Acylation of porcine pancreatic phospholipase A2 influences penetration and substrate head-group binding, depending on the position of the acylated lysine in the enzyme molecule

Biochemical and biological activities of the venom of the Chinese pitviper Zhaoermia mangshanensis, with the complete amino acid sequence and phylogenetic analysis of a novel Arg49 phospholipase A2 myotoxin

Zhaoermia mangshanensis (formerly Trimeresurus mangshanensis, Ermia mangshanensis) represents a monotypic genus of pitviper known only from Mt Mang in China's Hunan Province, and is among the largest and most spectacular of Asian venomous snakes. The venom of Zhaoermia exhibits high coagulant activity on bovine and human fibrinogen and human plasma, high phosphodiesterase and arginine ester hydrolytic activity, and moderate to low l-amino acid oxidase, kallikrein, caseinolytic, phospholipase A(2) (PLA(2)), haemorrhagic and myotoxic activities. The approximate i.p. LD(50) of the venom in mice was estimated to be 4 mg/kg. We purified the major toxin of Zhaoermia venom by gel-filtration, cation-exchange chromatography and HPLC. The toxin, a homodimer with an experimental monomeric mass of 13,972 Da, induced edema and myonecrosis in mice, but was devoid of detectable PLA(2) catalytic activity. Its complete amino acid sequence is composed of 121 amino acid residues cross-linked by seven disulfide bridges, and shows more than 80% identity to two Lys49-PLA(2)s from distantly related Asian pitvipers, Protobothrops mucrosquamatus and Calloselasma rhodostoma. Phylogenetic analysis of the novel toxin, zhaoermiatoxin, confirmed that it is rooted within a comprehensive sample of Lys49-PLA(2)s despite having an arginine residue in position 49, suggesting a secondary Lys49-->Arg substitution which did not alter the catalytic inactivity of the molecule.

Structural basis of the anionic interface preference and kcat* activation of pancreatic phospholipase A2

Pancreatic phospholipase A(2) (PLA2) shows a strong preference for the binding to the anionic interface and a consequent allosteric activation. In this paper, we show that virtually all the preference is mediated through 3 (Lys-53, -56, and -120) of the 12 cationic residues of bovine pancreatic PLA2. The lysine-to-methionine substitution enhances the binding of the enzyme to the zwitterionic interface, and for the K53,56,120M triple mutant at the zwitterionic interface is comparable to that for the wild type (WT) at the anionic interface. In the isomorphous crystal structure, the backbone folding of K53,56M K120,121A and WT are virtually identical, yet a significant change in the side chains of certain residues, away from the site of substitution, mostly at the putative contact site with the interface (i-face), is discernible. Such reciprocity, also supported by the spectroscopic results for the free and bound forms of the enzyme, is expected because a distal structural change that perturbs the interfacial binding could also affect the i-face. The results show that lysine-to-methionine substitution induces a structural change that promotes the binding of PLA2 to the interface as well as the substrate binding to the enzyme at the interface. The kinetic results are consistent with a model in which the interfacial Michaelis complex exists in two forms, and the complex that undergoes the chemical step is formed by the charge compensation of Lys-53 and -56. Analysis of the incremental changes in the kinetic parameters shows that the charge compensation of Lys-53 and -56 contributes to the activation and that of Lys-120 contributes only to the structural change that promotes the stability of the Michaelis complex at the interface. The charge compensation effects on these three residues also account for the differences in the anionic interface preference of the evolutionarily divergent secreted PLA2.

Control of the chemical step by leucine-31 of pancreatic phospholipase A2

A well-defined region of pancreatic and other secreted phospholipase A2 (PLA2), which we call the i-face, makes a molecular contact with the interface to facilitate and control the events and processivity of the interfacial catalytic turnover cycles. The structural features of the i-face and its allosteric relationship to the active site remain to be identified. As a part of the calcium binding (26-34) loop, Leu-31 is located on the surface near the substrate binding slot of PLA2. Analysis of the primary rate and equilibrium parameters of the Leu-31 substitution mutants of the pig pancreatic PLA2 shows that the only significant effect of the substitution is to impair the chemical step at the zwitterionic interface in the presence of added NaCl, and only a modest effect is seen on kcat at the anionic interface. Leu-31 substitutions have little effect on the binding of the enzyme to the interface; the affinity for certain substrate mimics is modestly influenced in W3F, L31W double mutant. The fluorescence emission results with the double mutant show that the microenvironment of Trp-31 is qualitatively different at the zwitterionic versus anionic interfaces. At both of the interfaces Trp-31 is not shielded from the bulk aqueous environment as it remains readily accessible to acrylamide and water. The NaCl-induced change in the Trp-31 emission spectrum of the double mutant on the zwitterionic interface is similar to that seen on the binding to the anionic interface. Together, the kinetic and spectroscopic results show that the form of PLA2 at the zwitterionic interface (Ez) is distinguishably different from the catalytically more efficient form at the anionic interface (Ea). This finding provides a structural basis for the two-state model for kcat activation by the anionic interface. In conjunction with earlier results we suggest that neutralization of certain cationic residues of PLA2 exerts a control on the calcium loop through residue 31.

