Characterization of pancreatic lipase-related protein 2 isolated from human pancreatic juice

Human pancreatic lipase-related protein 2 (HPLRP2) was identified for the first time in pancreatic juice using specific anti-peptide antibodies and purified to homogeneity. Antibodies were raised in the rabbit using a synthetic peptide from the HPLRP2 protein sequence deduced from cDNA. Western blotting analysis showed that these antibodies did not react with classical human pancreatic lipase (HPL) or human pancreatic lipase-related protein 1 (HPLRP1) but cross-reacted with native rat PLRP2 (RPLRP2), as well as with recombinant rat and guinea-pig PLRP2 (GPLRP2). Immunoaffinity chromatography was performed on immobilized anti-recombinant HPLRP2 polyclonal antibodies to purify native HPLRP2 after conventional chromatographic steps including gel filtration and chromatrography on an anion-exchanger. The substrate specificity of HPLRP2 was investigated using various triglycerides, phospholipids and galactolipids as substrates. The lipase activity on triglycerides was inhibited by bile salts and weakly restored by colipase. The phospholipase activity of HPLRP2 on phospholipid micelles was very low. A significant level of galactolipase activity was measured using monogalactosyldiglyceride monomolecular films. These data suggest that the main physiological function of HPLRP2 is the hydrolysis of galactolipids, which are the main lipids present in vegetable food.     

Reactivation of the totally inactive pancreatic lipase RP1 by structure-predicted point mutations

Both classical pancreatic lipase (DPL) and pancreatic lipase-related protein 1 (DPLRP1) have been found to be secreted by dog exocrine pancreas. These two proteins were purified to homogeneity from canine pancreatic juice and no significant catalytic activity was observed with dog PLRP1 on any of the substrates tested: di- and tri-glycerides, phospholipids, etc. DPLRP1 was crystallized and its structure solved by molecular replacement and refined at a resolution of 2.10 Å. Its structure is similar to that of the classical PL structures in the absence of any inhibitors or micelles. The lid domain that controls the access to the active site was found to have a closed conformation. An amino-acid substitution (Ala 178 Val) in the DPLRP1 may result in a steric clash with one of the acyl chains observed in the structures of a C11 alkyl phosphonate inhibitor, a transition state analogue, bound to the classical PL. This substitution was suspected of being responsible for the absence of DPLRP1 activity. The presence of Val and Ala residues in positions 178 and 180, respectively, are characteristic of all the known PLRP1, whereas Ala and Pro residues are always present in the same positions in all the other members of the PL gene family. Introducing the double mutation Val 178 Ala and Ala 180 Pro into the human pancreatic RP1 (HPLRP1) gene yielded a well expressed and folded enzyme in insect cells. This enzyme is kinetically active on triglycerides. Our findings on DPLRP1 and HPLRP1 are therefore likely to apply to all the RP1 lipases. Proteins 32:523–531, 1998. © 1998 Wiley-Liss, Inc.     

The open lid mediates pancreatic lipase function

Pancreatic triglyceride lipase (PTL) and the homologous pancreatic lipase related protein 2 (PLRP2) provide a unique opportunity to understand the molecular mechanism of lipolysis. They differ in substrate specificity, sensitivity to bile salts, and colipase dependence despite their close amino acid and tertiary structure identity. One important structure, present in both lipases, is the lid which occupies different positions in the inactive and active forms of PTL. We investigated the role of the lid in lipase function by site-specific mutagenesis. By exchanging the lids between PTL and PLRP2, we created two chimeric lipases. Additionally, we made multiple substitution mutations in the PTL lid. PLRP2 with the PTL lid had kinetic properties similar to PLRP2. PTL with the PLRP2 lid was greatly impaired and had no activity at micellar bile salt concentrations even in the presence of colipase. Both chimeras showed interfacial activation suggesting that the closed lid position was maintained. A series of substitution mutations were made in positions Arg257 and Asp258. These mutations demonstrated the importance of these two residues to maintaining the normal activity, triglyceride acyl chain specificity, and colipase interaction of PTL. The preserved interfacial activation in the chimeras, the similar crystal structure of the two lids in the closed position, and the importance of Arg257 and Asp258 in mediating the open conformation of the lid argue that the position of the open lid influences the differences in activity against triglycerides, in sensitivity to bile salts, and in colipase dependence between PTL and PLRP2.     

