of lipolytic enzymes has opened up new paths of research in the field of interfacial enzymology. Lipases are water-soluble enzymes hydrolyzing insoluble triglyceride substrates, and studies on these enzymes have led to the development of specific interfacial kinetic models. Structure-function studies on lipases have thrown light on the interfacial recognition sites present in the molecular structure of these enzymes, the conformational changes occurring in the presence of lipids and amphiphiles, and the stability of the enzymes present at interfaces. The pH-dependent activity, substrate specificity and inhibition of these enzymes can all result from both "classical" interactions between a substrate or inhibitor and the active site, as well as from the adsorption of the enzymes at the surface of aggregated substrate particles such as oil drops, lipid bilayers or monomolecular lipid films. The adsorption step can provide an alternative target for improving substrate specificity and developing specific enzyme inhibitors. Several data obtained with gastric lipase, classical pancreatic lipase, pancreatic lipase-related protein 2 and phosphatidylserine-specific phospholipase A1 were chosen here to illustrate these specific features of interfacial enzymology.

Possible roles of S···O and S···N interactions in the functions and evolution of phospholipase A2

To investigate possible roles of S···X (X= O, N, S) interactions in the functions and evolution of a protein, two types of database analyses were carried out for a vertebrate phospholipase A₂ (PLA₂) family. A comprehensive search for close S···X contacts in the structures retrieved from protein data bank (PDB) revealed that there are four common S···O interactions and one common S···N interaction for the PLA₂ domain group (PLA₂-DG), while an additional three S···O interactions were found for the snake PLA₂ domain group (sPLA₂-DG). On the other hand, a phylogenetic analysis on the conservation of the observed S···O and S···N interactions over various amino acid sequences of sPLA₂-DG demonstrated probable clustering of the interactions on the dendrogram. Most of the interactions characterized for PLA₂ were found to reside in the vicinity of the active site and to be able to tolerate the conformational changes due to the substrate binding. These observations suggested that the S···X interactions play some role in the functions and evolution of the PLA₂ family.

Atomic resolution structure of the double mutant (K53,56M) of bovine pancreatic phospholipase A2

The structure of the double mutant K53,56M has previously been refined at 1.9 A resolution using room-temperature data. The present paper reports the crystal structure of the same mutant K53,56M refined against 1.1 A data collected using synchrotron radiation. A total of 116 main-chain atoms from 29 residues and 44 side chains are modelled in alternate conformations. Most of the interfacial binding residues are found to be disordered and alternate conformations could be recognized. The second calcium ion-binding site residue Glu92 adopts two alternate conformations. The minor and major conformations of Glu92 correspond to the second calcium ion bound and unbound states.

On the importance of anandamide structural features for its interactions with DPPC bilayers: effects on PLA2 activity

The acylethanolamide anandamide (AEA) occurs in a variety of mammalian tissues and, as a result of its action on cannabinoid receptors, exhibits several cannabimimetic activities. Moreover, some of its effects are mediated through interaction with an ion channel-type vanilloid receptor. However, the chemical features of AEA suggest that some of its biological effects could be related to physical interactions with the lipidic part of the membrane. The present work studies the effect of AEA-induced structural modifications of the dipalmitoylphosphatidylcholine (DPPC) bilayer on phospholipase A2 (PLA2) activity, which is strictly dependent on lipid bilayer features. This study, performed by 2-dimethylamino-(6-lauroyl)-naphthalene fluorescence, demonstrates that the effect of AEA on PLA2 activity is concentration-dependent. In fact, at low AEA/DPPC molar ratios (from R = 0.001 to R = 0.04), there is an increase of the enzymatic activity, which is completely inhibited for R = 0.1. X-ray diffraction data indicate that the AEA affects DPPC membrane structural properties in a concentration-dependent manner. Because the biphasic effect of increasing AEA concentrations on PLA2 activity is related to the induced modifications of membrane bilayer structural properties, we suggest that AEA-phospholipid interactions may be important to produce, at least in part, some of the similarly biphasic responses of some physiological activities to increasing concentrations of AEA.

