p-Nitrophenyl N-alkylcarbamates with different alkyl chains were used as substrates to determine separately the carbamylation and decarbamylation rates of the lipases from Staphylococcus hyicus and S. aureus. Both enzymes are reversibly inhibited by these compounds due to a rapid carbamylation of their active site serines followed by a slow decarbamylation. The carbamylation reaction is strongly pH-dependent and the pH profile suggests that an unprotonated histidine is required for this reaction. In contrast, the decarbamylation is pH-independent suggesting the presence of a hydrogen bond between the active site histidine and the carbamyl moiety. S. hyicus lipase preferably reacts with medium to long chain carbamates with an optimum for eight carbon atoms. In contrast, S. aureus lipase is highly specific for short chain carbamates. These results are in agreement with the respective substrate preferences of both lipases toward natural lipids. The decarbamylation rates of both enzymes hardly depend on the alkyl chain length, and from this it is concluded that chain length selectivity is expressed in the first step of catalysis. Both the carbamylation and decarbamylation reaction rates of S. hyicus lipase are enhanced in the presence of micelles, the activation effect being most pronounced in the first step. For the S. aureus lipase only a small influence of interfaces on both reaction steps was observed. These results are discussed in view of a possible role of a lid covering the active site. Kinetic experiments in the presence and absence of calcium strongly suggest that calcium ions are important for the structural stabilization of the unmodified as well as of the carbamylated enzymes. This structural function of calcium was supported by urea unfolding experiments, from which it appeared that for both enzymes the free energy for unfolding is significantly lower in the absence of calcium. In conclusion our results show that kinetic differences between both lipases reside in the acylation step, and that calcium is important for the structural stabilization of the unmodified, and moreover, the acylated enzymes.
Staphylococcus hyicus lipase is a serine hydrolase. In order to identify the active site histidine of S. hyicus lipase we have chemically modified S. hyicus lipase with 1-bromo-octan-2-one. The enzyme is rapidly inactivated by this inhibitor with a half-time of 578 s at pH 6.5 and 30 degrees C. Addition of the enzyme's cofactor calcium increases the inactivation rate approx. 2-fold. When n-hexadecylphosphocholine, a non-hydrolysable substrate analogue, is added the inactivation rate decreases about 3-fold, suggesting that a residue in the active site of S. hyicus lipase is involved in the inactivation reaction. Inactivation of S. hyicus lipase with 14C-labelled 1-bromo-octan-2-one shows that 1.4 moles of inhibitor per mole of lipase are incorporated. The results of an electrospray mass spectrometric study of the inactivated enzyme are consistent with this finding. In order to identify the modified residue, both the inactivated and the unmodified lipase were digested with cyanogen bromide followed by trypsin. The resulting peptides were analysed using HPLC and fast atom bombardment mass spectrometry. The results allow the modified residue to be assigned to the peptide Gly597-Lys612. Collision induced dissociation mass spectrometry allowed the modified residue to be identified as His-600. From these results we conclude that this residue forms part of the catalytic triad of S. hyicus lipase.
Title: 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 De Haas GH, Dijkman R, Boots JW, Verheij HM Ref: Biochimica & Biophysica Acta, 1257:87, 1995 : PubMed
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.
1,2-Dioctylcarbamoylglycero-3-O-p-nitrophenyl alkylphosphonates, with alkyl being methyl or octyl, were synthesised and tested as irreversible inhibitors of cutinase from Fusarium solani pisi and Staphylococcus hyicus lipase. Rapid inactivation of these enzymes occurred with a concomitant release of one mole of p-nitrophenol per mole of enzyme. With both lipases a higher reactivity was observed when the alkyl substituent on the phosphonate is a methyl rather than an octyl chain. Both lipases are highly selective for the chirality of these compounds at glycerol and at phosphorus. Rapid inactivation at an inhibitor concentration of 0.1 mol% in 100 mM NaTDOC (t 1/2 < 60 min.) occurred when the glycerol moiety had the (R) configuration, while inhibitors of the (S) configuration react 4-10-fold more slowly. The isomer with the p-nitrophenyl octylphosphonate attached to the secondary hydroxyl group of glycerol hardly inhibited (t 1/2 > 1 day) the lipases. These results reflect the known positional- and stereopreference of these enzymes which preferentially release the fatty acid at sn-3 of natural triacylglycerols. The enzymes appeared to be even more selective for the chirality at phosphorus, the differences in reactivity of the faster and slower reacting isomers being as high as about 250-fold for the methylphosphonates and about 60-fold for the octylphosphonates. These phosphonates can be regarded as true active site-directed inhibitors. The inhibited enzymes can be considered as analogues of the tetrahedral intermediate in the acylation step that occurs during triacylglycerol hydrolysis.
Single cyrstals of a lipase from Staphylococcus hyicus have been obtained using a combination of 18 to 24% dimethylsulfoxide and 10% isopropanol as a precipitant. The crystals grow at 4 degrees C in 2-3 months. They belong to the orthorhombic space group P212121 with a = 73.31 A, b = 77.96 A, and c = 169.81 A, with two protein molecules per asymmetrical unit. The crystals diffract to at least 2.8 A resolution and are suitable for an X-ray structure analysis.