Human pancreatic lipase-related protein 2 is a galactolipase

Human pancreatic lipase-related protein 2 (HPLRP2) was found to be expressed in the pancreas, but its biochemical properties were not investigated in detail. A recombinant HPLRP2 was produced in insect cells and the yeast Pichia pastoris and purified by cation exchange chromatography. Its substrate specificity was investigated using pH-stat and monomolecular film techniques and various lipid substrates (triglycerides, diglycerides, phospholipids, and galactolipids). Lipase activity of HPLRP2 on trioctanoin was inhibited by bile salts and poorly restored by adding colipase. In vivo, HPLRP2 therefore seems unlikely to show any lipase activity on dietary fat. In human pancreatic lipase (HPL), residues R256, D257, Y267, and K268 are involved in the stabilization of the open conformation of the lid domain, which interacts with colipase. These residues are not conserved in HPLRP2. When the corresponding mutations (R256G, D257G, Y267F, and K268E) are introduced into HPL, the effects of colipase are drastically reduced in the presence of bile salts. This may explain why colipase has such weak effects on HPLRP2. HPLRP2 displayed a very low level of activity on phospholipid micelles and monomolecular films. Its activity on monogalactosyldiglyceride monomolecular film, which was much higher, was similar to the activity of guinea pig pancreatic lipase related-protein 2, which shows the highest galactolipase activity ever measured. The physiological role of HPLRP2 suggested by the present results is the digestion of galactolipids, the most abundant lipids occurring in plant cells, and therefore, in the vegetables that are part of the human diet.     

Porcine pancreatic lipase related protein 2 has high triglyceride lipase activity in the absence of colipase

Efficient dietary fat digestion is essential for newborns who consume more dietary fat per body weight than at any other time of life. In many mammalian newborns, pancreatic lipase related protein 2 (PLRP2) is the predominant duodenal lipase. Pigs may be an exception since PLRP2 expression has been documented in the intestine but not in the pancreas. Because of the differences in tissue-specific expression, we hypothesized that the kinetic properties of porcine PLRP2 would differ from those of other mammals. To characterize its properties, recombinant porcine PLRP2 was expressed in HEK293T cells and purified to homogeneity. Porcine PLRP2 had activity against tributyrin, trioctanoin and triolein. The activity was not inhibited by bile salts and colipase, which is required for the activity of pancreatic triglyceride lipase (PTL), minimally stimulated PLRP2 activity. Similar to PLRP2 from other species, PLRP2 from pigs had activity against galactolipids and phospholipids. Importantly, porcine PLRP2 hydrolyzed a variety of dietary substrates including pasteurized human mother's milk and infant formula and its activity was comparable to that of PTL. In conclusion, porcine PLRP2 has broad substrate specificity and has high triglyceride lipase activity even in the absence of colipase. The data suggest that porcine PLRP2 would be a suitable lipase for inclusion in recombinant preparations for pancreatic enzyme replacement therapy.     

The triglyceride lipases of the pancreas

Pancreatic triglyceride lipase (PTL) and its protein cofactor, colipase, are required for efficient dietary triglyceride digestion. In addition to PTL, pancreatic acinar cells synthesize two pancreatic lipase related proteins (PLRP1 and PLRP2), which have a high degree of sequence and structural homology with PTL. PLRP1 has no known activity. PTL and PLRP2 differ in substrate specificity, behavior in bile salts and dependence on colipase. Each protein has a globular amino-terminal (N-terminal) domain, which contains the catalytic site for PTL and PLRP2, and a beta-sandwich carboxyl-terminal (C-terminal) domain, which includes the predominant colipase-binding site for PTL. Inactive and active conformations of PTL have been described. They differ in the position of a surface loop, the lid domain, and of the beta5-loop. In the inactive conformation, the lid covers the active site and, upon activation by bile salt micelles and colipase or by lipid-water interfaces, the lid moves dramatically to open and configure the active site. After the lid movement, PTL and colipase create a large hydrophobic plateau that can interact with the lipid-water interface. A hydrophobic surface loop in the C-terminal domain, the beta5' loop, may also contribute to the interfacial-binding domain of the PTL-colipase complex.     