Advanced strategies in liposomal cancer therapy: problems and prospects of active and tumor specific drug release

Tumor specific drug delivery has become increasingly interesting in cancer therapy, as the use of chemotherapeutics is often limited due to severe side effects. Conventional drug delivery systems have shown low efficiency and a continuous search for more advanced drug delivery principles is therefore of great importance. In the first part of this review, we present current strategies in the drug delivery field, focusing on site-specific triggered drug release from liposomes in cancerous tissue. Currently marketed drug delivery systems lack the ability to actively release the carried drug and rely on passive diffusion or slow non-specific degradation of the liposomal carrier. To obtain elevated tumor-to-normal tissue drug ratios, it is important to develop drug delivery strategies where the liposomal carriers are actively degraded specifically in the tumor tissue. Many promising strategies have emerged ranging from externally triggered light- and thermosensitive liposomes to receptor targeted, pH- and enzymatically triggered liposomes relying on an endogenous trigger mechanism in the cancerous tissue. However, even though several of these strategies were introduced three decades ago, none of them have yet led to marketed drugs and are still far from achieving this goal. The most advanced and prospective technologies are probably the prodrug strategies where non-toxic drugs are carried and activated specifically in the malignant tissue by overexpressed enzymes. In the second part of this paper, we review our own work, exploiting secretory phospholipase A2 as a site-specific trigger and prodrug activator in cancer therapy. We present novel prodrug lipids together with biophysical investigations of liposome systems, constituted by these new lipids and demonstrate their degradability by secretory phospholipase A2. We furthermore give examples of the biological performance of the enzymatically degradable liposomes as advanced drug delivery systems.

Investigating porcine pancreatic phospholipase A2 action on vesicles and supported planar bilayers using a quartz crystal microbalance with dissipation

We present an investigation of the activity of porcine pancreatic phospholipase A2 towards phospholipids. The phospholipids are presented in three different ways, namely as tethered vesicles, intact surface-bound vesicles, and supported planar bilayers (SPBs). The process is followed using a quartz crystal microbalance which measures both the frequency shift and the energy dissipation factor. This technique is very sensitive not only to the mass of the material deposited on the crystal, but also to its viscoelasticity. The breakdown of the phospholipid vesicles and bilayers consequently gives rise to very large signal changes. Enzyme binding is separated from vesicle hydrolysis using nonhydrolyzable ether lipids. Intact and tethered vesicles give rise to the same profile, indicating that direct immobilization of the vesicles does not affect hydrolysis significantly. The data fit well to a Voight-based model describing the change in film structure with time. Initial enzyme binding to intact vesicles is accompanied by a significant increase in layer thickness as well as a decrease in viscosity and shear modulus. This effect, which is less pronounced in SPBs, is probably mainly due to the accumulation of hydrolysis products in the vesicle prior to rupture of the vesicles and release of bound water, since it disappears when lysolipid is included in the vesicles prior to hydrolysis.

Interfacial kinetic analysis of the tumour suppressor phosphatase, PTEN: evidence for activation by anionic phospholipids

