Identification of a potent and selective LAPTc inhibitor by RapidFire-Mass Spectrometry, with antichagasic activity

BACKGROUND

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi and leads to ~10,000 deaths each year. Nifurtimox and benznidazole are the only two drugs available but have significant adverse effects and limited efficacy. New chemotherapeutic agents are urgently required. Here we identified inhibitors of the acidic M17 leucyl-aminopeptidase from T. cruzi (LAPTc) that show promise as novel starting points for Chagas disease drug discovery.

METHODOLOGY/PRINCIPAL FINDINGS

A RapidFire-MS screen with a protease-focused compound library identified novel LAPTc inhibitors. Twenty-eight hits were progressed to the dose-response studies, from which 12 molecules inhibited LAPTc with IC50 < 34 μM. Of these, compound 4 was the most potent hit and mode of inhibition studies indicate that compound 4 is a competitive LAPTc inhibitor, with Ki 0.27 μM. Compound 4 is selective with respect to human LAP3, showing a selectivity index of >500. Compound 4 exhibited sub-micromolar activity against intracellular T. cruzi amastigotes, and while the selectivity-window against the host cells was narrow, no toxicity was observed for un-infected HepG2 cells. In silico modelling of the LAPTc-compound 4 interaction is consistent with the competitive mode of inhibition. Molecular dynamics simulations reproduce the experimental binding strength (-8.95 kcal/mol), and indicate a binding mode based mainly on hydrophobic interactions with active site residues without metal cation coordination.

CONCLUSIONS/SIGNIFICANCE

Our data indicates that these new LAPTc inhibitors should be considered for further development as antiparasitic agents for the treatment of Chagas disease.

Proteases influence colony aggregation behavior in Vibrio cholerae

Aggregation behavior provides bacteria protection from harsh environments and threats to survival. Two uncharacterized proteases, LapX and Lap, are important for Vibrio cholerae liquid-based aggregation. Here, we determined that LapX is a serine protease with a preference for cleavage after glutamate and glutamine residues in the P1 position, which processes a physiologically based peptide substrate with a catalytic efficiency of 180 ± 80 M-1s-1. The activity with a LapX substrate identified by a multiplex substrate profiling by mass spectrometry screen was 590 ± 20 M-1s-1. Lap shares high sequence identity with an aminopeptidase (termed VpAP) from Vibrio proteolyticus and contains an inhibitory bacterial prepeptidase C-terminal domain that, when eliminated, increases catalytic efficiency on leucine p-nitroanilide nearly four-fold from 5.4 ± 4.1 × 104 M-1s-1 to 20.3 ± 4.3 × 104 M-1s-1. We demonstrate that LapX processes Lap to its mature form and thus amplifies Lap activity. The increase is approximately eighteen-fold for full-length Lap (95.7 ± 5.6 × 104 M-1s-1) and six-fold for Lap lacking the prepeptidase C-terminal domain (11.3 ± 1.9 × 105 M-1s-1). In addition, substrate profiling reveals preferences for these two proteases that could inform in vivo function. Furthermore, purified LapX and Lap restore the timing of the V. cholerae aggregation program to a mutant lacking the lapX and lap genes. Both proteases must be present to restore WT timing, and thus they appear to act sequentially: LapX acts on Lap, and Lap acts on the substrate involved in aggregation.

Unveiling the Catalytic Mechanism of a Processive Metalloaminopeptidase

Intracellular leucine aminopeptidases (PepA) are metalloproteases from the family M17. These enzymes catalyze peptide bond cleavage, removing N-terminal residues from peptide and protein substrates, with consequences for protein homeostasis and quality control. While general mechanistic studies using model substrates have been conducted on PepA enzymes from various organisms, specific information about their substrate preferences and promiscuity, choice of metal, activation mechanisms, and the steps that limit steady-state turnover remain unexplored. Here, we dissected the catalytic and chemical mechanisms of PaPepA: a leucine aminopeptidase from Pseudomonas aeruginosa. Cleavage assays using peptides and small-molecule substrate mimics allowed us to propose a mechanism for catalysis. Steady-state and pre-steady-state kinetics, pH rate profiles, solvent kinetic isotope effects, and biophysical techniques were used to evaluate metal binding and activation. This revealed that metal binding to a tight affinity site is insufficient for enzyme activity; binding to a weaker affinity site is essential for catalysis. Progress curves for peptide hydrolysis and crystal structures of free and inhibitor-bound PaPepA revealed that PaPepA cleaves peptide substrates in a processive manner. We propose three distinct modes for activity regulation: tight packing of PaPepA in a hexameric assembly controls substrate length and reaction processivity; the product leucine acts as an inhibitor, and the high concentration of metal ions required for activation limits catalytic turnover. Our work uncovers catalysis by a metalloaminopeptidase, revealing the intricacies of metal activation and substrate selection. This will pave the way for a deeper understanding of metalloenzymes and processive peptidases/proteases.

A promising new target to control fasciolosis: Fasciola hepatica leucine aminopeptidase 2

Parasite M17 leucine aminopeptidases (LAPs) have been associated with critical roles in different key functions such as the nutrition, migration, and invasion of the natural host. Native or recombinant LAP used as a vaccine antigen has proved effective to elicit protection against Fasciola hepatica infection in sheep, pointing to potential vaccine candidates against fascioliasis in ruminant species. Previously, the FhLAP1, abundantly secreted in vitro by the mature adult parasite was used as a vaccine antigen obtaining promising protection results against F. hepatica challenge in small ruminants. Here, we report the biochemical characterization of a second recombinant LAP (FhLAP2) associated with the juvenile stage of F. hepatica. FhLAP2 showed aminopeptidase activity using different synthetic substrates, including leucine, arginine, and methionine and was increased in the presence of Mn+ 2 and Mg+ 2. The activity was inhibited by bestatin, 1,10-phenanthroline, and EDTA, specific inhibitors of aminopeptidase and/or metalloproteases. Finally, the recombinant FhLAP2 functional form was tested in combination with Freund's incomplete adjuvant in an immunization trial in mice followed by an experimental challenge with F. hepatica metacercariae. The immunization with FhLAP2/FIA resulted in a significant reduction of parasite recovery compared to control groups. The immunized group elicited total specific IgG and subclasses IgG1 and IgG2 antibody responses. This study highlights the potential of a new candidate vaccine formulation with potential applications in natural ruminant hosts, especially those targeting the juvenile stage.

Partial protection with a chimeric tetraspanin-leucine aminopeptidase subunit vaccine against Opisthorchis viverrini infection in hamsters

Opisthorchiasis is a serious public health problem in East Asia and Europe. The pathology involves hepatobiliary abnormalities such as cholangitis, choledocholithiasis and tissue fibrosis that can develop into cholangiocarcinoma. Prevention of infection is difficult as multiple social and behavioral factors are involved, thus, progress on a prophylactic vaccine against opisthorchiasis is urgently needed. Opisthorchis viverrini tetraspanin-2 (Ov-TSP-2) was previously described as a potential vaccine candidate conferring partial protection against O. viverrini infections in hamsters. In this study, we generated a recombinant chimeric form of the large extracellular loop of Ov-TSP-2 and O. viverrini leucine aminopeptidase, designated rOv-TSP-2-LAP. Hamsters were vaccinated with 100 and 200 µg of rOv-TSP-2-LAP formulated with alum-CpG adjuvant via intraperitoneal injection and evaluated the level of protection against O. viverrini infection. Our results demonstrated that the number of worms recovered from hamsters vaccinated with either 100 or 200 µg of rOv-TSP-2-LAP were significantly reduced by 27% compared to the adjuvant control group. Furthermore, the average length of worms recovered from animals vaccinated with 200 μg of rOv-TSP-2-LAP was significantly shorter than those from the control adjuvant group. Immunized hamsters showed significantly increased serum levels of anti-rOv-TSP-2 IgG and IgG1 compared to adjuvant control group, suggesting that rOv-TSP-2-LAP vaccination induces a mixed Th1/Th2 immune response in hamsters. Therefore, the development of a suitable vaccine against opisthorchiasis requires further work involving new vaccine technologies to improve immunogenicity and protective efficacy.