Further biochemical characterization of human pancreatic lipase-related protein 2 expressed in yeast cells

Recombinant human pancreatic lipase-related protein 2 (rHPLRP2) was produced in the protease A-deficient yeast Pichia pastoris. A major protein with a molecular mass of 50 kDa was purified from the culture medium using SP-Sepharose and Mono Q chromatography. The protein was found to be highly sensitive to the proteolytic cleavage of a peptide bond in the lid domain. The proteolytic cleavage process occurring in the lid affected both the lipase and phospholipase activities of rHPLRP2. The substrate specificity of the nonproteolyzed rHPLRP2 was investigated using pH-stat and monomolecular film techniques and various substrates (glycerides, phospholipids, and galactolipids). All of the enzyme activities were maximum at alkaline pH values and decreased in the pH 5-7 range corresponding to the physiological conditions occurring in the duodenum. rHPLRP2 was found to act preferentially on substrates forming small aggregates in solution (monoglycerides, egg phosphatidylcholine, and galactolipids) rather than on emulsified substrates such as triolein and diolein. The activity of rHPLRP2 on monogalactosyldiglyceride and digalactosyldiglyceride monomolecular films was determined and compared with that of guinea pig pancreatic lipase-related protein 2, which shows a large deletion in the lid domain. The presence of a full-length lid domain in rHPLRP2 makes it possible for enzyme activity to occur at higher surface pressures. The finding that the inhibition of nonproteolyzed rHPLRP2 by tetrahydrolipstatin and diethyl-p-nitrophenyl phosphate does not involve any bile salt requirements suggests that the rHPLRP2 lid adopts an open conformation in aqueous media.     

Activation of horse PLRP2 by bile salts does not require colipase

Although structurally similar to pancreatic lipase (PL), the key enzyme of intestinal fat digestion, pancreatic lipase-related protein type 2 (PLRP2) differs from PL in certain functional properties. Notably, PLRP2 has a broader substrate specificity than PL, and unlike that of PL, its activity is not restored by colipase in the presence of bile salts. In the studies presented here, the activation mechanism of horse PLRP2 was studied through active site-directed inhibition experiments, and the results demonstrate fundamental differences with that of PL. The opening of the horse PLRP2 flap occurs as soon as bile salt monomers are present, is accelerated in the presence of micelles, and does not require the presence of colipase. Moreover, in contrast to PL, horse PLRP2 is able to directly interact with a bile salt micelle to form an active binary complex, without the micelle being presented by colipase, as evidenced by molecular sieving experiments. These findings, together with the sensitivity of the horse PLRP2 flap to partial proteolysis, are indicative of a higher flexibility of the flap of horse PLRP2 relative to PL. From these results, it can be concluded that PLRP2 can adopt an active conformation in the intestine, which could be important for the further understanding of the physiological role of PLRP2. Finally, this work emphasizes the essential role of colipase in lipase catalysis at the lipid-water interface in the presence of bile.     

Human pancreatic lipase-related protein 2: tissular localization along the digestive tract and quantification in pancreatic juice using a specific ELISA

Human pancreatic lipase-related protein 2 (HPLRP2) was previously found to be secreted by the exocrine pancreas. HPLRP2 shows a high level of activity on galactolipids, and might be involved in the digestion of these common vegetable lipids. Specific antibodies were raised in rabbits using a synthetic HPLRP2 peptide selected for its weak amino acid homology with the corresponding peptides of classical human pancreatic lipase (HPL) and human pancreatic lipase-related protein 1 (HPLRP1). ELISA and Western blotting data showed that these antibodies did not react with HPL or HPLRP1. Various tissues from the digestive tract were subjected to Western blotting analysis with the specific anti-peptide HPLRP2 antibody and the expression of HPLRP2 was detected in the pancreas and colon. An ELISA was developed for specifically measuring the HPLRP2 levels in pure pancreatic juice. This procedure was performed using the anti-peptide HPLRP2 antibody as the captor antibody and a biotinylated anti-HPLRP2 polyclonal antibody as the detector antibody. The lowest HPLRP2 quantification limit was found to be 50 microg/L and the reference range for the present assay was 50 microg-500 microg/L. HPL and HPLRP2 levels were measured using specific ELISAs in pancreatic juice from patients with and without pancreatic disorders. Patients with chronic calcifying pancreatitis (CCP) had significantly lower levels of both HPL and HPLRP2 than the controls subjects. The mean HPLRP2 to HPL ratio was estimated to be 28.30% (w/w) and 23.96% (w/w) in controls subjects and CCP patients, respectively, and the difference was not significant. The levels of HPL and HPLRP2 are therefore similarly reduced in both healthy patients and CCP patients.     