We investigated the kinetic behaviour and substrate specificity of PTEN (phosphatase and tensin homologue deleted on chromosome 10) using unilamellar vesicles containing substrate lipids in a background of phosphatidylcholine. PTEN displays the characteristics expected of an interfacial enzyme, since the rate of enzyme activity is dependent on the surface concentration of the substrate lipids used (mol fraction), as well as the bulk concentration. Surface-dilution analysis revealed the catalytic efficiency of PTEN for PtdIns(3,4,5) P (3) to be 200-fold greater than for either PtdIns(3,4) P (2) or PtdIns(3,5) P (2), and 1000-fold greater than for PtdIns3 P. The interfacial K (m) value of PTEN for PtdIns(3,4,5) P (3) was very low, reflecting the small proportions of this lipid that are present in cellular membranes. The catalytic-centre activity ( k (cat)) for PtdIns(3,4,5) P (3) was at least 200-fold greater than that for the water-soluble substrate Ins(1,3,4,5) P (4). The preference for lipid substrates may result from an interfacial activation of the enzyme, rather than processive catalysis of vesicular substrates. Moreover, both PtdIns(4,5) P (2) and univalent salts stimulated the activity of PTEN for PtdIns(3,4,5) P (3), but profoundly inhibited activity against Ins(1,3,4,5) P (4). The stimulatory effect of PtdIns(4,5) P (2) was greater in magnitude and more potent in comparison with other anionic phospholipid species. A mutation in the lipid-binding C2 domain (M-CBR3) that is biologically inactive did not alter overall catalytic efficiency in this model, but decreased the efficiency of the interfacial binding step, demonstrating its importance in the catalytic mechanism of PTEN.

Different modulation of phospholipase A2 activity by saturated and monounsaturated N-acylethanolamines

The physiological functions of N-acylethanolamines (NAEs) are poorly understood, although many functions were suggested for these naturally occurring membrane components of plants and animals. The binding with cannabinoid receptors CB1 and CB2 was demonstrated for some NAEs, such as anandamide. However, the chemical nature of these molecules suggests that some of their biological effects on biomembranes could be related, at least partially, to physical interactions with the lipid bilayer. The present work studies the effect of saturated and monounsaturated NAEs on phospholipase A2 (PLA2) activity, which is dependent on lipid bilayer features. The present study, performed by 2-dimethylamino-(6-lauroyl)-naphthalene (Laurdan) fluorescence, demonstrates that the acyl chain length and the presence of a single double bond are crucial for the enzymatic activity modulation by NAEs. In fact, saturated NAEs with 10 carbon atoms don't affect the PLA2 activity, while NAEs with 12 and 16 carbon atoms largely activate the enzyme. On the other hand, an acyl chain length of 18 carbon atoms, with or without the presence of a double bond, only slightly affects the enzymatic activity. A structural model for NAE-lipid interactions is proposed in order to explain the differences in PLA2 activity modulation by these fatty acid derivatives.

Modulation of the activity of secretory phospholipase A2 by antimicrobial peptides

The antimicrobial peptides magainin 2, indolicidin, and temporins B and L were found to modulate the hydrolytic activity of secretory phospholipase A(2) (sPLA(2)) from bee venom and in human lacrimal fluid. More specifically, hydrolysis of phosphatidylcholine (PC) liposomes by bee venom sPLA(2) at 10 micro M Ca(2+) was attenuated by these peptides while augmented product formation was observed in the presence of 5 mM Ca(2+). The activity of sPLA(2) towards anionic liposomes was significantly enhanced by the antimicrobial peptides at low [Ca(2+)] and was further enhanced in the presence of 5 mM Ca(2+). Similarly, with 5 mM Ca(2+) the hydrolysis of anionic liposomes was enhanced significantly by human lacrimal fluid sPLA(2), while that of PC liposomes was attenuated. These results indicate that concerted action of antimicrobial peptides and sPLA(2) could improve the efficiency of the innate response to infections. Interestingly, inclusion of a cationic gemini surfactant in the vesicles showed an essentially similar pattern on sPLA(2) activity, suggesting that the modulation of the enzyme activity by the antimicrobial peptides may involve also charge properties of the substrate surface.