Structure of a Lys49-phospholipase A2 homologue isolated from the venom of Bothrops nummifer (jumping viper)

Lys49-Phospholipase A2 (Lys49-PLA2) homologues damage membranes by a Ca2+-independent mechanism which does not involve catalytic activity. We have solved the structure of myotoxin-I, a Lys49-PLA2 homologue isolated from the venom of Bothrops nummifer (jumping viper) at 2.4 A resolution using molecular replacement techniques. The final model has been refined to a final R-factor of 18.4% (R-free = 23.2%), and shows excellent geometry. The myotoxin-I from Bothrops nummifer is dimeric in the crystalline state as has been observed for other Lys49-PLA2 homologues. In addition, a continuous electron density in the active site and substrate binding channel could be successfully modeled as a fatty-acid molecule.

Cationic residues 53 and 56 control the anion-induced interfacial k*cat activation of pancreatic phospholipase A2

Added NaCl or anionic amphiphiles increase the rate of hydrolysis of dispersions of zwitterionic phospholipid by pancreatic phospholipase A2 (PLA2). Two effects of the negative charge at the interface have been dissected: enhanced binding of the enzyme to the interface, and k*cat activation of the enzyme at the interface [Berg et al. (1997) Biochemistry 36, 14512-14530]. Results reported here show that the structural basis for the k*cat activation is predominantly through cationic K53 and K56 in bovine pancreatic PLA2 with the anionic interface. The maximum rate at saturating diheptanoylphosphatidylcholine micelles, VMapp, for WT, K56M, and K53M in 4 M NaCl is in the 800-1300 s-1 range. In contrast, VMapp at 0.1 M NaCl is considerably higher for K56M (400 s-1) and K53M (230 s-1) compared to the rate with WT (30 s-1) or K56E (45 s-1). The rate of hydrolysis of anionic dimyristoylphosphatidylmethanol vesicles is virtually the same with all these mutants (200-300 s-1) and it is not affected by added NaCl. The chemical step for the hydrolysis of anionic and zwitterionic substrates remains rate-limiting in the presence or absence of added NaCl. A modest (approximately 10-fold) effect of K56M substitution or of added NaCl is seen on the binding of the enzyme to the interface; however, the binding of the substrate or a substrate mimic to the active site of the enzyme at the interface is not affected by more than a factor of 2. Magnitudes of the primary rate and equilibrium parameters at the zwitterionic and anionic interfaces show that the effect of mutation or of added NaCl is primarily on k*cat at the zwitterionic interface. These results are interpreted in terms of a two-state model for the interfacial allosteric activation, where the enzyme-substrate complex at the zwitterionic interface becomes catalytically active only after the positive charge on cationic K56 and K53 has been removed by mutation or neutralized by anionic charges in the interface.

Targeting of porcine pancreatic phospholipase A2 to human platelets. Introduction of an RGD sequence and acyl-group by chemical modification

In the present study we prepared by chemical modification a series of porcine pancreatic phospholipase A2 (PLA) derivatives, that bind to the activated glycoprotein (GP) IIb/IIIa complex and hydrolyse phospholipids in the outer leaflet of the platelet membrane. To the native enzyme, an RGD-containing peptide was coupled to introduce affinity for GPIIb/IIIa in combination with lauric acid to improve binding to the membrane. As controls, derivatives containing only one of these modifications were prepared. Acylation of the enzyme improved the affinity for densely packed phospholipids, as deduced by kinetic analyses. After stimulation of platelets, the RGD-containing PLAs bound to GPIIb/IIIa since GRGDS peptide and a monoclonal antibody against the complex interfered with binding. No binding was found with native PLA. The binding seen with lauric acid PLA was not mediated by GPIIb/IIIa. All modified PLAs induced 1-3% hydrolysis of [3H]arachidonic-acid-labelled phospholipids in resting platelets. After activation with alpha-thrombin, hydrolysis increased to 17%, corresponding to about 90% of [3H]arachidonate-labelled phospholipids in the outer leaflet of the plasma membrane. RGD-containing PLAs were more active than lauroyl PLA, and their activity was mediated via GPIIb/IIIa since GRGDS inhibited release of [3H]arachidonic acid. Acylation of the RGD-containing PLAs did not further improve the hydrolytic properties. We conclude that chemical modification of PLA leads to a targetted hydrolytic action and could be a basis for the design of enzymes that specifically destroy activated platelets.

