Competitive inhibition of lipolytic enzymes. VII. The interaction of pancreatic phospholipase A2 with micellar lipid/water interfaces of competitive inhibitors

Introduction of a C-terminal aromatic sequence from snake venom phospholipases A2 into the porcine pancreatic isozyme dramatically changes the interfacial kinetics

Porcine pancreatic phospholipase A2 (PLA2) was modified by single and multiple site-directed mutations at sites thought to be involved in interfacial binding. Charged and polar residues in the C-terminal region were replaced by aromatic residues on the basis of an analogy with snake venom PLA2s, which display high affinity for a zwitterionic interface. The PLA2 variants constructed were N117W, N117W/D119Y and K116Y/N117W/D119Y. Titration with micelles of a zwitterionic substrate suggests that the variants N117W and K116Y/N117W/D119Y possess improved ability to bind to the micellar substrate interface, relative to the wild-type enzyme. Improved interfacial binding was confirmed by direct binding studies with micelles of a zwitterionic substrate analogue, indicating up to five times higher affinity for both variants. Interfacial binding is not improved for the variant N117W/D119Y. Maximal enzyme velocities (Vapp./max) with the zwitterionic substrate were between 25 and 75% of that of the wild-type enzyme. However, competitive inhibition and direct binding studies with a strong inhibitor revealed that the affinity for substrate present at the interface (Km*) is perturbed by the mutations made. For the variant N117W, the slight decrease observed in Vapp./max is most likely made up of a 24-fold reduction in catalytic turnover (kcat) and 18-fold improved substrate binding (Km*).

Catalytic role of the active site histidine of porcine pancreatic phospholipase A2 probed by the variants H48Q, H48N and H48K

The catalytic contribution of His48 in the active site of porcine pancreatic phospholipase A2 was examined using site-directed mutagenesis. Replacement of His48 by lysine (H48K) gives rise to a protein having a distorted lipid binding pocket. Activity of this variant drops below the detection limit which is 10(7)-fold lower than that of the wild-type enzyme. On the other hand, the presence of glutamine (H48Q) or asparagine (H48N) at this position does not affect the structural integrity of the enzyme as can be derived from the preserved lipid binding properties of these variants. However, the substitutions H48Q and H48N strongly reduce the turnover number, i.e. by a factor of 10(5). Residual activity is totally lost after addition of a competitive inhibitor. We conclude that proper lipid binding on its own accelerates ester bond hydrolysis by a factor of 10(2). With the selected variants, we were also able to dissect the contribution of the hydrogen bond between Asp99 and His48 on conformational stability, being 5.2 kJ/mol. Another hydrogen bond with His48 is formed when the competitive inhibitor (R)-2-dodecanoylamino-hexanol-1-phosphoglycol interacts with the enzyme. Its contribution to binding of the inhibitor in the presence of an interface was found to be 5.7 kJ/mol.

Competitive inhibition of lipolytic enzymes. XI. Estimation of the interfacial dissociation constants of porcine pancreatic phospholipase A2 for substrate and inhibitor in the absence of detergents

Based on the strong inhibitory properties of (R)-2-decanoylamino-octanol-1-phosphocholine and its phosphoglycol analogue for porcine pancreatic phospholipase A2, the corresponding 2-decanoyloxy derivatives have been synthesised in both enantiomeric forms and their substrate properties for the enzyme were analysed. The high aqueous solubility in the absence of detergents, combined with low critical micelle concentrations of both the amide- and ester phospholipids allowed the estimation of the interfacial dissociation constants of the enzyme-substrate and enzyme-inhibitor complexes by kinetic and direct binding techniques.

Stereospecificity of the interaction of porcine pancreatic phospholipase A2 with micellar and monomeric inhibitors. A time-resolved fluorescence study of the tryptophan residue