Role of the structural domains in the functional properties of pancreatic lipase-related protein 2

Although structurally similar, classic pancreatic lipase (PL) and pancreatic lipase-related protein (PLRP)2, expressed in the pancreas of several species, differ in substrate specificity, sensitivity to bile salts and colipase dependence. In order to investigate the role of the two domains of PLRP2 in the function of the protein, two chimeric proteins were designed by swapping the N and C structural domains between the horse PL (Nc and Cc domains) and the horse PLRP2 (N2 and C2 domains). NcC2 and N2Cc proteins were expressed in insect cells, purified by one-step chromatography, and characterized. NcC2 displays the same specific activity as PL, whereas N2Cc has the same as that PLRP2. In contrast to N2Cc, NcC2 is highly sensitive to interfacial denaturation. The lipolytic activity of both chimeric proteins is inhibited by bile salts and is not restored by colipase. Only N2Cc is found to be a strong inhibitor of PL activity, due to competition for colipase binding. Active site-directed inhibition experiments demonstrate that activation of N2Cc occurs in the presence of bile salt and does not require colipase, as does PLRP2. The inability of PLRP2 to form a high-affinity complex with colipase is only due to the C-terminal domain. Indeed, the N-terminal domain can interact with the colipase. PLRP2 properties such as substrate selectivity, specific activity, bile salt-dependent activation and interfacial stability depend on the nature of the N-terminal domain.     

Probing the opening of the pancreatic lipase lid using site-directed spin labeling and EPR spectroscopy

Access to the active site of human pancreatic lipase (HPL) is controlled by a surface loop (the lid) that undergoes a conformational change in the presence of amphiphiles and lipid substrate. The question of how and when the lid opens still remains to be elucidated, however. A paramagnetic probe was covalently bound to the lid via the D249C mutation, and electron paramagnetic resonance (EPR) spectroscopy was used to monitor the conformational change in solution. Two EPR spectral components, corresponding to distinct mobilities of the probe, were attributed to the closed and open conformations of the HPL lid, based on experiments performed with the E600 inhibitor. The open conformation of the lid was observed in solution at supramicellar bile salt concentrations. Colipase alone did not induce lid opening but increased the relative proportions of the open conformation in the presence of bile salts. The opening of the lid was found to be a reversible process. Using various colipase to lipase molar ratios, a correlation between the proportion of the open conformation and the catalytic activity of HPL was observed.     

Pancreatic lipase and pancreatic lipase-related protein 2, but not pancreatic lipase-related protein 1, hydrolyze retinyl palmitate in physiological conditions

The major sources of vitamin A in the human diet are retinyl esters (mainly retinyl palmitate) and provitamin A carotenoids. It has been shown that classical pancreatic lipase (PL) is involved in the luminal hydrolysis of retinyl palmitate (RP), but it is not known whether pancreatic lipase-related proteins 1 (PLRP1) and 2 (PLRP2), two other lipases recovered in the human pancreatic juice, are also involved. The aim of this study was to assess whether RP acts a substrate for these lipase-related proteins. Pure horse PL, horse PLRP2 and dog PLRP1 were incubated with RP solubilized in its physiological vehicles, i.e., triglyceride-rich lipid droplets, mixed micelles and vesicles. High performance liquid chromatography (HPLC) was used to assess RP hydrolysis by the free retinol released in the incubation medium. Incubation of RP-containing emulsions with horse PL and colipase resulted in RP hydrolysis (0.051+/-0.01 micromol/min/mg). This hydrolysis was abolished when colipase was not added to the medium. PLRP2 and PLRP1 were unable to hydrolyze RP solubilized in emulsions, regardless of whether colipase was added to the medium. PL hydrolyzed RP solubilized in mixed micelles as well (0.074+/-0.014 micromol/min/mg). Again, this hydrolysis was abolished in the absence of colipase. PLRP2 hydrolyzed RP solubilized in micelles but less efficiently than PL (0.023+/-0.005 micromol/min/mg). Colipase had no effect on this hydrolysis. PLRP1 was unable to hydrolyze RP solubilized in micelles, regardless of whether colipase was present or absent. Both PL and PLRP2 hydrolyzed RP solubilized in a vesicle rich-solution, and a synergic phenomenon between the two lipases was enlighten. Taken together, these results show that (1) PL hydrolyzes RP whether RP is solubilized in emulsions or in mixed micelles, (2) PLRP2 hydrolyzes RP only when RP is solubilized in mixed micelles, and (3) PLRP1 is unable to hydrolyze RP regardless of whether RP is solubilized in emulsions or in mixed micelles.     