Quantitation of Methionine Sulfoxide in Milk and Milk-Based Beverages-Minimizing Artificial Oxidation by Anaerobic Enzymatic Hydrolysis

Protein oxidation in milk products may entail flavor changes through reactions at methionine residues. However, little is known about the extent of methionine oxidation in milk and milk products. In the present study, a method for quantitation of methionine, methionine sulfoxide, and methionine sulfone by a stable isotope dilution assay using HILIC-ESI-MS/MS was established. For the quantitation of protein-bound analytes, anaerobic enzymatic hydrolysis was optimized to suppress artificial methionine oxidation. Moreover, the method allowed for monitoring of artificial oxidation by coincubation of the labeled probe [²H₈]methionine. The percentage of oxidized methionine was low in UHT milk (up to 1.6%) and evaporated milk (up to 8.8%), but higher in beverages such as cocoa milk drinks (up to 19.0%) and coffee milk drinks (up to 32.8%), resulting in methionine sulfoxide concentrations of up to 6.7 g/kg protein in the latter. These products are important dietary sources of methionine sulfoxide. Model studies revealed that methionine residues can be oxidized strongly in the presence of phenolic compounds such as catechin, caffeic acid, and gallic acid, which are present in cocoa and coffee and may account for the high extent of oxidation in commercial samples.

[Proteolysis of simple glyprolines by leucine aminopeptidase and enzymes from nasal slime, brain membranes, and rat blood]

Proteolysis of Pro-Gly-Pro-Leu, Pro-Gly-Pro-Gly and Pro-Gly-Pro were studied comparatively to Met-Glu-His-Phe-Pro-Gly-Pro (semax). It is shown that all three peptides are considerably more stable to proteolysis by N-leucine-aminopeptidase (EC 3.4.11.1, Sigma, type VI, 9.2 units/mg), and by enzymes of nasal slime, brain microsomal fractions, and rat blood. Metabolites of the proteolysis showed that semax derives His-Phe-Pro-Gly-Pro only, Pro-Gly-Pro-Leu forms Gly-Pro-Leu, Pro-Gly-Pro and Gly-Pro, Pro-Gly-Pro-Gly gives Pro-Gly-Pro and Gly-Pro, and Pro-Gly-Pro forms Gly-Pro.

Biochemical properties of recombinant leucine aminopeptidase II from Bacillus stearothermophilus and potential applications in the hydrolysis of Chinese anchovy (Engraulis japonicus) proteins

The effects of various factors on the activity and conformation of recombinant leucine aminopeptidase II (rLAP II) from Bacillus stearothermophilus and its potential utilization in the hydrolysis of anchovy proteins were investigated. The optimal temperature and pH of rLAP II were 55 °C and 8.0 in phosphate buffer, and its activity was strongly stimulated by Co(2+). Conformational studies indicated that maintaining the α-helical structure had a critical effect on rLAP II activity. rLAP II was used to hydrolyze anchovy proteins, and it exhibited high specificity for peptides with molecular weight between 6000 and 1000 Da and positive coordination with endogenous enzymes and commercial Flavourzyme. Its use will enhance protein hydrolysis in species of aquatic animals. rLAP II could potentially be used to remove bitterness in the protein hydrolysis industry.

Biochemical properties and potential applications of recombinant leucine aminopeptidase from Bacillus kaustophilus CCRC 11223

Experiments were carried out to investigate the effects of various factors on the activity and conformation of recombinant leucine aminopeptidase of Bacillus kaustophilus CCRC 11223 (BkLAP) and potential utilization of BkLAP in the hydrolysis of anchovy protein. Optimal temperature and pH of BkLAP were 70 °C and 8.0 in potassium-phosphate buffer, respectively, and the activity was strongly stimulated by Ni²⁺, followed by Mn²⁺ and Co²⁺. Conformational studies via circular dichroism spectroscopy indicated that various factors could influence the secondary structure of BkLAP to different extents and further induce the changes in enzymatic activity. The secondary structure of BkLAP was slightly modified by Ni²⁺ at the concentration of 1×10⁻⁴ M, however, significant changes on the secondary structures of the enzyme were observed when Hg²⁺ was added to the concentration of 1×10⁻⁴ M. The potential application of BkLAP was evaluated through combination with the commercial or endogenous enzyme to hydrolysis the anchovy protein. Results showed that combining the BkLAP with other enzymes could significantly increase the degree of hydrolysis and amino acid component of hydrolysate. In this regard, BkLAP is a potential enzyme that can be used in the protein hydrolysate industry.

Identification of an aminopeptidase from the skeletal muscle of grass carp (Ctenopharyngodon idellus)

Aminopeptidases play important roles in turnover of proteins, metabolism of hormones and neurotransmission, cell maturation and immunological regulations. In the present study, an aminopeptidase was purified to homogeneity from the skeletal muscle of grass carp by ammonium sulfate fractionation and sequential chromatographic steps, including DEAE-Sephacel, Sephacryl S-200, hydroxyapatite and Phenyl-Sepharose. The purified enzyme revealed a molecular mass of approximately 105 kDa both on SDS-PAGE and on gel filtration of Superdex 200. The enzymatic activity toward synthetic substrates was optimal at 40°C and pH 7.0-7.5. Metal-chelating agents such as EDTA and EGTA effectively inhibited the enzyme activity while inhibitors to serine, asparatic and cysteine proteinases did not show much effect, suggesting its belonging to metalloproteinase family. A specific aminopeptidase inhibitor bestatin was most effective in suppressing the enzymatic activity and performed in a competitive fashion. The enzymatic activity was slightly enhanced by metal ions of Mg2+ and Mn2+ while inhibited to different extents by Co2+, Cu2+, Zn2+ and Ca2+. Sulfhydryl reagent was necessary to maintain its activity. Purified enzyme demonstrated amidolytic activity most effectively against synthetic aminopeptidase substrate Leu-methylcoumarylamide (MCA) while N-terminal-blocked substrates and myofibrillar proteins were not hydrolyzed. The enzyme purified in the present study was quite possibly a leucine aminopeptidase (LAP) and functions during muscular protein metabolism.