Crystal structure of human group X secreted phospholipase A2. Electrostatically neutral interfacial surface targets zwitterionic membranes

The crystal structure of human group X (hGX) secreted phospholipase A2 (sPLA2) has been solved to a resolution of 1.97 A. As expected the protein fold is similar to previously reported sPLA2 structures. The active site architecture, including the positions of the catalytic residues and the first and second shell water around the Ca2+ cofactor, are highly conserved and remarkably similar to the group IB and group IIA enzymes. Differences are seen in the structures following the (1-12)-N-terminal helix and at the C terminus. These regions are proposed to interact with the substrate membrane surface. The opening to the active site slot is considerably larger in hGX than in human group IIA sPLA2. Furthermore, the electrostatic surface potential of the hGX interfacial-binding surface does not resemble that of the human group IIA sPLA2; the former is highly neutral, whereas the latter is highly cationic. The cationic residues on this face of group IB and IIA enzymes have been implicated in membrane binding and in k(cat*) allostery. In contrast, hGX does not show activation by the anionic charge at the lipid interface when acting on phospholipid vesicles or short-chain phospholipid micelles. Together, the crystal structure and kinetic results of hGX supports the conclusion that it is as active on zwitterionic as on anionic interfaces, and thus it is predicted to target the zwitterionic membrane surfaces of mammalian cells.

Processive interfacial catalytic turnover by Bacillus cereus sphingomyelinase on sphingomyelin vesicles

Sphingomyelinase (SMase), a water-soluble enzyme from Bacillus cereus, is shown to bind with high affinity to vesicles of sphingomyelin (SM) but not to vesicles of phosphatidylcholine (PC). The reaction progress by SMase bound to SM vesicles occurs in the scooting mode with virtually infinite processivity of the successive interfacial turnover cycles. Three conditions for the microscopic steady state during the reaction progress at the interface are satisfied: the bound SMase does not leave the interface even after all the SM in the outer layer is converted to ceramide; the SMase-treated vesicles remain intact; and the ceramide product does not exchange with SM present in excess vesicles or in the inner layer of the hydrolyzed vesicle. Within these constraints, on accessibility and replenishment of the substrate, the extent of hydrolysis in the scooting mode reaction progress is a measure of the number of vesicles containing enzyme. The slope of the Poisson distribution plot, for the enzyme per vesicle versus the logarithm of the fraction of the total accessible substrate remaining unhydrolyzed in excess vesicles, shows that a single 32 kDa subunit of SMase is fully catalytically active. The maximum initial rate of hydrolysis, at the limit of the maximum possible substrate mol fraction, X(S)*=1, is 400 s(-1) in H(2)O and 220 s(-1) in D(2)O, which is consistent with the rate-limiting chemical step. The integrated reaction progress suggests that the ceramide product does not codisperse ideally on the hydrolyzed vesicles. Furthermore, complex reaction progress seen with covesicles of SM+PC are attributed to slow secondary changes in the partially hydrolyzed SM vesicles.

A snake venom phospholipase A2 blocks malaria parasite development in the mosquito midgut by inhibiting ookinete association with the midgut surface

Oocyst formation is a critical stage in the development of the malaria parasite in the mosquito. We have discovered that the phospholipase A2 (PLA2) from the venom of the eastern diamondback rattlesnake (Crotalus adamanteus) inhibits oocyst formation when added to infected chicken blood and fed to mosquitoes. A similar transmission-blocking activity was demonstrated for PLA2s from the venom of other snakes and from the honeybee. This effect is seen both with the avian malaria parasite Plasmodium gallinaceum and with the human parasite Plasmodium falciparum developing in their respective mosquito hosts. The inhibition occurs even in the presence of an irreversible inhibitor of the active site of PLA2, indicating that the hydrolytic activity of the enzyme is not required for the antiparasitic effect. Inhibition is also seen when the enzyme is fed to mosquitoes together with ookinetes, suggesting that the inhibition occurs after ookinete maturation. PLA2 has no direct effect on the parasite. However, pretreatment of midguts with PLA2 (catalytically active or inactive) dramatically lowers the level of ookinete/midgut association in vitro. It appears, therefore, that PLA2 is acting by associating with the midgut surface and preventing ookinete attachment to this surface. Thus, PLA2 is an excellent candidate for expression in transgenic mosquitoes as a means of inhibiting the transmission of malaria.