High expression in adult horse of PLRP2 displaying a low phospholipase activity

The physiological role of the two lipase-related proteins, PLRP1 and PLRP2, still remains obscure although some propositions have been made concerning PLRP2. In this paper, we report the presence of high amounts of PLRP2 in adult horse pancreas whereas no PLRP1 could be detected. As well, a non-parallel expression of PLRP2 and PLRP1 is observed in adult cat and dog, since no PLRP2 could be detected in these two species. In adult ox, neither PLRP2 nor PLRP1 could be found. These findings are in favor of a different regulation of the expression of the genes encoding pancreatic lipase and the related proteins according to the species. The cDNA encoding horse PLRP2 has been cloned and the protein expressed in insect cells. Both native and recombinant PLRP2 display the same catalytic properties. They possess a moderate lipase activity, inhibited by bile salts and not restored by colipase. Interestingly, they differ from PLRP2 from other species by their very low phospholipase activity indicating that PLRP2 could not be considered as a general phospholipase as previously postulated. This work highlights the variability of the properties of PLRP2 and rises the question of the physiological function of this protein in adult according to the species.     

Inhibition of phospholipase A1, lipase and galactolipase activities of pancreatic lipase-related protein 2 by methyl arachidonyl fluorophosphonate (MAFP)

Methyl arachidonyl fluorophosphonate (MAFP) is a known inhibitor of cytosolic phospholipase A2 and some other serine enzymes. MAFP was found here to be an irreversible inhibitor of human pancreatic lipase-related protein 2 (HPLRP2), an enzyme displaying lipase, phospholipase A1 and galactolipase activities. In the presence of MAFP, mass spectrometry analysis of HPLRP2 revealed a mass increase of 351Da, suggesting a covalent binding of MAFP to the active site serine residue. When HPLRP2 was pre-incubated with MAFP before measuring residual activity, a direct inhibition of HPLRP2 occurred, confirming that HPLRP2 has an active site freely accessible to solvent and differs from most lipases in solution. HPLRP2 activities on tributyrin (TC4), phosphatidylcholine (PC) and monogalactosyl dioctanoylglycerol (C8-MGDG) were equally inhibited under these conditions. Bile salts were not required to trigger the inhibition, but they significantly increased the rate of HPLRP2 inhibition, probably because of MAFP micellar solubilization. Since HPLRP2 is active on various substrates that self-organize differently in the presence of water, HPLRP2 inhibition by MAFP was tested in the presence of these substrates after adding MAFP in the course of the lipolysis reaction. In this case, the rates of inhibition of lipase, phospholipase A1 and galactolipase activities were not equivalent (triglycerides>PC>MGDG), suggesting different enzyme/inhibitor partitioning between the aqueous phase and lipid aggregates. The inhibition by MAFP of a well identified phospholipase A1 (HPLRP2), present in pancreatic juice and also in human monocytes, indicates that MAFP cannot be used for discriminating phospholipase A2 from A1 activities at the cellular level.     

Lipolysis of natural long chain and synthetic medium chain galactolipids by pancreatic lipase-related protein 2

Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the most abundant lipids in nature, mainly as important components of plant leaves and chloroplast membranes. Pancreatic lipase-related protein 2 (PLRP2) was previously found to express galactolipase activity, and it is assumed to be the main enzyme involved in the digestion of these common vegetable lipids in the gastrointestinal tract. Most of the previous in vitro studies were however performed with medium chain synthetic galactolipids as substrates. It was shown here that recombinant guinea pig (Cavia porcellus) as well as human PLRP2 hydrolyzed at high rates natural DGDG and MGDG extracted from spinach leaves. Their specific activities were estimated by combining the pH-stat technique, thin layer chromatography coupled to scanning densitometry and gas chromatography. The optimum assay conditions for hydrolysis of these natural long chain galactolipids were investigated and the optimum bile salt to substrate ratio was found to be different from that established with synthetic medium chains MGDG and DGDG. Nevertheless the length of acyl chains and the nature of the galactosyl polar head of the galactolipid did not have major effects on the specific activities of PLRP2, which were found to be very high on both medium chain [1786 ± 100 to 5420 ± 85 U/mg] and long chain [1756 ± 208 to 4167 ± 167 U/mg] galactolipids. Fatty acid composition analysis of natural MGDG, DGDG and their lipolysis products revealed that PLRP2 only hydrolyzed one ester bond at the sn-1 position of galactolipids. PLRP2 might be used to produce lipid and free fatty acid fractions enriched in either 16:3 n − 3 or 18:3 n − 3 fatty acids, both found at high levels in galactolipids.     