Ecoenzymatic stoichiometry in relation to productivity for freshwater biofilm and plankton communities

The degradation of detrital organic matter and assimilation of carbon (C), nitrogen (N), and phosphorus (P) by heterotrophic microbial communities is mediated by enzymes released into the environment (ecoenzymes). For the attached microbial communities of soils and freshwater sediments, the activities of β-glucosidase, β-N-acetylglucosaminidase, leucine aminopeptidase, and phosphatase show consistent stoichiometric patterns. To determine whether similar constraints apply to planktonic communities, we assembled data from nine studies that include measurements of these enzyme activities along with microbial productivity. By normalizing enzyme activity to productivity, we directly compared the ecoenzymatic stoichiometry of aquatic biofilm and bacterioplankton communities. The relationships between β-glucosidase and α-glucosidase and β-glucosidase and β-N-acetylglucosaminidase were statistically indistinguishable for the two community types, while the relationships between β-glucosidase and phosphatase and β-glucosidase and leucine aminopeptidase significantly differed. For β-glucosidase vs. phosphatase, the differences in slope (biofilm 0.65, plankton 1.05) corresponded with differences in the mean elemental C:P ratio of microbial biomass (60 and 106, respectively). For β-glucosidase vs. leucine aminopeptidase, differences in slope (0.80 and 1.02) did not correspond to differences in the mean elemental C:N of biomass (8.6 and 6.6). β-N-Acetylglucosaminidase activity in biofilms was significantly greater than that of plankton, suggesting that aminosaccharides were a relatively more important N source for biofilms, perhaps because fungi are more abundant. The slopes of β-glucosidase vs. (β-N-acetylglucosaminidase + leucine aminopeptidase) regressions (biofilm 1.07, plankton 0.94) corresponded more closely to the estimated difference in mean biomass C:N. Despite major differences in physical structure and trophic organization, biofilm and plankton communities have similar ecoenzymatic stoichiometry in relation to productivity and biomass composition. These relationships can be integrated into the stoichiometric and metabolic theories of ecology and used to analyze community metabolism in relation to resource constraints.

Cloning, expression, crystallization and preliminary X-ray crystallographic analysis of leucine aminopeptidase (LAP) from the pepA gene of Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo) causes the serious disease bacterial blight in rice. The pepA (Xoo0834) gene from Xoo is one of around 100 genes that have been selected for the design of antibacterial drugs. The pepA gene encodes leucine aminopeptidase (LAP), an exopeptidase that catalyzes the hydrolysis of leucine residues from the N-terminus of a protein or peptide. This enzyme was expressed in Escherichia coli, purified and crystallized, and preliminary X-ray structural studies have been carried out. The LAP crystal diffracted to 2.6 A resolution and belonged to the cubic space group P2(1)3. The unit-cell volume of the crystal was compatible with the presence of two monomers in the asymmetric unit.

Purification and properties of major midgut leucyl aminopeptidase of Morimus funereus (Coleoptera, Cerambycidae) larvae

The major leucyl aminopeptidase (LAP) from the midgut of Morimus funereus larvae was purified and characterised. Specific LAP activity was increased 292-fold by purification of the crude midgut extract. The purified enzyme had a pH optimum of 7.5 (optimum pH range 7.0-8.5) and preferentially hydrolysed p-nitroanilides containing hydrophobic amino acids in the active site, with the highest V(max)/K(M) ratio for leucine-p-nitroanilide (LpNA). Among a number of inhibitors tested, the most efficient were 1,10-phenanthroline having a K(i) value of 0.12 mM and cysteine with K(i) value of 0.31 mM, while EGTA stimulated LAP activity. Zn(2+), Mg(2+) and Mn(2+) all showed bi-modal effects on LAP activity (activated at low concentrations and inhibited at high concentrations). The purified LAP (after gel filtration on Superose 6 column) had molecular mass of 400 kDa with an isoelectric point of 6.2. Sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed one band of 67 kDa, suggesting that the enzyme is a hexamer. Six peptide sequences from protein band were obtained using ESI/MS-MS analysis. Comparison of the obtained peptide sequences with the EMBL-EBI sequence analysis toolbox and the BLASTP database showed a high degree of identity with other insect aminopeptidases.

Enzymatic degradation of beta- and mixed alpha,beta-oligopeptides

One of the main and most astonishing characteristics of peptides comprised of beta-amino acids with proteinogenic side chains is their extraordinarily high stability towards enzymatic degradation. So far, only certain microbial enzymes have been shown to cleave N-terminal beta(3)-homoamino acid residues from peptides. In this work, the L-aminopeptidase-D-amidase/esterase (DmpA) from Ochrobactrum anthropi LMG7991 is compared to two closely related beta-peptidyl aminopeptidases (BapA), which originate from Sphingosinicella strains, and to microsomal leucine aminopeptidase (LAP) as a reference. All four enzymes are aminopeptidases cleaving N-terminal amino acids from small peptides. Degradation experiments reveal that DmpA and both BapA enzymes exhibit unique, but clearly distinct substrate specificities and preferences. DmpA also cleaves beta- and mixed alpha,beta-peptides and amides, but a short side chain of the N-terminal beta-amino acid residue seems to be a prerequisite, since only peptides carrying N-terminal betahGly and beta(3)hAla are hydrolyzed with good efficiencies. Both beta-peptidyl aminopeptidases cleave beta-amino acids from a variety of beta-peptides and mixed alpha,beta-peptides, but they do not accept alpha-amino acids in the N-terminal position. Astonishingly, DmpA exhibited much higher catalytical rates for the mixed dipeptide carnosine (H-betahGly-His-OH) than for any other substrate described until now.

A synthetic method for diversification of the P1' substituent in phosphinic dipeptides as a tool for exploration of the specificity of the S1' binding pockets of leucine aminopeptidases

A novel, general, and versatile method of diversification of the P1' position in phosphinic pseudodipeptides, presumable inhibitors of proteolytic enzymes, was elaborated. The procedure was based on parallel derivatization of the amino group in the suitably protected phosphinate building blocks with appropriate alkyl and aryl halides. This synthetic strategy represents an original approach to phosphinic dipeptide chemistry. Its usefulness was confirmed by obtaining a series of P1' modified phosphinic dipeptides, inhibitors of cytosolic leucine aminopeptidase, through computer-aided design basing on the structure of homophenylalanyl-phenylalanine analogue (hPheP[CH(2)]Phe) bound in the enzyme active site as a lead structure. In this approach novel interactions between inhibitor P1' fragment and the S1' region of the enzyme, particularly hydrogen bonding involving Asn330 and Asp332 enzyme residues, were predicted. The details of the design, synthesis, and activity evaluation toward cytosolic leucine aminopeptidase and aminopeptidase N are discussed. Although the potency of the lead compound has not been improved, marked selectivity of the synthesized inhibitors toward both studied enzymes was observed.

Characterization of the Plasmodium falciparum M17 Leucyl Aminopeptidase

Amino acids generated from the catabolism of hemoglobin by intra-erythrocytic malaria parasites are not only essential for protein synthesis but also function in maintaining an osmotically stable environment, and creating a gradient by which amino acids that are rare or not present in hemoglobin are drawn into the parasite from host serum. We have proposed that a Plasmodium falciparum M17 leucyl aminopeptidase (PfLAP) generates and regulates the internal pool of free amino acids and therefore represents a target for novel antimalarial drugs. This enzyme has been expressed in insect cells as a functional 320-kDa homo-hexamer that is optimally active at neutral or alkaline pH, is dependent on metal ions for activity, and exhibits a substrate preference for N-terminally exposed hydrophobic amino acids, particularly leucine. PfLAP is produced by all stages in the intra-erythrocytic developmental cycle of malaria but was most highly expressed by trophozoites, a stage at which hemoglobin degradation and parasite protein synthesis are elevated. The enzyme was located by immunohistochemical methods and by transfecting malaria cells with a PfLAP-green fluorescent protein construct, to the cytosolic compartment of the cell at all developmental stages, including segregated merozoites. Amino acid dipeptide analogs, such as bestatin and its derivatives, are potent inhibitors of the protease and also block the growth of P. falciparum malaria parasites in culture. This study provides a biochemical basis for the antimalarial activity of aminopeptidase inhibitors. Availability of functionally active recombinant PfLAP, coupled with a simple enzymatic readout, will aid medicinal chemistry and/or high throughput approaches for the future design/discovery of new antimalarial drugs.

