Kinetic characterization of phospholipase A2 modified by manoalogue

Biochemical characterization of selective inhibitors of human group IIA secreted phospholipase A(2) and hyaluronic acid-linked inhibitor conjugates

We explored the inhibition mode of group IIA secreted phospholipase A(2) (GIIA sPLA(2)) selective inhibitors and tested their ability to inhibit GIIA sPLA(2) activity as chemical conjugates with hyaluronic acid (HA). Analogues of a benzo-fused indole sPLA(2) inhibitor were developed in which the carboxylate group on the inhibitor scaffold, which has been shown to coordinate to a Ca(2+) ligand in the enzyme active site, was replaced with other functionality. Replacing the carboxylate group with amine, amide, or hydroxyl groups had no effect on human GIIA (hGIIA) sPLA(2) inhibition potency but dramatically lowered inhibition potency against hGV and hGX sPLA(2)s. An alkylation protection assay was used to probe active site binding of carboxylate and noncarboxylate inhibitors in the presence and absence of Ca(2+) and/or lipid vesicles. We observed that carboxylate-containing inhibitors bind the hGIIA sPLA(2) active site with low nanomolar affinity, but only when Ca(2+) is present. Noncarboxylate, GIIA sPLA(2) selective inhibitors also bind the hGIIA sPLA(2) active site in the nanomolar range. However, binding for GIIA sPLA(2) selective inhibitors was dependent on the presence of a lipid membrane and not Ca(2+). These results indicate that GIIA sPLA(2) selective inhibitors exert their inhibitory effects by binding to the hGIIA sPLA(2) active site. An HA-linked GIIA inhibitor conjugate was developed using peptide coupling conditions and found to be less potent and selective against hGIIA sPLA(2) than the unconjugated inhibitor. Compounds reported in this study are some of the most potent and selective GIIA sPLA(2) active site binding inhibitors reported to date.

Assay of phospholipases A(2) and their inhibitors by kinetic analysis in the scooting mode

Several cellular processes are regulated by interfacial catalysis on biomembrane surfaces. Phospholipases A₂ (PLA₂) are interesting not only as prototypes for interfacial catalysis, but also because they mobilize precursors for the biosynthesis of eicosanoids and platelet activating factor, and these agents ultimately control a wide range of secretory and inflammatory processes. Since PLA₂ carry out their catalytic function at membrane surfaces, the kinetics of these enzymes depends on what the enzyme ‘sees’ at the interface, and thus the observed rate is profoundly influenced by the organization and dynamics of the lipidwater interface (‘quality of the interface’). In this review we elaborate the advantages of monitoring interfacial catalysis in the scooting mode, that is, under the conditions where the enzyme remains bound to vesicles for several thousand catalytic turnover cycles. Such a highly processive catalytic turnover in the scooting mode is useful for a rigorous and quantitative characterization of the kinetics of interfacial catalysis. This analysis is now extended to provide insights into designing strategy for PLA₂ assays and screens for their inhibitors.

Biochemistry and physiology of mammalian secreted phospholipases A2

Phospholipases A(2) (PLA2s) are esterases that hydrolyze the sn-2 ester of glycerophospholipids and constitute one of the largest families of lipid hydrolyzing enzymes. The mammalian genome contains 10 enzymatically active secreted PLA2s (sPLA2s) and two sPLA2-related proteins devoid of lipolytic enzymatic activity. In addition to the well-established functions of one of these enzymes in digestion of dietary phospholipids and another in host defense against bacterial infections, accumulating evidence shows that some of these sPLA2s are involved in arachidonic acid release from cellular phospholipids for the biosynthesis of eicosanoids, especially during inflammation. More speculative results suggest the involvement of one or more sPLA2s in promoting atherosclerosis and cancer. In addition, the mammalian genome encodes several types of sPLA2-binding proteins, and mounting evidence shows that sPLA2s may have functions related to binding to cellular target proteins in a manner independent of their lipolytic enzymatic activity.