Phospholipase A2 myotoxins from Bothrops snake venoms

Several myotoxins have been isolated from Bothrops snake venoms during the last 10 years. All of them are group II basic phospholipases A2, although some lack enzymatic activity (i.e. Lys-49 variants). These myotoxins appear as an antigenically related family of proteins occurring in many, but not all, Bothrops venoms, bearing a close structural and antigenic relationship to toxins found in other crotalid venoms of the genera Agkistrodon and Trimeresurus. Myotoxins are quantitatively important venom components in some Bothrops species. Intramuscular injection of Bothrops myotoxins leads to a rapid series of drastic degenerative events, probably initiated at the plasma membrane level, which culminate in a selective skeletal muscle necrosis. This in vivo specificity contrasts with the ability of myotoxins to lyse many types of cells in culture. Muscle damage, as well as cytolysis and liposome disruption, occur in conditions where phospholipase A2 activity is inhibited, although enzymatic activity might enhance myotoxin actions. A membrane receptor for Bothrops myotoxins has not been identified yet. A working hypothesis on the mechanism of action is proposed. Current evidence suggests that these toxins interact with biological membranes via a molecular region distinct from their known catalytic site. The active region is likely to be formed by a combination of basic and hydrophobic amino acid residues near the C-terminus of the protein, which allow electrostatic interaction and bilayer penetration. These events may lead to membrane destabilization and loss of selective permeability to ions such as calcium, both of which appear to be important mediators in the process of muscle necrosis.

An extended binding pocket determines the polar head group specificity of porcine pancreatic phospholipase A2

Porcine pancreatic phospholipase A2 (PLA2) was studied by site-directed mutagenesis. Arg53 and/or Lys56 were replaced by a methionine (R53M or K56M, respectively) in combination with the Tyr69-->Phe (Y69F) substitution. These substitutions improved the activity on micellar and monomeric zwitterionic substrates and reduced the activity on negatively charged substrates compared to the Y69F mutant. With the neutral substrate 1,2-didodecanoyl-sn-glycerol-3-dimethyl phosphate (Lau2GroMe2P) a 20-fold increase of activity was observed for the 69F53M56M mutant, whereas this mutant showed a lower activity than native PLA2 on zwitterionic substrates. Thus the ratio Lau2GroMe2P/Lau2GroPCho has become 65 times higher for 69F53M56M compared to native phospholipase A2, illustrating that the substrate specificity has changed enormously. The methionine substitutions were also prepared in a 69F mutant in which a part of the surface loop (residues 62-66) was deleted. Also in this deletion mutant these substitutions showed a similar effect as the substitutions in the native 69F mutant. Furthermore it was shown that deletion of the loop increases the activity on micellar lecithins and negatively charged micellar substrates, but reduces the activity on Lau2GroMe2P. Therefore it can be concluded that the loop is important for the recognition of substrates. We also show that the loop plays a role in the dimerization of these proteins. Dimerization may account for the high activities observed for some mutants acting on monomeric substrate.

Structural elements of secretory phospholipases A2 involved in the binding to M-type receptors

Specific membrane receptors for secretory phospholipases A2 (sPLA2s) have been initially identified with novel snake venom sPLA2s called OS1 and OS2. One of these sPLA2 receptors (muscle (M)-type, 180 kDa) has a very high affinity for OS1 and OS2 and a high affinity for pancreatic and inflammatory-type mammalian sPLA2s, which might be the natural endogenous ligands of PLA2 receptors. Primary structures of OS1 and OS2 were determined and compared with sequences of other sPLA2s that bind less tightly or do not bind to the M-type receptor. In addition, the binding properties of pancreatic sPLA2 mutants to the M-type receptor have been analyzed. Residues within or close to the Ca(2+)-binding loop of pancreatic sPLA2 are crucially involved in the binding step, although the presence of Ca2+ that is essential for the enzymatic activity is not required for binding to the receptor. These residues include Gly-30 and Asp-49, which are conserved in all sPLA2s. Leu-31 is also essential for binding of pancreatic sPLA2 to its receptor. Many other mutations have been considered. Those occurring in the N-terminal alpha helices and the pancreatic loop do not change binding to the M-type receptor. Conversion of pancreatic prophospholipase to phospholipase is essential for the acquisition of binding properties to the M-type receptor.