Binding structure of the leucine aminopeptidase inhibitor microginin FR1

Natural bioactive compounds are of general interest for pharmaceutical research because they may serve as leads in drug development campaigns. Among them, microginins are linear peptides known to inhibit various exopeptidases. The crystal structure of microginin FR1 from Microcystis sp. bound to bovine lens leucine aminopeptidase was established at 1.73 Angstrom resolution. The observed binding structure could be beneficial for the design of potent aminopeptidase inhibitors.

Residues threonine 346 and leucine 352 are critical for the proper function of Bacillus kaustophilus leucine aminopeptidase

The importance of Thr-346 and Leu-352 residues in Bacillus kaustophilus leucine aminopeptidase (BkLAP) was explored by site-directed mutagenesis. The impact of substitutions at these positions was evaluated with His6-BkLAP fusion proteins expressed in Escherichia coli. Substitution of Thr-346 with Tyr, Arg, and Leu, respectively, resulted in a dramatic reduction in LAP activity. A complete loss of activity was observed in L352E and L352R variants with the exception of L352 V, which retained approximately 60% of the wild-type activity. Zinc content analysis and protein modeling suggested that Thr-346 and Leu-352 of BkLAP play a role in maintaining the coordination environment for the zinc-binding residues.

Identification of glutamate residues important for catalytic activity of Bacillus stearothermophilus leucine aminopeptidase II

Each of four conserved glutamate residues of Bacillus stearothermophilus leucine aminopeptidase II (BsLAPII) was replaced with aspartate, lysine, and leucine respectively by site-directed mutagenesis. The over-expressed wild-type and mutant enzymes were purified to homogeneity by nickel-chelate chromatography and the molecular mass of the subunit was determined to be 44.5 kDa by SDS-PAGE. The specific activity for the Glu-316 and Glu-340 mutants was completely abolished, while Glu-249 mutants showed comparable activity to that of the wild-type BsLAPII. Compared with the wild-type enzyme, the E250D and E250L mutant enzymes retained less than 18% of the enzyme activity and exhibited a dramatic decrease in the value of k (cat)/K (m). These observations indicate that Glu-250, Glu-316, and Glu-340 residues are critical for the catalytic activity of BsLAPII.

Modulation of Streptomyces Leucine Aminopeptidase by Calcium

Streptomyces griseus leucine aminopeptidase (SGAP), which has two zinc atoms in its active site, is clinically important as a model for understanding the structure and mechanism of action of other metallopeptidases. SGAP is a calcium-activated and calcium-stabilized enzyme, and its activation by calcium correlates with substrate specificity. In our previous study, we found a non-calcium-modulated leucine aminopeptidase secreted by Streptomyces septatus, the primary structure of which showed 71% identity with SGAP. In this study, we constructed chimeras of SGAP and S. septatus aminopeptidase by using an in vivo DNA shuffling system and several mutant enzymes by site-directed mutagenesis to identify the key residues in this modulation by calcium. We identified the key residues Asp-173 and Asp-174 of SGAP associated with both SGAP activation and stabilization by calcium. We also showed that the known calcium-binding site, which is composed of Asp-3, Ile-4, Asp-262, and Asp-266 of SGAP, only contributes to SGAP stabilization by calcium. Furthermore, we identified an important residue, Glu-196, that functions in cooperation with Asp-173, Asp-174, and calcium to increase the catalytic activity of SGAP.

Metal Ion Substitution in the Catalytic Site Greatly Affects the Binding of Sulfhydryl-Containing Compounds to Leucyl Aminopeptidase,

Bovine lens leucyl aminopeptidase (blLAP), a homohexameric metallopeptidase preferring bulky and hydrophobic amino acids at the N-terminus of (di)peptides, contains two Zn(2+) ions per subunit that are essential for catalytic activity. They may be replaced by other divalent cations with different exchange kinetics. The protein readily exchangeable site (site 1) can be occupied by Zn(2+), Mn(2+), Mg(2+), or Co(2+), while the tight binding site (site 2) can be occupied by Zn(2+) or Co(2+). We recently reported that introduction of Mn(2+) into site 1 generates a novel activity of blLAP toward CysGly [Cappiello, M., et al. (2004) Biochem. J. 378, 35-44], which in contrast is not hydrolyzed by the (Zn/Zn) enzyme. This finding, while disclosing a potential specific role for blLAP in glutathione metabolism, raised a question about the features required for molecules to be a substrate for the enzyme. To clarify the interaction of the enzyme with sulfhydryl-containing derivatives, (Zn/Zn)- and (Mn/Zn)blLAP forms were prepared and functional-structural studies were undertaken. Thus, a kinetic analysis of various compounds with both enzyme forms was performed; the crystal structure of (Zn/Zn)blLAP in complex with the peptidomimetic derivative Zofenoprilat was determined, and a modeling study on the CysGly-(Zn/Zn)blLAP complex was carried out. This combined approach provided insight into the interaction of blLAP with sulfhydryl-containing derivatives, showing that the metal exchange in site 1 modulates binding to these molecules that may result in enzyme substrates or inhibitors, depending on the nature of the metal.

Physical Nature of Interactions within the Active Site of Cytosine-5-methyltransferase

The physical nature of interactions within the active site of cytosine-5-methyltransferase (CMT) was studied using a variation-perturbation energy decomposition scheme defining a sequence of approximate intermolecular interaction energy models. These models have been used to analyze the catalytic activity of residues constituting cytosine-5-methyltransferase active site as well their role in the binding group of de novo designed inhibitors. Our results indicate that Glu119, Arg163, and Arg165 appear to play the dominant role in stabilizing the protonated transition state structure and their influence can be qualitatively approximated by electrostatic interactions alone. The stabilization of neutral structures of the alternative reaction pathway is small, which might suggest the protonated pathway as preferred by the enzyme. Exchange and delocalization terms are negligible in most cases, or they cancel each other to some extent. Interactions of inhibitors with the CMT active site are dominated by electrostatic multipole contributions in analogy with previously studied transition state analogue inhibitors of leucyl aminopeptidase.

The role of Glu196 in the environment around the substrate binding site of leucine aminopeptidase fromStreptomyces griseus

To investigate the role of Glu196 of leucine aminopeptidase from Streptomyces griseus (SGAP) in SGAP activation by calcium and substrate specificity, we constructed E196X SGAP by saturation mutagenesis. Most mutations led to the abrogation of SGAP activation by calcium, and substitution with Lys led to a marked increase in activity toward Asp-p-nitroanilide (pNA) and a decrease in that toward Lys-pNA. A similar result was obtained from the investigation using non-calcium-activated enzyme from Streptomyces septatus (SSAP). These results indicate that Glu196 of SGAP is associated with the environment around the substrate binding site besides its role in SGAP activation by calcium.