Interaction of group IA phospholipase A2 with metal ions and phospholipid vesicles probed with deuterium exchange mass spectrometry

Deuterium exchange mass spectrometric evaluation of the cobra venom (Naja naja naja) group IA phospholipase A 2 (GIA PLA 2) was carried out in the presence of metal ions Ca (2+) and Ba (2+) and phospholipid vesicles. Novel conditions for digesting highly disulfide bonded proteins and a methodology for studying protein-lipid interactions using deuterium exchange have been developed. The enzyme exhibits unexpectedly slow rates of exchange in the two large alpha-helices of residues 43-53 and 89-101, which suggests that these alpha-helices are highly rigidified by the four disulfide bonds in this region. The binding of Ca (2+) or Ba (2+) ions decreased the deuterium exchange rates for five regions of the protein (residues 24-27, 29-40, 43-53, 103-110, and 111-114). The magnitude of the changes was the same for both ions with the exception of regions of residues 24-27 and 103-110 which showed greater changes for Ca (2+). The crystal structure of the N. naja naja GIA PLA 2 contains a single Ca (2+) bound in the catalytic site, but the crystal structures of related PLA 2s contain a second Ca (2+) binding site. The deuterium exchange studies reported here clearly show that in solution the GIA PLA 2 does in fact bind two Ca (2+) ions. With dimyristoylphosphatidylcholine (DMPC) phospholipid vesicles with 100 microM Ca (2+) present at 0 degrees C, significant areas on the i-face of the enzyme showed decreases in the rate of exchange. These areas included regions of residues 3-8, 18-21, and 56-64 which include Tyr-3, Trp-61, Tyr-63, and Phe-64 proposed to penetrate the membrane surface. These regions also contained Phe-5 and Trp-19, proposed to bind the fatty acyl tails of substrate.

Production of synthetically created phospholipase A(2) variants with industrial impact

Phospholipases A(2) (PLA(2)) play an important role for the production of lysophospholipids. Presently they are mainly obtained from porcine or bovine pancreas but these mammalian sources are not accepted in several fields of application. To make accessible a non-mammalian PLA(2) to industrial application, synthetic genes encoding PLA(2) from honey bee (Apis mellifera) with modified N-termini were constructed and expressed in Escherichia coli. While expression of the gene with an N-terminal leader sequence to direct the protein into the periplasm failed, four variants with slightly modified N-termini (I1A-PLA(2), I1V-PLA(2), His(6)-tagged PLA(2) and PLA(2) still containing the start methionine) were successfully expressed. In all cases, the PLA(2) variants were produced as inclusion bodies. Their protein content amounted to 26-35% of total cell protein. The optimized renaturation procedure and subsequent purification by cation-exchange chromatography yielded pure active enzymes in yields of 4-11 mg L(-1). The recombinant PLA(2) variants showed activities, far-UV CD and fluorescence spectra similar to the glycosylated PLA(2) isolated from the venom glands of honey bee (bv-PLA(2)). The thermodynamic stabilities of the recombinant enzymes calculated from the transition curves of guanidine hydrochloride induced unfolding were also nearly identical to the stability of bv-PLA(2). For the variant I1A-PLA(2) high-cell density fermentation in 10 L-scale using mineral salt medium was shown to increase the volumetric enzyme yield considerably.

Docking phospholipase A2 on membranes using electrostatic potential-modulated spin relaxation magnetic resonance

A method involving electron paramagnetic resonance spectroscopy of a site-selectively spin-labeled peripheral membrane protein in the presence and absence of membranes and of a water-soluble spin relaxant (chromium oxalate) has been developed to determine how bee venom phospholipase A2 sits on the membrane. Theory based on the Poisson-Boltzmann equation shows that the rate of spin relaxation of a protein-bound nitroxide by a membrane-impermeant spin relaxant depends on the distance (up to tens of angstroms) from the spin probe to the membrane. The measurements define the interfacial binding surface of this secreted phospholipase A2.

Localization of structural elements of bee venom phospholipase A2 involved in N-type receptor binding and neurotoxicity

We have shown previously that neurotoxic venom secretory phospholipases A2 (sPLA2s) have specific receptors in brain membranes called N-type receptors that are likely to play a role in the molecular events leading to neurotoxicity of these proteins. The sPLA2 found in honey bee venom is neurotoxic and binds to this receptor with high affinity. In this paper, we have used a number of mutants of bee venom sPLA2 produced in Escherichia coli to determine the structural elements of this protein that are involved in its binding to N-type receptors. Mutations in the interfacial binding surface, in the Ca2+-binding loop and in the hydrophobic channel lead to a dramatic decrease in binding to N-type receptors, whereas mutations of surface residues localized in other parts of the sPLA2 structure do not significantly modify the binding properties. Neurotoxicity experiments show that mutants with low affinity for N-type receptors are devoid of neurotoxic properties, even though some of them retain high enzymatic activity. These results provide further evidence for the involvement of N-type receptors in neurotoxic processes associated with venom sPLA2s and identify the surface region surrounding the hydrophobic channel of bee venom sPLA2 as the N-type receptor recognition domain.