A structure-function study of bovine pancreatic phospholipase A2 using polymerized mixed liposomes

A new combinatorial approach that includes the genetic variation of protein structure and the chemical modification of phospholipid structure in polymerized mixed liposomes was used to delineate the structure-function relationships in the interfacial catalysis of bovine pancreatic phospholipase A2 (PLA2). Based on previous structural and mutational studies, several bovine PLA2 mutants were generated in which a positive charge of putatively important lysyl side chains was reversed (K10E, K53E, K56E, and K116E) or neutralized (K56Q and K116Q). Kinetic parameters of bovine wild type and mutant PLA2s determined using polymerized mixed liposomes consisting of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphoethanolamine (or -phosphoglycerol) and 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol showed that Lys-53 is involved specifically in the interaction with a substrate bound in the active site. Also, these results showed that Lys-10 and Lys-116 are involved in the interaction of bovine PLA2 with anionic interfaces but not in the interaction with the active site-bound substrate. In particular, Lys-116 makes more significant contribution than Lys-10 by approximately 1.0 kcal/mol to the binding to anionic interfaces. Most importantly, Lys-56 was shown to participate in the interaction with both the active site-bound substrate and anionic interfaces. These findings establish Lys-56 and Lys-116 as essential residues for the binding of bovine pancreatic PLA2 to anionic interfaces. Lastly, our structure-function analysis based on the use of polymerized mixed liposomes was further supported by equilibrium binding measurements of these proteins using 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol polymerized liposomes and by kinetic analyses using monomeric substrates, 1,2-dihexanoyl-sn-glycero-3-phosphoethanolamine and -phosphoglycerol.

The electrostatic basis for the interfacial binding of secretory phospholipases A2

Biochemical and structural data suggest that electrostatic forces play a critical role in the binding of secretory phospholipases A2 to substrate aggregates (micelles, vesicles, monolayers, and membranes). This initial binding (adsorption) of the enzyme to the interface is kinetically distinct from the subsequent binding of substrate to the buried active site. Thus, in the absence of specific active-site interactions, electrostatic forces operating at the molecular surface may orient and hold the enzyme at the interface. We have calculated the electrostatic potentials for 10 species of secretory phospholipases A2 whose atomic coordinates have been determined by x-ray crystallography. Most of these enzymes show a marked electrostatic sidedness that is accentuated to a variable degree by the presence of the essential cofactor calcium ion. This asymmetry suggests a discrete interfacial binding region on the protein's surface, the location of which is in general agreement with proposals derived from the results of chemical modification, mutational, and crystallographic experiments.

Competitive inhibition of lipolytic enzymes. X. Further delineation of the active site of pancreatic phospholipases A2 from pig, ox and horse by comparing the inhibitory power of a number of (R)-2-acylamino phospholipid analogues

Two series of (R)-phospholipid analogues, each containing a n-propyl group at the C-1 position and various acylamino functions at the C-2 position have been synthesized and their inhibitory properties towards three mammalian pancreatic phospholipases A2 have been determined. The members of the first series of analogues all contained the zwitter-ionic phosphocholine headgroup which in the second series was replaced by the anionic phosphoglycol function. In the saturated 2-acylamino phospholipids the length of the acyl chain ranged from 8 to 18 carbon atoms. The unsaturated 2-acylamino analogues possessed a chain length of 11 or 18 carbon atoms and contained one, two, three or four double bonds. For inhibitors with a saturated acylamino group, the phospholipases A2 from pig, ox and horse show a sharp optimum in inhibitory power Z for an acyl chain length of 10 carbon atoms. The inhibitory behaviour of the unsaturated acylamino analogues is more complex: both the zwitter-ionic and the anionic inhibitors demonstrate an increase in Z with an increasing number of cis-double bonds but the degree of improvement is dependent on the position of the double bonds. Subsequently the influence of polar groups at carbon position 12 of the dodecanoylamino phospholipids on Z was analyzed. Substitution of the terminal methyl group by an OH-function lowers the inhibitory potency of the three enzymes by a factor of 4 to 5 both in the phosphocholine and phosphoglycol series. Replacement of the methyl group by potentially charged functions (-NH2, -COOH) resulted in a complete loss of inhibitory properties. Blocking of the amino group and carboxyl function by t-butyloxycarbonylation and esterification, respectively, fully restored the inhibitory power. Finally we investigated how changes in the polar headgroup and the presence of aromatic rings at the C-1 or C-2 position influenced the inhibitory potency of the analogues.