Leucine aminopeptidases: diversity in structure and function

Leucine aminopeptidases (LAPs) are metallopeptidases that cleave N-terminal residues from proteins and peptides. While hydrolyzing Leu substrates, LAPs often have a broader specificity. LAPs are members of the M1 or M17 peptidase families, and therefore the LAP nomenclature is complex. LAPs are often viewed as cell maintenance enzymes with critical roles in turnover of peptides. In mammals, the M17 and M1 enzymes with LAP activity contribute to processing peptides for MHC I antigen presentation, processing of bioactive peptides (oxytocin, vasopressin, enkephalins), and vesicle trafficking to the plasma membrane. In microbes, the M17 LAPs have a role in proteolysis and have also acquired the ability to bind DNA. This property enables LAPs to serve as transcriptional repressors to control pyrimidine, alginate and cholera toxin biosynthesis, as well as mediate site-specific recombination events in plasmids and phages. In plants the roles of the M17 LAPs and the peptidases related to M1 LAPs are being elucidated. Roles in defense, membrane transport of auxin receptors, and meiosis have been implicated.

Alteration of leucine aminopeptidase from Streptomyces septatus TH-2 to phenylalanine aminopeptidase by site-directed mutagenesis

To tailor leucine aminopeptidase from Streptomyces septatus TH-2 (SSAP) to become a convenient biocatalyst, we are interested in Phe221 of SSAP, which is thought to interact with the side chain of the N-terminal residue of the substrate. By using saturation mutagenesis, the feasibility of altering the performance of SSAP was evaluated. The hydrolytic activities of 19 mutants were investigated using aminoacyl p-nitroanilide (pNA) derivatives as substrates. Replacement of Phe221 resulted in changes in the activities of all the mutants. Three of these mutants, F221G, F221A, and F221S, specifically hydrolyzed L-Phe-pNA, and F221I SSAP exhibited hydrolytic activity with L-Leu-pNA exceeding that of the wild type. Although the hydrolytic activities with peptide substrates decreased, the hydrolytic activities with amide and methyl ester substrates were proportional to the changes in the hydrolytic activities with pNA derivatives. Furthermore, based on a comparative kinetic study, the mechanism underlying the alteration in the preference of SSAP from leucine to phenylalanine is discussed.

Fasting and refeeding cause rapid changes in intestinal tissue mass and digestive enzyme capacities of Atlantic salmon (Salmo salar L.)

Fasting and refeeding effects on gastrointestinal morphology and digestive enzyme activities of Atlantic salmon, held in tanks of seawater at 9 degrees C and 31 per thousand salinity, were addressed in two trials. Trial 1: Fish (mean body mass 1190 g) were fasted for 40 days and intestines sampled at day 0, 2, 4, 11, 19 and 40. Trial 2: Fish (1334 g), fasted for 50 days, were refed and sampled at day 0, 3 and 7. Mass, length, protein, and maltase, lactase, and leucine aminopeptidase (LAP) activities were analyzed for stomach (ST), pyloric caeca (PC), proximal (PI), mid (MI), and distal intestine (DI). PC contributed 50% of gastrointestinal mass and 75% of enzyme capacity. Fasting decreased mass and enzyme capacities by 20-50% within two days, and 40-75% after 40 days. In PC, specific brush border membrane (BBM) maltase activity decreased whereas BBM LAP increased during fasting. Upon refeeding, enzyme capacities were mostly regenerated after one week. The results suggest that refeeding should start slowly with about 25% of estimated feed requirement during the first 3 days, but may then be stepped up rapidly. Investigations of digestive processes of fed fish should only be performed when intestines are feed-filled to avoid bias due to effects of fasting.

Histidines 345 and 378 of Bacillus stearothermophilus leucine aminopeptidase II are essential for the catalytic activity of the enzyme

The conserved histidine residues, His-191, His-227, His-345, and His-378, in Bacillus stearothermophilus leucine aminopeptidase II (LAPII) were replaced with leucine by site-directed mutagenesis. The overexpressed wild-type and mutant enzymes have been purified by nickel-chelate chromatography and their molecular masses were approximately 44.5 kDa. Under assay conditions, no LAP activity was detected in H345L and H378L. Although the Km value for H191L increased more than 30% with respect to the wild-type LAPII, alteration in this residue did not lead to a significant change on the catalytic efficiency. The 39% decrease in Kcat/Km for H227L was partly caused by a 3.9-fold increase in Km value. Based on these results, it is suggested that His-345 and His-378 play a crucial role in the catalytic reaction of B. stearothermophilus LAPII.

Construction and one-step purification of Bacillus kaustophilus leucine aminopeptidase fused to the starch-binding domain of Bacillus sp. strain TS-23 α-amylase

The starch-binding domain of Bacillus sp. strain TS-23 alpha-amylase was introduced into the C-terminal end of Bacillus kaustophilus leucine aminopeptidase (BkLAP) to generate a chimeric enzyme (BkLAPsbd) with raw-starch-binding activity. BkLAPsbd, with an apparent molecular mass of approximately 65 kDa, was overexpressed in Escherichia coli M15 cells and purified to homogeneity by nickel-chelate chromatography. Native PAGE and chromatographic analyses revealed that the purified fusion protein has a hexameric structure. The half-life for BkLAPsbd was 12 min at 70 degrees C, while less than 20% of wild-type enzyme activity retained at the same heating condition. Compared with the wild-type enzyme, the 60% decrease in the catalytic efficiency of BkLAPsbd was due to a 91% increase in K (m) value. Starch-binding assays showed that the K (d) and B (max) values for the fusion enzyme were 2.3 microM and 0.35 micromol/g, respectively. The adsorption of the crude BkLAPsbd onto raw starch was affected by starch concentration, pH, and temperature. The adsorbed enzyme could be eluted from the adsorbent by 2% soluble starch in 20 mM Tris-HCl buffer (pH 8.0). About 49% of BkLAPsbd in the crude extract was recovered through one adsorption-elution cycle with a purification of 11.4-fold.

Purification and characterization of hyperthermotolerant leucine aminopeptidase from Geobacillus thermoleovorans 47b

A thermophilic bacterium, which we designated as Geobacillus thermoleovorans 47b was isolated from a hot spring in Beppu, Oita Prefecture, Japan, on the basis of its ability to grow on bitter peptides as a sole carbon and nitrogen source. The cell-free extract from G. thermoleovorans 47b contained leucine aminopeptidase (LAP; EC 3.4.11.10), which was purified 164-fold to homogeneity in seven steps, using ammonium sulfate fractionation followed by the column chromatography using DEAE-Toyopearl, hydroxyapatite, MonoQ and Superdex 200 PC gel filtration, followed again by MonoQ and hydroxyapatite. The enzyme was a single polypeptide with a molecular mass of 42,977.2 Da, as determined by matrix-assisted laser desorption ionization and time-of-flight mass spectrometry, and was found to be thermostable at 90 degrees C for up to 1 h. Its optimal pH and temperature were observed to be 7.6-7.8 and 60 degrees C, respectively, and it had high activity towards the substrates Leu-p-nitroanilide (p-NA)(100%), Arg-p-NA (56.3%) and LeuGlyGly (486%). The K(m) and V(max) values for Leu-p-NA and LeuGlyGly were 0.658 mM and 25.0 mM and 236.2 micromol min(-1) mg(-1) protein and 1,149 micromol min(-1) mg(-1) protein, respectively. The turnover rate (k(cat)) and catalytic efficiency (k(cat)/ K(m)) for Leu-p-NA and LeuGlyGly were 10,179 s(-1) and 49,543 s(-1) and 15,470 mM(-1 ) s(-1) and 1981.7 mM(-1 ) s(-1), respectively. The enzyme was strongly inhibited by EDTA, 1,10-phenanthroline, dithiothreitol, beta-mercaptoethanol, iodoacetate and bestatin; and its apoenzyme was found to be reactivated by Co(2+) .