Closed conformation of the active site loop of rabbit muscle triosephosphate isomerase in the absence of substrate: evidence of conformational heterogeneity

The active site loop of triosephosphate isomerase (TIM) exhibits a hinged-lid motion, alternating between the two well defined "open" and "closed" conformations. Until now the closed conformation had only been observed in protein complexes with substrate analogues. Here, we present the first rabbit muscle apo TIM structure, refined to 1.5A resolution, in which the active site loop is either in the open or in the closed conformation in different subunits of the enzyme. In the closed conformation described here, the lid loop residues participate in stabilizing hydrogen bonds characteristic of holo TIM structures, whereas chemical interactions observed in the open loop conformation are similar to those found in the apo structures of TIM. In the closed conformation, a number of water molecules are observed at the projected ligand atom positions that are hydrogen bonded to the active site residues. Additives used during crystallization (DMSO and Tris molecules and magnesium atoms) were modeled in the electron density maps. However, no specific binding of these molecules is observed at, or close to, the active site and the lid loop. To further investigate this unusual closed conformation of the apo enzyme, two more rabbit muscle TIM structures, one in the same and another in a different crystal form, were determined. These structures present the open lid conformation only, indicating that the closed conformation cannot be explained by crystal contact effects. To rationalize why the active site loop is closed in the absence of ligand in one of the subunits, extensive comparison with previously solved TIM structures was carried out, supported by the bulk of available experimental information about enzyme kinetics and reaction mechanism of TIM. The observation of both open and closed lid conformations in TIM crystals might be related to a persistent conformational heterogeneity of this protein in solution.     

Lipase-catalysed hydrolysis of short-chain substrates in solution and in emulsion: a kinetic study.

We have studied the enzymatic hydrolysis of solutions and emulsions of vinyl propionate, vinyl butyrate and tripropionin by lipases of various origin and specificity. Kinetic studies of the hydrolysis of short-chain substrates by microbial triacylglycerol lipases from Rhizopus oryzae, Mucor miehei, Candida rugosa, Candida antarctica A and by (phospho)lipase from guinea-pig pancreas show that these lipolytic enzymes follow the Michaelis-Menten model. Surprisingly, the activity against solutions of tripropionin and vinyl esters ranges from 70% to 90% of that determined against emulsions. In contrast, a non-hyperbolic (sigmoidal) dependence of enzyme activity on ester concentration is found with human pancreatic lipase, triacylglycerol lipase from Humicola lanuginosa (Thermomyces lanuginosa) and partial acylglycerol lipase from Penicillium camembertii and the same substrates. In all cases, no abrupt jump in activity (interfacial activation) is observed at substrate concentration corresponding to the solubility limit of the esters. Maximal lipolytic activity is always obtained in the presence of emulsified ester. Despite progress in the understanding of structure-function of lipases, interpretation of the mode of action of lipases active against solutions of short-chain substrates remains difficult. Actually, it is not known whether these enzymes, which possess a lid structure, are in open or/and closed conformation in the bulk phase and whether the opening of the lid that gives access to the catalytic triad is triggered by interaction of the enzyme molecule with monomeric substrates or/and multimolecular aggregates (micelles) both present in the bulk phase. From the comparison of the behaviour of lipases used in this study which, in some cases, follow the Michaelis-Menten model and, in others, deviate from classical kinetics, it appears that the activity of classical lipases against soluble short-chain vinyl esters and tripropionin depends not only on specific interaction with single substrate molecules at the catalytic site of the enzyme but also on physico-chemical parameters related to the state of association of the substrate dispersed in the aqueous phase. It is assumed that the interaction of lipase with soluble multimolecular aggregates of tripropionin or short-chain vinyl esters or the formation of enzyme-substrate mixed micelles with ester bound to lipase, might represent a crucial step that triggers the structural transition to the open enzyme conformation by displacement of the lid.     