Molecular dynamics study of phospholipase A2 on a membrane surface

The desolvation of lipid molecules in a complex of the enzyme human synovial phospholipase A2 with a lipid membrane is investigated as a mechanism that enhances the overall activity of the enzyme. For this purpose the interaction of the enzyme phospholipase A2 with a dilauryl-phosphatityl-ethanolamin (DLPE) membrane monolayer surface has been studied by means of molecular dynamics simulations. Two enzyme-membrane complexes, a loose and a tight complex, are considered. For comparison, simulations are also carried out for the enzyme in aqueous solution. The conformation, dynamics, and energetics of the three systems are compared, and the interactions between the protein and lipid molecules are analyzed. Free energies of solvation are calculated for the lipid molecules in the enzyme-membrane interface. Along with the calculated dielectric susceptibility at this interface, the results show the desolvation of lipids in a tightly bound, but not in a loosely bound protein-membrane complex. The desolvated lipids are found to interact mainly with hydrophobic protein residues, including Leu-2, Val-3, Ala-18, Leu-19, Phe-24, Val-31, and Phe-70. The results also explain why the turnover rate of phospholipase A2 complexed to a membrane is enhanced after a critical amount of negatively charged reaction product is accumulated.

Active site of bee venom phospholipase A2: the role of histidine-34, aspartate-64 and tyrosine-87

In bee venom phospholipase A2, histidine-34 probably functions as a Brønsted base to deprotonate the attacking water. Aspartate-64 and tyrosine-87 form a hydrogen bonding network with histidine-34. We have prepared mutants at these positions and studied their kinetic properties. The mutant in which histidine-34 is changed to glutamine is catalytically inactive, while the mutants in which aspartate-64 is changed to asparagine or alanine (interfacial turnover numbers are reduced by 50-100-fold) or in which tyrosine-87 is changed to phenylalanine (no change in turnover number) retain good activity. The interfacial Michaelis constants are changed by less than 10-fold for all mutants. Molecular simulations suggest that mutation of aspartate-64 and tyrosine-87 should yield enzymes that retain a native-like structure and support catalysis. The pKa of the histidine-34 imidazole was deduced from the pH-rate profile and from the pH dependence of the rate of histidine-34 alkylation by 2-bromo-4'-nitroacetophenone. The pKa is increased about one-half unit by the tyrosine-87 mutation and reduced about one-half unit by the aspartate-64 to asparagine mutation, while in the aspartate-64 to alanine mutant the pKa is unchanged. These pKas are generally consistent with results of electrostatic calculations and suggest that the hydrogen bond between aspartate-64 and histidine-34 is not unusually strong. The hydrogen bonding network linking tyrosine-87 to aspartate-64 and aspartate-64 to histidine-34 is not critical for catalysis.

Identification of two specific lysines responsible for the inhibition of phospholipase A2 by manoalide

Manoalide, a natural product of sponge, irreversibly inhibits phospholipase A2 (PLA2) by reacting with lysine residues. Cobra venom PLA2 mutants were constructed in which four of the six lysine residues were independently replaced by arginine or methionine, which cannot react with manoalide. The mutants were overexpressed in Escherichia coli, renatured, and purified. The enzyme mutants lacking Lys-6 (K6R and K6M) or Lys-79 (K79R) were inhibited only 40% by manoalide while the native cobra venom PLA2 was inhibited 80% under the same conditions. This means that the manoalide modification of either Lys-6 or Lys-79 accounted for only half of the manoalide inhibition. The double mutant (K6R79R) was not inhibited by manoalide at all. Lys-56 (K56R) and Lys-65 (K65R) mutants were inhibited to the same extent as the native enzyme which indicates that these residues are not responsible for any of the inhibitory effects produced by manoalide. These results demonstrate that the reaction of manoalide with both Lys-6 and Lys-79 can account for all of its inhibition of cobra venom PLA2. The inhibition of PLA2 and its mutants with manoalide did not affect the activity of the enzyme toward monomeric substrate, which suggests that manoalide does not modify the catalytic site residues, that it does not block access to this site, and that its inhibition requires an interface. Furthermore, as with native PLA2, the activation of phosphatidylethanolamine hydrolysis by phosphorylcholine-containing compounds was exhibited by all of the mutants suggesting that none of the lysines examined are essential for this activation.