alpha-Aminoalkylphosphonates as a tool in experimental optimisation of P1 side chain shape of potential inhibitors in S1 pocket of leucine- and neutral aminopeptidases

The synthesis and biological activity studies of the series of structurally different alpha-aminoalkylphosphonates were performed in order to optimise the shape of the side chain of the potential inhibitors in S1 pocket of leucine aminopeptidase [E.C.3.4.11.1]. Analysis of a series of compounds with aromatic, aliphatic and alicyclic P1 side chains enabled to find out the structural features, optimal for that fragment of inhibitors of LAP. The most active among all investigated compounds were the phosphonic analogues of homo-tyrosine (K(i)=120 nM) and homo-phenylalanine (K(i)=140 nM), which even as racemic mixtures were better inhibitors in comparison with the best till now-phosphonic analogue of l-leucine (230 nM). Additional comparison of the inhibitory activity obtained for aminopeptidase N (APN, E.C.3.4.11.2) give insight into structural preferences of both enzymes.

A Water Sluice Is Generated in the Active Site of Bovine Lens Leucine Aminopeptidase

Nature has provided the binuclear zinc based active site of bovine lens leucine aminopeptidase (blLAP) with two water channels: one for substrate docking and a much smaller one (function unknown) above Zn1. In addition, Zn1 possesses an unusual pentacoordinate geometry with a loosely bound carbonyl ligand (Ala333). Extensive DFT calculations on a model of the active site provide first mechanistic implications for these structural features. The weakly bound carbonyl ligand is capable of functioning as a "traffic cop" to direct water molecules coming from the small channel into the heart of the active site. A water sluice is thus generated that is capable of repeatedly providing a series of nucleophilic active "Zn-OH" functionalities.

Characterization of a TET-like Aminopeptidase Complex from the Hyperthermophilic Archaeon Pyrococcus horikoshii

Pyrococcus horikoshii open reading frame PH1527 encodes a 39014 Da protein that shares about 30% identity with endoglucanases and members of the M42 peptidase family. Analytical ultracentrifugation and electron microscopy studies showed that the purified recombinant protein forms stable, large dodecameric complexes with a tetrahedral shape similar to the one described for DAP, a deblocking aminopeptidase that was characterized in the same organism. The two related proteins were named PhTET1 (for DAP) and PhTET2 (for PH1527). The substrate specificity and the mode of action of the PhTET2 complex were studied in detail and compared to those of PhTET1 and other assigned M42 peptidases. When assayed with short chromogenic peptides, PhTET2 was found to be an aminopeptidase, with a clear preference for leucine as the N-terminal amino acid. However, the enzyme can cleave moderately long polypeptide substrates of various compositions in a fairly unspecific manner. The hydrolytic mechanism was found to be nonprocessive. The enzyme has neither carboxypeptidase nor endoproteolytic activities, and it is devoid of N-terminal deblocking activity. PhTET2 was inhibited in the presence of EDTA and bestatin, and cobalt was found to be an activating metal. The PhTET2 protein is a highly thermostable enzyme that displays optimal activity around 100 degrees C over a broad pH array.

Inactivation of Bacillus stearothermophilus leucine aminopeptidase II by hydrogen peroxide and site-directed mutagenesis of methionine residues on the enzyme

Leucine aminopeptidases (LAPs) are exopeptidases that remove the N-terminal L-leucine from peptide substrates. Oxidative stability assay showed that the recombinant Bacillus stearothermophilus LAP II (rLAPII) was sensitive to oxidative damage by hydrogen peroxide at the elevated temperature. The H2O2-treated enzyme experienced obvious changes in the secondary structure when the oxidant concentration increased to 300 mM. To investigate the role of methionine residues on the oxidative inactivation, each of the five methionine residues in the rLAPII was replaced with leucine by site-directed mutagenesis. The mutant enzymes with an apparent Mr of approximately 44.5 kDa were overexpressed in Escherichia coli and were purified to homogeneity by nickel-chelate chromatography. The specific activities for Met82Leu, Met88Leu, Met254Leu, and Met382Leu were similar to that of the wild-type enzyme, whereas a reduced activity was observed in Met136Leu. The 50% decrease in the catalytic efficiency (kcat/Km) for Met136Leu was caused by 47% decrease in kcat value. As compared with the wild-type enzyme, all mutant proteins were more sensitive to the oxidant, implying that the methionine residues of B. stearothermophilus LAP II are important for the protection of the enzyme from oxidative inactivation.

Origins of the Activity of PAL and LAP Enzyme Inhibitors: Toward Ab Initio Binding Affinity Prediction

Interaction energies of phenylalanine ammonia-lyase (PAL) active site residues with a series of PAL inhibitors have been partitioned into electrostatic, exchange, delocalization, and correlation components and compared with analogous results obtained previously for leucine aminopeptidase (LAP). In the latter metalloenzyme, either of the two charged residues controls entirely relative inhibitor binding energies, while at least four residues are required to determine ligand relative stabilization in neutral PAL. Significant correlation with experimental inhibitory activity was found between the stabilization energy at gradually decreasing levels of theory (MP2, SCF) down to the first-order Heitler-London term. Contrary to the LAP case, where the electrostatic term was sufficient to reproduce experimentally observed trends, in the case of PAL, exchange repulsion effects also have to be considered. Computational protocol presented herein constitutes a promising way to incorporate the first principle calculation's accuracy into the process of rational binding affinity prediction, revealing the physical nature of the interactions, where successive approximations can be introduced in a systematic and justifiable manner.

Pathway for Degradation of Peptides Generated by Proteasomes

The degradation of cellular proteins by proteasomes generates peptides 2-24 residues long, which are hydrolyzed rapidly to amino acids. To define the final steps in this pathway and the responsible peptidases, we fractionated by size the peptides generated by proteasomes from beta-[14C]casein and studied in HeLa cell extracts the degradation of the 9-17 residue fraction and also of synthetic deca- and dodecapeptide libraries, because peptides of this size serve as precursors to MHC class I antigenic peptides. Their hydrolysis was followed by measuring the generation of smaller peptides or of new amino groups using fluorescamine. The 14C-labeled peptides released by 20 S proteasomes could not be degraded further by proteasomes. However, their degradation in the extracts and that of the peptide libraries was completely blocked by o-phenanthroline and thus required metallopeptidases. One such endopeptidase, thimet oligopeptidase (TOP), which was recently shown to degrade many antigenic precursors in the cytosol, was found to play a major role in degrading proteasome products. Inhibition or immunodepletion of TOP decreased their degradation and that of the peptide libraries by 30-50%. Pure TOP failed to degrade proteasome products 18-24 residues long but degraded the 9-17 residue fraction to peptides of 6-9 residues. When aminopeptidases in the cell extract were inhibited with bestatin, the 9-17 residue proteasome products were also converted to peptides of 6-9 residues, instead of smaller products. Accordingly, the cytosolic aminopeptidase, leucine aminopeptidase, could not degrade the 9-17 residue fraction but hydrolyzed the peptides generated by TOP to smaller products, recapitulating the process in cell extracts. Inactivation of both TOP and aminopeptidases blocked the degradation of proteasome products and peptide libraries nearly completely. Thus, degradation of most 9-17 residue proteasome products is initiated by endoproteolytic cleavages, primarily by TOP, and the resulting 6-9 residue fragments are further digested to amino acids by aminopeptidases.