The binding effect of enantiomeric substrate analogs under micellar form on the local conformation and dynamics of the N-terminal region of porcine pancreas phospholipase A2 was examined by time-resolved fluorescence measurements of its single tryptophan residue (Trp3). The complexity of the fluorescence intensity decay of the unliganded protein (four excited-state lifetime populations) suggests a conformational heterogeneity of the N-terminal region of the protein. A considerable simplification of the excited-state lifetime profile was specifically observed in the complex with one of the stereoisomers [(R)-2-tetradecanoylamino)-hexanol-phosphocholine] at low inhibitor/protein molar ratio of approximately 9. This indicates the existence of a definite conformation of the N-terminal region of the protein in the complex. No effect was detected for the S-enantiomer. In parallel, the rotational mobility of the Trp residue in the complex with the R-enantiomer was reduced. At a higher inhibitor/protein molar ratio of approximately 130, the stereospecificity of the interaction was lost and complexes were formed with both stereoisomers. These complexes were, however, not similar to the specific one either in terms of the local Trp3 environment or of the volume of the rotating unit. The local effects of low amounts of monomeric inhibitors added to a preformed protein/micelle complex of a phospholipase A2 double mutant in which a Trp residue was genetically inserted near the active site at position 31 while the natural Trp3 was replaced by Phe [Kuipers, O., Vincent, M., Brochon, J. C., Verheij, H. M., de Haas, G. H. & Gallay, J. (1991) Biochemistry 30, 8771-8785], were also monitored by time-resolved fluorescence of this single Trp residue. A stereospecific dependence of the local perturbations was again observed. These results support the idea that the active conformation of the protein is reached in solution only after formation of a ternary complex: protein-interface-inhibitor.

Competitive inhibition of lipolytic enzymes. IX. A comparative study on the inhibition of pancreatic phospholipases A2 from different sources by (R)-2-acylamino phospholipid analogues

The inhibitory power (Z) of a number of (R)-1-alkyl-2-acylamino phospholipid analogues was determined for three mammalian phospholipases A2 from pig, ox and horse pancreas. All three enzymes display a clear preference for anionic (phosphoglycol) inhibitors over the zwitterionic (phosphocholine) derivatives; this effect is most pronounced for the bovine enzyme. Upon variation of the 1-alkyl chain length, the bovine and equine phospholipases, like the porcine enzyme in previous studies, show an optimum in Z for a six-carbon alkyl group. The introduction of a double bond in the 2-acylamino group generally improves the inhibitory power as compared with a fully saturated acyl chain. For the horse enzyme, the presence of an (R)-2-undecenoylamino group in the phosphocholine- and phosphoglycol-containing inhibitors resulted in affinities which are nearly 4 and 5 orders of magnitude higher, respectively, than for the substrate molecule. Direct determination of the dissociation constant Ki* of several inhibitors incorporated in a host lipid/water interface of non-inhibitory n-octadecenylphosphocholine micelles, was performed by ultraviolet difference spectroscopy. The progressive binding of a single inhibitor molecule into the active site of the three enzymes was followed quantitatively by an increasing tyrosine perturbation. With moderately strong competitive inhibitors (Z values ranging from about 50 to 10,000), quantitative values for Ki* were obtained. Extrapolation of the experimentally found linear relationship between Z and 1/Ki* yields predicted Ki* numbers for the much stronger inhibitors with Z values between 10,000 and 100,000.

Arginine 53 is involved in head-group specificity of the active site of porcine pancreatic phospholipase A2

The X-ray structure of a mutant porcine pancreatic phospholipase A2 inhibitor complex [Thunnissen et al. (1990) Nature 347, 689-691] has been determined. This structure shows several interactions between the sn-2-acyl chain and the phosphate moiety of the inhibitor at sn-3 and the protein. The interactions of the remaining part of the polar head group are less clear. Because Arg53 is in close proximity to the head group, we tested the importance of charge at position 53 on enzymatic activity and specificity. Arg53 has been replaced by a glutamine and a glutamic acid in mutants R53Q and R53E, respectively. The effects of the mutations were tested with both zwitterionic and anionic substrates. With monomeric, zwitterionic, (R,S)-1,2-dihexanoyldithiopropyl-3-phosphocholine as substrate, the mutants R53Q and R53E display twofold and sevenfold, respectively, increased kcat/Km values, composed of increased kcat and decreased Km values. Tested on micelles of zwitterionic (R)-1,2-dioctanoylglycero-3-phosphocholine the mutants R53Q and R53E are more active than the native enzyme, whereas these mutations have an opposite effect on the activity on anionic (R)-1,2-dioctanoylglycero-3-phosphoglycol. Thus, whereas the native enzyme is 0.3 times as active on zwitterionic as on the anionic substrate, these ratios are 1.0 (R53Q) and 1.7 (R53E) for the mutants. No changes in activity were observed with the anionic substrate (R)-1,2-dioctanoylglycero-3-sulfate. Binding studies with substrate-derived inhibitors confirmed the increased affinity for zwitterionic phospholipids and the reduced affinity for anionic phospholipids. The kinetic and binding data indicate the involvement of the charge of residue 53 in head-group specificity and suggest a position of residue 53 closer to the choline or glycol than to the phosphate.