Multiple enzymatic activities of the human cytosolic 85-kDa phospholipase A2: hydrolytic reactions and acyl transfer to glycerol

The recombinant human 85-kDa cytosolic phospholipase A2 (cPLA2), when assayed in the presence of glycerol, catalyzes the transfer of acyl chains of radiolabeled phosphatidylcholine and para-substituted phenyl esters of fatty acids to glycerol, in addition to hydrolyzing these substrates. The product of the transacylation reaction is monoacylglycerol (MAG), and the acyl chain is predominantly esterified (> or = 95%) to a primary hydroxyl group of glycerol (sn-1/3); the stereochemistry is not known. Increasing concentrations of glycerol accelerate enzyme turnover both by providing an additional mechanistic pathway for the enzyme-substrate complex to form products and by increasing the intrinsic hydrolytic and transacylation activities of the enzyme. Significant enzymatic hydrolysis of sn-1/3-arachidonylmonoacylglycerol was measured, while sn-1/3-alpha-linolenoyl- and sn-2-arachidonylmonoacylglycerols were not detectably hydrolyzed. 1,3-Propanediol also serves as an acyl acceptor for the enzyme. cPLA2 hydrolyzes analog of lysophosphatidylcholine that lacks the sn-2 hydroxyl group. The enzyme will hydrolyze sn-1-acyl chains of rac-1-(arachidonyl, alpha-linolenoyl, palmitoyl)-2-O-hexadecyl-glycero-3-phosphocholine lipids and transfer the acyl chain to glycerol. Thus, cPLA2 has phospholipase A1 activity but only if an ether linkage rather than an ester linkage is present at the sn-2 position, and it is shown that the sn-1 acyl chains of both enantiomers of phosphatidylcholine are hydrolyzed. Phenyl [14C]-alpha-linolenate and five para-substituted phenyl esters of [3H]-alpha-linolenic acid with pKa values ranging from 7.2 to 10.2 for the phenol leaving groups were incorporated into 1,2-ditetradecyl-sn-glycero-3-phosphomethanol/Triton X-100 mixed micelles as substrates for the transacylation/hydrolysis reactions of the enzyme. Average product ratios, which are defined as the amount of monoacylglycerol formed to phenyl ester hydrolyzed, were 2.1 +/- 0.1 (n = 5) for the para-substituted phenyl esters and 2.0 +/- 0.3 (n = 7) for phenyl alpha-linolenate. The similarity of the ratios, despite the range of pKa values for the leaving groups, is consistent with the formation of a common enzyme intermediate that partitions to give either fatty acid or MAG. That intermediate may be a covalent acyl enzyme. Finally, the acyl chain specificity of cPLA2 was investigated to better understand the preference of the enzyme for phospholipids with sn-2-arachidonyl chains.

Influence of chemistry in immobilization of cobra venom phospholipase A2: implications as to mechanism

Phospholipase A2 from Naja naja kaouthia venom was covalently coupled onto agarose beads using two different chemistries. The effect of micellar competitive inhibitors in the coupling media was evaluated. Enzyme bound to N-hydroxysuccinimide-activated agarose, which is reactive primarily toward epsilon-amino groups, had 20% activity retention against micellar diheptanoylphosphatidylcholine (DiC7-PC). Enzyme bound through carboxylic groups, using a modification of the carbodiimide method, had 50% retention. Similar relative activities were observed, for both conjugates, in monomeric dihexanoyl-PC and in mixed micelles of Triton X-100 with dipalmitoyl-PC or dioleoylphosphatidylethanolamine. The soluble form of the enzyme showed premicellar activation against monomeric DiC7-PC, while the immobilized form showed interfacial recognition at concentrations around the critical micellar concentration. These results suggest that the enzyme activity lost upon immobilization is a result of the inherent chemical modification of the enzyme and that enzyme oligomerization and interfacial recognition are not cause-effect phenomena.

High-level expression in Escherichia coli and rapid purification of enzymatically active honey bee venom phospholipase A2

Bee venom phospholipase A2 (BV-PLA2) is a hydrolytic enzyme that specifically cleaves the sn-2 acyl bond of phospholipids at the lipid/water interface. The same enzyme is also believed to be responsible for some systemic anaphylactic reactions in bee venom sensitized individuals. To study the structure/function relationships of this enzyme and to define the molecular determinants responsible for its allergenic potential, a synthetic gene encoding the mature form of BV-PLA2 was expressed in Escherichia coli. This enzyme was produced as a fusion protein with a 6xHis-tag on its amino-terminus yielding 40-50 mg of fusion protein per 1 of culture after metal ion affinity chromatography. A kallikrein protease recognition site was engineered between the 6xHis-tag and the amino-terminus of the enzyme allowing isolation of the protein with its correct N-terminus. Recombinant affinity purified BV-PLA2 was refolded, purified to homogeneity, and cleaved with kallikrein, resulting in a final yield of 8-9 mg of active enzyme per 1 of culture. The enzymatic and immunological properties of the recombinant BV-PLA2 are identical to enzyme isolated from bee venom indicating a native-like folding of the protein.