A pyrrolidine-based specific inhibitor of cytosolic phospholipase A(2)alpha blocks arachidonic acid release in a variety of mammalian cells

We analyzed a recently reported (K. Seno, T. Okuno, K. Nishi, Y. Murakami, F. Watanabe, T. Matsuur, M. Wada, Y. Fujii, M. Yamada, T. Ogawa, T. Okada, H. Hashizume, M. Kii, S.-H. Hara, S. Hagishita, S. Nakamoto, J. Med. Chem. 43 (2000)) pyrrolidine-based inhibitor, pyrrolidine-1, against the human group IV cytosolic phospholipase A(2) alpha-isoform (cPLA(2)alpha). Pyrrolidine-1 inhibits cPLA(2)alpha by 50% when present at approx. 0.002 mole fraction in the interface in a number of in vitro assays. It is much less potent on the cPLA(2)gamma isoform, calcium-independent group VI PLA(2) and groups IIA, X, and V secreted PLA(2)s. Pyrrolidine-1 blocked all of the arachidonic acid released in Ca(2+) ionophore-stimulated CHO cells stably transfected with cPLA(2)alpha, in zymosan- and okadaic acid-stimulated mouse peritoneal macrophages, and in ATP- and Ca(2+) ionophore-stimulated MDCK cells.

Effect of the lipid interface on the catalytic activity and spectroscopic properties of a fungal lipase

Lipase from the fungi Thermomyces (formerly Humicola) lanuginosa (TlL) is widely used in industry. This interfacial enzyme is inactive under aqueous conditions, but catalytic activation is induced on binding to a lipid-water interface. In order for protein engineering to design more efficient mutants of TlL for specific applications, it is important to characterize its interfacial catalysis. A complete analysis of steady-state kinetics for the hydrolysis of a soluble substrate by TlL has been developed using an interface different from the substrate. Small vesicles of 1-palmitoyl-2-oleoylglycero-sn-3-phosphoglycerol (POPG) or other anionic phospholipids are a neutral diluent interface for the partitioning of substrate and enzyme. TlL binds to these interfaces in an active or open form, thus implying a displacement of the helical lid away from the active site. A study of the influence of substrate and diluent concentration dependence of the rate of hydrolysis provides a basis for the determination of the primary interfacial catalytic parameters. The interfacial activation is not supported by zwitterionic vesicles or by large anionic vesicles of 100 nm diameter, although TlL binds to these interfaces. Using a combination of fluorescence-based techniques applied to several mutants of TlL with different tryptophan residues we have shown that TlL binds to phospholipid vesicles in different forms rendering different catalytic activities, and that the open lid conformation is achieved and stabilized by a combination of electrostatic and hydrophobic interactions between the enzyme's lipid-binding face and the interface.

Exogenously added human group X secreted phospholipase A(2) but not the group IB, IIA, and V enzymes efficiently release arachidonic acid from adherent mammalian cells

Mammalian secreted phospholipases A(2) (sPLA2s) comprise a group of at least eight enzymes, including the recently identified group X sPLA2. A bacterial expression system was developed to produce human group X sPLA2 (hGX). Inhibition studies show that the sPLA2 inhibitor LY311727 binds modestly more tightly to human group IIA sPLA2 than to hGX and that a pyrazole-based inhibitor of group IIA sPLA2 is much less active against hGX. The phospholipid head group preference of vesicle-bound hGX was determined. hGX binds tightly to phosphatidylcholine vesicles, which is thought to be required to act efficiently on cells. Tryptophan 67 hGX makes a significant contribution to interfacial binding to zwitterionic vesicles. As little as 10 ng/ml hGX releases arachidonic acid for cyclooxygenase-2- dependent prostaglandin E(2) generation when added exogenously to adherent mammalian cells. In contrast, human group IIA, rat group V, and mouse group IB sPLA2s are virtually inactive at releasing arachidonate when added exogenously to adherent cells. Dislodging cells from the growth surface enhances the ability of all the sPLA2s to release fatty acids. Studies with CHO-K1 cell mutants show that binding of sPLA2s to glycosaminoglycans is not the basis for poor plasma membrane hydrolysis by group IB, IIA, and V sPLA2s.