Human pancreatic lipase: colipase dependence and interfacial binding of lid domain mutants

Five key amino acid residues from human pancreatic lipase (HPL) are mutated in some pancreatic lipase-related proteins 2 (PLRP2) that are not reactivated by colipase in the presence of bile salts. One of these residues (Y403) is involved in a direct interaction between the HPL C-terminal domain and colipase. The other four residues (R256, D257, Y267, and K268) are involved in the interactions stabilizing the open conformation of the lid domain, which also interacts with colipase. Here we produced and characterized three HPL mutants: HPL Y403N, an HPL four-site mutant (R256G, D257G, Y267F, and K268E), and an HPL five-site mutant (R256G, D257G, Y267F, K268E, and Y403N), in which the HPL amino acids were replaced by those present in human PLRP2. Colipase reactivated both the HPL Y403N mutant and HPL, and Y403 is therefore not essential for lipase-colipase interactions. Both the HPL four-site and five-site mutants showed low activity on trioctanoin, were inhibited by bile salts (sodium taurodeoxycholate, NaTDC) and were not reactivated by colipase. The interfacial binding of the HPL four-site mutant to a trioctanoin emulsion was suppressed in the presence of 4 mM NaTDC and was not restored by addition of colipase. Protein blotting/protein overlay immunoassay revealed that the HPL four-site mutant-colipase interactions are not abolished, and therefore, the absence of reactivation of the HPL four-site mutant is probably due to a lid domain conformation that prevents the interfacial binding of the lipase-colipase complex. The effects of colipase were also studied with HPL(-lid), an HPL mutant showing an 18-residue deletion within the lid domain, which therefore has only one colipase interaction site. HPL(-lid) showed a low activity on trioctanoin, was inhibited by bile salts, and recovered its lipase activity in the presence of colipase. Reactivation of HPL(-lid) by colipase was associated with a strong interfacial binding of the mutant to a trioctanoin emulsion. The lid domain is therefore not essential for either the interfacial binding of HPL or the lipase-colipase interactions.     

X-ray Crystallographic and MD Simulation Studies on the Mechanism of Interfacial Activation of a Family I.3 Lipase with Two Lids

The interfacial activation mechanism of family I.3 lipase from Pseudomonas sp. MIS38 (PML), which has two α-helical lids (lid1 and lid2), was investigated using a combination of X-ray crystallography and molecular dynamics (MD) simulation. The crystal structure of PML in an open conformation was determined at 2.1 Å resolution in the presence of Ca²⁺ and Triton X-100. Comparison of this structure with that in the closed conformation indicates that both lids greatly change their positions and lid1 is anchored by the calcium ion (Ca1) in the open conformation. This structure was not seriously changed even when the protein was dialyzed extensively against the Ca²⁺-free buffer containing Triton X-100 before crystallization, indicating that the open conformation is fairly stable unless a micellar substance is removed. The crystal structure of the PML derivative, in which the active site serine residue (Ser207) is diethylphosphorylated by soaking the crystal of PML in the open conformation in a solution containing diethyl p-nitrophenyl phosphate, was also determined. This structure greatly resembles that in the open conformation, indicating that PML structure in the open conformation represents that in the active form. MD simulation of PML in the open conformation in the absence of micelles showed that lid2 closes first, while lid1 maintains its open conformation. Likewise, MD simulation of PML in the closed conformation in the absence of Ca²⁺ and in the presence of octane or trilaurin micelles showed that lid1 opens, while lid2 remains closed. These results suggest that Ca1 functions as a hook for stabilization of a fully opened conformation of lid1 and for initiation of subsequent opening of lid2.     

Crystal structure of a mono- and diacylglycerol lipase from Malassezia globosa reveals a novel lid conformation and insights into the substrate specificity

Most lipases contain a lid domain to shield the hydrophobic binding site from the water environment. The lid, mostly in helical form, can undergo a conformational change to expose the active cleft during the interfacial activation. Here we report the crystal structures of Malassezia globosa LIP1 (SMG1) at 1.45 and 2.60 ? resolution in two crystal forms. The structures present SMG1 in its closed form, with a novel lid in loop conformation. SMG1 is one of the few members in the fungal lipase family that has been found to be strictly specific for mono- and diacylglycerol. To date, the mechanism for this substrate specificity remains largely unknown. To investigate the substrate binding properties, we built a model of SMG1 in open conformation. Based on this model, we found that the two bulky hydrophobic residues adjacent to the catalytic site and the N-terminal hinge region of the lid both may act as steric hindrances for triacylglycerols binding. These unique structural features of SMG1 will provide a better understanding on the substrate specificity of mono- and diacylglycerol lipases and a platform for further functional study of this enzyme.