Function of the signal peptide and N- and C-terminal propeptides in the leucine aminopeptidase from Aeromonas proteolytica

The leucine aminopeptidase from Aeromonas proteolytica (also known as Vibrio proteolyticus) (AAP) is a metalloenzyme with broad substrate specificity. The open reading frame (ORF) for AAP encodes a 54 kDa enzyme, however, the extracellular enzyme has a molecular weight of 43 kDa. This form of AAP is further processed to a mature, thermostable 32 kDa form but the exact nature of this process is unknown. Over-expression of different forms of AAP in Escherichia coli (with AAP's native leader sequence, with and without the N- and/or C-terminal propeptides, and as fusion protein) has allowed a model for the processing of wild-type AAP to be proposed. The role of the A. proteolytica signal peptide in protein secretion as well as comparison to other known signal peptides reveals a close resemblance of the A. proteolytica signal peptide to the outer membrane protein (OmpA) signal peptide. Over-expression of the full 54 kDa AAP enzyme provides an enzyme that is significantly less active, due to a cooperative inhibitory interaction between both propeptides. Over-expression of AAP lacking its C-terminal propeptide provided an enzyme with an identical kcat value to wild-type AAP but exhibited a larger Km value, suggesting competitive inhibition of AAP by the N-terminal propeptide (Ki approximately 0.13 nM). The recombinant 32 kDa form of AAP was characterized by kinetic and spectroscopic methods and was shown to be identical to mature, wild-type AAP. Therefore, the ease of purification and processing of rAAP along with the fact that large quantities can be obtained now allow new detailed mechanistic studies to be performed on AAP through site-directed mutagenesis.

Identification of the catalytic residues in the double-zinc aminopeptidase from Streptomyces griseus

The aminopeptidase from Streptomyces griseus (SGAP) has been cloned and expressed in Escherichia coli. By growing the cells in the presence of 1 M sorbitol at 18 degrees C, the protein was obtained in a soluble and active form. The amino acid sequence of the recombinant SGAP contained four amino acids differing from the previously published sequence. Re-sequencing of the native protein indicated that asparagines 70 and 184 are in fact aspartic acids as in the recombinant protein. Based on the crystal structure of SGAP, Glu131 and Tyr246 were proposed to be the catalytic residues. Replacements of Glu131 resulted in loss of activity of 4-5 orders of magnitude, consistent with Glu131 acting as the general base residue. Mutations in Tyr246 resulted in about 100-fold reduction of activity, suggesting that this residue is involved in the stabilization of the transition state intermediate.

The Catalytic Role of Glutamate 151 in the Leucine Aminopeptidase from Aeromonas proteolytica

Glutamate 151 has been proposed to act as the general acid/base during the peptide hydrolysis reaction catalyzed by the co-catalytic metallohydrolase from Aeromonas proteolytica (AAP). However, to date, no direct evidence has been reported for the role of Glu-151 during catalytic turnover by AAP. In order to elucidate the catalytic role of Glu-151, altered AAP enzymes have been prepared in which Glu-151 has been substituted with a glutamine, an alanine, and an aspartate. The Michaelis constant (K(m)) does not change upon substitution to aspartate or glutamine, but the rate of the reaction changes drastically in the following order: glutamate (100% activity), aspartate (0.05%), glutamine (0.004%), and alanine (0%). Examination of the pH dependence of the kinetic constants k(cat) and K(m) revealed a change in the pK(a) of a group that ionizes at pH 4.8 in recombinant leucine aminopeptidase (rAAP) to 4.2 for E151D-AAP. The remaining pK(a) values at 5.2, 7.5, and 9.9 do not change. Proton inventory studies indicate that one proton is transferred in the rate-limiting step of the reaction at pH 10.50 for both rAAP and E151D-AAP, but at pH 6.50 two protons and general solvation effects are responsible for the observed effects in the reaction catalyzed by rAAP and E151D-AAP, respectively. Based on these data, Glu-151 is intrinsically involved in the peptide hydrolysis reaction catalyzed by AAP and can be assigned the role of a general acid and base.

New role for leucyl aminopeptidase in glutathione turnover

A manganese-dependent cysteinyl-glycine hydrolysing activity has been purified to electrophoretic homogeneity from bovine lens. The characterization of the purified enzyme (molecular mass of the native protein, molecular mass of the subunit and extensive primary structure analysis) allowed the unequivocal attribution of the cysteinyl-glycine hydrolysing activity, which is usually associated with alanyl aminopeptidase (EC 3.4.11.2) or membrane-bound dipeptidase (EC 3.4.13.19), to LAP (leucyl aminopeptidase; EC 3.4.11.1). Analysis of the pH dependence of Cys-Gly hydrolysis catalysed by LAP, supported by a molecular modelling approach to the enzyme-substrate conformation, gave insights into the ability of the enzyme to recognize Cys-Gly as a substrate. Due to the effectiveness of LAP in hydrolysing Cys-Gly (K(m)=0.57 mM, kcat=6.0x10(3) min(-1) at pH 7.4 and 25 degrees C) with respect to other dipeptide substrates, a new role for this enzyme in glutathione turnover is proposed.

Stereoselective synthesis of 1-aminoalkanephosphonic acids with two chiral centers and their activity towards leucine aminopeptidase

The stereoselective synthesis of 1-amino-2-alkylalkanephosphonic acids, namely, compounds bearing two chiral centers, was achieved by the condensation of hypophosphorous acid salts of (R)(+) or (S)(-)-N-alpha-methylbenzylamine with the appropriate aldehydes in isopropanol. Simultaneous deprotection and oxidation by the action of bromine water provided equimolar mixtures of the RS:RR and SR:SS diastereomers of desired acids. They appeared to act as moderate inhibitors of kidney leucine aminopeptidase with potency dependent on the absolute configuration of both centers of chirality.

Generating oxidation-resistant variants of Bacillus kaustophilus leucine aminopeptidase by substitution of the critical methionine residues with leucine

Bacillus kaustophilus leucine aminopeptidase (bkLAP) was sensitive to oxidative damage by hydrogen peroxide. To improve its oxidative stability, the oxidation-sensitive methionine residues in the enzyme were replaced with leucine by site-directed mutagenesis. The variants, each with an apparent molecular mass of approximately 54 kDa, were overexpressed in recombinant Escherichia coli M15 cells and purified to homogeneity by nickel-chelate chromatography. The specific activity for M282L, M285L, M289L and M321L decreased by more than 43%, while M400L, M426L, M445L, and M485L showed 191, 79, 313, and 103%, respectively, higher activity than the wild-type enzyme. Although the mutations did not cause significant changes in the K(m) value, more than 67.8% increase in the value of k(cat)/K(m) was observed in the M400L, M426L, M445L and M485L. In the presence of 50 mM H2O2, most variants were more stable with respect to the wild-type enzyme, indicating that the oxidative stability of the enzyme can be improved by engineering the methionine residues.