The electrostatics of lipid surfaces

Charged lipids constitute a substantial fraction of all membrane lipids. Their charges vary in quantity and distribution within their headgroup regions. In long range interactions, their charges' value and electrostatic potential in the vicinity of the membrane surface can be approximated by the Guy-Chapman theory. This theory treats the interface as a charged structureless plain surrounded by uniform environments. However, if one considers intermolecular interactions, such assumptions need to be revised. The interface is in reality a thick region containing the residual charges of lipid headgroups. Their arrangement depends on the type of lipid present in the membrane. The variety of lipids and their biological functions suggests that charge distribution determines the extent and type of interaction with surface associated molecules. Numerous examples show that protein behavior at the lipid bilayer surface is determined by the type of lipid present, indicating protein specificity towards certain surface locations and local properties (determined by lipid composition) of a particular type. Such specificity is achieved by a combination of electrostatic, hydrophobic and enthropic effects. Comparing lipid biological activity, it can be stated that residual charge distribution is one of the factors of intermolecular recognition leading to the specific interaction of lipid molecules and selected proteins in various processes, particularly those involved with signal transduction pathways. Such specificity enables a variety of processes occurring simultaneously on the same membrane surface to function without cross-reaction interference.

Significance of reduced dimensionality in reaction kinetics: impact of multi-site particles

This review examines novel kinetic properties of enzymes on membrane surfaces or states of restricted diffusion. A leading feature is the presence of multiple enzymes and/or substrates per particle. In these states, enzymes can be influenced by parameters such as the number of substrates or enzymes per particle, particle size, the rates of exchange of substrate or enzyme from the particle, or substrate diffusion to the particle. These steps are independent of the enzyme site parameters which are described by classical enzymology. The results make it clear that non-classical behaviors are important to biological systems, are the basis for some enzyme expression levels and are determinants of cellular design. To identify more unique functions of these states, descriptions of catalysis in the non-solution state should become a part of kinetic education in biology.

1,3-Dioxane-4,6-dione-5-carboxamide-based inhibitors of human group IIA phospholipase A: X-ray structure of the complex and interfacial selection of inhibitors from a structural library

A library of 109 1,3-dioxane-4,6-dione-5-carboxamides was prepared by solution-phase methods as potential inhibitors of human group IIa phospholipase A2. Tight binding inhibitors were found by an interfacial affinity selection method. The crystal structure of the secreted phospholipase A2 containing one of the inhibitors was determined, and it reveals the inhibitor-calcium bidendate coordination.

Lipid exchange between mixed micelles of phospholipid and triton X-100

If phospholipase catalyzed hydrolysis of phospholipid dissolved in a detergent mixed micelle is limited to the phospholipid carried by a single micelle, then hydrolysis ceases upon exhaustion of that pool. However, if the rate of phospholipid exchange between micelles exceeds the catalytic rate then all of the phospholipid is available for hydrolysis. To determine phospholipid availability we studied the exchange of 1,2-dioleoyl-sn-glycero-3-phosphocholine between mixed micelles of phospholipid and non-ionic Triton detergents by both stopped-flow fluorescence-recovery and nuclear magnetic resonance-relaxation techniques. Stopped-flow analysis was performed by combining mixed micelles of Triton and phospholipid with mixed micelles that contained the fluorescent phospholipid 1-palmitoyl-2-(12-[{7-nitro-2-1, 3-benzoxadiazo-4-yl}amino]dodecanoyl)-sn-glycero-3-phosphocholine (P-2-NBD-PC). The concentration dependence of fluorescence recovery suggested a second-order exchange mechanism that was saturable. The true second-order rate constant depends on the specific mechanism for exchange, which was not determined in this study, but the rate constant will be on the order of 106 to 107 M-1s-1. Incorporation of 1-palmitoyl-2-(16-doxylstearoyl)phosphatidylcholine into micelles increased the rate of proton relaxation and gave a limiting relaxation time of 1.3 ms. The results demonstrate that phospholipid exchange was rapid and that the phospholipid content of a single micelle did not limit the rate of phospholipid hydrolysis by phospholipases.