A study of the effect on nucleophilic hydrolytic activity of pancreatic elastase, trypsin, chymotrypsin, and leucine aminopeptidase by boronic acids in the presence of arabinogalactan: a subsequent study on the hydrolytic activity of chymotrypsin by boronic acids in the presence of mono-, di-, and trisaccharides

The hydrolytic activity of trypsin, chymotrypsin, elastase, and leucine aminopeptidase, is inhibited by different boronic acids. However, all the enzymes are inhibited by the compound CbzAla(boro)Gly(OH)(2). Therefore, these additives can control the nucleophilic hydrolytic activity of these enzymes.

The most potent organophosphorus inhibitors of leucine aminopeptidase. Structure-based design, chemistry, and activity

A new class of very potent inhibitors of cytosol leucine aminopeptidase (LAP), a member of the metalloprotease family, is described. The X-ray structure of bovine lens leucine aminopeptidase complexed with the phosphonic acid analogue of leucine (LeuP) was used for structure-based design of novel LAP inhibitors and for the analysis of their interactions with the enzyme binding site. The inhibitors were designed by modification of phosphonic group in the LeuP structure toward finding the substituents bound at the S' side of the enzyme. This resulted in two classes of compounds, the phosphonamidate and phosphinate dipeptide analogues, which were synthesized and evaluated as inhibitors of the enzyme. The in vitro kinetic studies for the phosphinate dipeptide analogues revealed that these compounds belong to the group of the most effective LAP inhibitors found so far. Their further modification at the P1 position resulted in more active inhibitors, hPheP[CH(2)]Phe and hPheP[CH(2)]Tyr (K(i) values 66 nM and 67 nM, respectively, for the mixture of four diastereomers). The binding affinities of these inhibitors toward the enzyme are the highest, if considering all compounds containing a phosphorus atom that mimic the transition state of the reaction catalyzed by LAP. To evaluate selectivity of the designed LAP inhibitors, additional tests toward aminopeptidase N (APN) were performed. The key feature, which determines their selectivity, is structure at the P1' position. Aromatic and aliphatic substituents placed at this position strongly interact with the LAP S1' binding pocket, while a significant increase in binding affinity toward APN was observed for compounds containing aromatic versus leucine side chains at the P1' position. The most selective inhibitor, hPheP[CH(2)]Leu, binds to LAP with 15 times higher affinity than to APN. One of the studied compounds, hPheP[CH(2)]Tyr, appeared to be very potent inhibitor of APN (K(i) = 36 nM for the mixture of four diastereomers). The most promising LAP inhibitors designed by computer-aided approach, the phosphonamidate dipeptide analogues, were unstable at pH below 12, because of the P-N bond decomposition, which excluded the possibility of determination of their binding affinities toward LAP.

Partial purification and characterization of midgut leucyl aminopeptidase of Morimus funereus (Coleoptera: Cerambycidae) larvae

Exopeptidases of Morimus funereus larvae were partially purified and characterized. Specific leucyl aminopeptidase (LAP) activity was increased eight-fold by gel filtration of the crude midgut extract. The partially purified LAP had a molecular mass greater than 100 kDa with pH optima from 7.0-9.0 and no strict substrate specificity. M. funereus LAP preferentially hydrolyzed p-nitroanilides with hydrophobic amino acids in the active site, with a K(m) for leucine-p-nitroanilide of 0.21 mM. Zymogram analysis of an electropherogram obtained by native polyacrylamide gel electrophoresis revealed four enzymatically active proteinases using leucine-p-nitroanilide and methionine-p-nitroanilide as substrates and two enzymatically active proteinases using lysine-p-nitroanilide as a substrate. Although the optimal temperature of LAP activity was 40 degrees C, the enzyme was active over a broad temperature range from 2 to 60 degrees C. Among a number of inhibitors tested, heavy metals and 1,10-phenanthroline completely inhibited the enzyme, while methanol, ethanol and EGTA stimulated somewhat LAP activity.

An IFN-gamma-induced aminopeptidase in the ER, ERAP1, trims precursors to MHC class I-presented peptides

Precursors to major histocompatibility complex (MHC) class I-presented peptides with extra NH2-terminal residues can be efficiently trimmed to mature epitopes in the endoplasmic reticulum (ER). Here, we purified from liver microsomes a lumenal, soluble aminopeptidase that removes NH2-terminal residues from many antigenic precursors. It was identified as a metallopeptidase named "adipocyte-derived leucine" or "puromycin-insensitive leucine-specific" aminopeptidase. However, because we localized it to the ER, we propose it be renamed ER-aminopeptidase 1 (ERAP1). ERAP1 is inhibited by agents that block precursor trimming in ER vesicles and although it trimmed NH2-extended precursors, it spared presented peptides of 8 amino acid and less. Like other proteins involved in antigen presentation, ERAP1 is induced by interferon-gamma. When overexpressed in vivo, we found that ERAP1 stimulates the processing and presentation of an antigenic precursor in the ER.

The ER aminopeptidase ERAP1 enhances or limits antigen presentation by trimming epitopes to 8-9 residues

Endoplasmic reticulum (ER) aminopeptidase 1 (ERAP1) appears to be specialized to produce peptides presented on class I major histocompatibility complex molecules. We found that purified ERAP1 trimmed peptides that were ten residues or longer, but spared eight-residue peptides. In vivo, ERAP1 enhanced production of an eight-residue ovalbumin epitope from precursors extended on the NH2 terminus that were generated either in the ER or cytosol. Purified ERAP1 also trimmed nearly half the nine-residue peptides tested. By destroying such nine-residue peptides in normal human cells, ERAP1 reduced the overall supply of antigenic peptides. However, after interferon-gamma treatment, which causes proteasomes to produce more NH2-extended antigenic precursors, ERAP1 increased the supply of peptides for MHC class I antigen presentation.

Purification, characterization, and genetic analysis of a leucine aminopeptidase from Aspergillus sojae

Extracellular leucine aminopeptidase (LAP) from Aspergillus sojae was purified to protein homogeneity by sequential fast protein liquid chromatography steps. LAP had an apparent molecular mass of 37 kDa, of which approximately 3% was contributed by N-glycosylated carbohydrate. The purified enzyme was most active at pH 9 and 70 degrees C for 30 min. The enzyme preferentially hydrolyzed leucine p-nitroanilide followed by Phe, Lys, and Arg derivatives. The LAP activity was strongly inhibited by metal-chelating agents, and was largely restored by divalent cations like Zn(2+) and Co(2+). The lap gene and its corresponding cDNA fragment of the A. sojae were cloned using degenerated primers derived from internal amino acid sequences of the purified enzyme. lap is interrupted by three introns and is transcribed in a 1.3-kb mRNA that encodes a 377-amino-acid protein with a calculated molecular mass of 41.061 kDa. The mature LAP is preceded by a leader peptide of 77 amino acids, predicted to include an 18-amino-acid signal peptide and an extra sequence of 59 amino acids. Two putative N-glycosylation sites are identified in Asn-87 and Asn-288. Southern blot analysis suggested that lap is a single-copy gene in the A. sojae genome. The deduced amino acid sequence of A. sojae LAP shares only 11-33.1% identity with those of LAPs from 18 organisms.