Action of human group IIa secreted phospholipase A2 on cell membranes. Vesicle but not heparinoid binding determines rate of fatty acid release by exogenously added enzyme

Human group IIa phospholipase A2 (hIIa-PLA2) is a highly basic protein that is secreted from a number of cells during inflammation and may play a role in arachidonate liberation and in destruction of invading bacteria. It has been proposed that rodent group IIa PLA2 is anchored to cell surfaces via attachment to heparan sulfate proteoglycan and that this interaction facilitates lipolysis. hIIa-PLA2 contains 13 lysines, 2 histidines, and 10 arginines that fall into 10 clusters. A panel of 26 hIIa-PLA2 mutants were prepared in which 1-4 basic residues in each cluster were changed to glutamate or aspartate (charge reversal). A detailed analysis of the affinities of these mutants for anionic vesicles and for heparin and heparan sulfate in vitro and of the specific activities of these proteins for hydrolysis of vesicles in vitro and of living cell membranes reveal the following trends: 1) the affinity of hIIa-PLA2 for heparin and heparan sulfate is modulated not by a highly localized site of basic residues but by diffuse sites that partially overlap with the interfacial binding site. In contrast, only those residues on the interfacial binding site of hIIa-PLA2 are involved in binding to membranes; 2) the relative ability of these mutants to hydrolyze cellular phospholipids when enzymes were added exogenously to CHO-K1, NIH-3T3, and RAW 264.7 cells correlates with their relative in vitro affinity for vesicles and not with their affinity for heparin and heparan sulfate. 3) The rates of exogenous hIIa-PLA2-catalyzed fatty acid release from wild type CHO-K1 cells and two mutant lines, one lacking glycosaminoglycan and one lacking heparan sulfate, were similar. Thus basic residues that modulate interfacial binding are important for plasma membrane fatty acid release by exogenously added hIIa-PLA2. Binding of hIIa-PLA2 to cell surface heparan sulfate does not modulate plasma membrane phospholipid hydrolysis by exogenously added hIIa-PLA2.

Crystal structure of piratoxin-I: a calcium-independent, myotoxic phospholipase A2-homologue from Bothrops pirajai venom

The crystal structure of Piratoxin-I (PrTX-I) a Lys49 homologue isolated from the venom of Bothrops pirajai has been determined and refined at 2.8 A to a crystallographic residual of 19.7% (Rfree = 29.7%). Amino-acid sequence differences between catalytically active phospholipases and PrTX-I in the putative Ca2+-binding loop, specifically the substitutions Tyr28 --> Asn, Gly32 --> Leu and Asp49 --> Lys, result in an altered conformation of this loop. The analysis of the position of the epsilon-amino group of Lys49 in the PrTX-I structure indicates that it fills the site normally occupied by the calcium ion in the catalytically active phospholipases. In contrast to the homologous monomeric Lys49 variant from Agkistrodon piscivorus piscivorus (App), PrTX-I is present as a dimer in the crystalline state, as observed in the structures of myotoxin II from Bothrops asper and Bothropstoxin I from Bothrops jararacussu. The two molecules in the asymmetric unit in the crystal structure of PrTX-I are related by a nearly perfect two-fold symmetry axis, yet the dimeric structure is radically different from the dimeric structure of the phospholipase from Crotalus atrox. In the C. atrox structure the dimer interface occludes the active sites, whereas in the PrTX-I structure they are exposed to solvent.