Design, synthesis and catalytic activity of a serine protease synthetic model

The design, synthesis and catalytic properties of acyclic branched peptide carrier that possesses thecatalytic triad residues of the serine proteases isreported. The synthesis of the peptide model wastotally completed on solid support using threedifferent orthogonal amino protecting groups.Hydrolytic activity measurements againstSuc-Ala-Ala-Ala-pNA substrate showed that it ishydrolysed by the peptide model to a small extent.Despite this small hydrolytic activity, it is thefirst time, to our knowledge, that hydrolysis of such a substrate is reported by an enzyme model compound.Contrary, this enzyme model peptide showedconsiderable activity against the Boc-Ala-pNPsubstrate (k_cat = 0.414 min^–1 and K_m = 0.228 mm). These results suggest that thedesigned carrier brings in appropriate contact thecatalytic triad residues (Ser, His, Asp) resulting inthe obtained hydrolytic activity.     

Design, synthesis and catalytic activity of a serine protease synthetic model

Summary The design, synthesis and catalytic properties of a cyclic branched peptide carrier that possesses the catalytic triad residues of the serine proteases is reported. The synthesis of the peptide model was totally completed on solid support using three different orthogonal amino protecting groups. Hydrolytic activity measurements against Suc-Ala-Ala-Ala-pNA substrate showed that it is hydrolysed by the peptide model to a small extent. Despite this small hydrolytic activity, it is the first time, to our knowledge, that hydrolysis of such a substrate is reported by an enzyme model compound. Contrary, this enzyme model peptide showed considerable activity against the Boc-Ala-pNP substrate (k _cat =0.414 min⁻¹ andK _m =0.228 mm). These results suggest that the designed carrier brings in appropriate contact the catalytic triad residues (Ser, His, Asp) resulting in the obtained hydrolytic activity.     

Preparation of cyclic peptide libraries using intramolecular oxime formation.

A new method for the synthesis of cyclic head-to-side chain peptide libraries has been developed in which the key cyclization step involves reaction between a C-terminal ketone and an N-terminal hydroxylamine to form a macrocyclic oxime. This methodology efficiently delivers cyclic products that consist of mixtures of syn and anti isomers.     

Identification of tubulins as substrates of serine protease HtrA1 by mixture‐based oriented peptide library screening

Serine protease HtrA1 belongs to a family of chymotrypsin‐like proteases that were first identified in bacteria and later in mammalian systems. These proteases were identified as components of protein quality control in prokaryotic systems and as regulators of diverse signaling pathways in mammalian systems. In particular, HtrA1 is implicated in trophoblast cell migration and invasion, tumor progression, chemotherapy‐induced cytotoxicity, osteoarthritis, age‐related macular degeneration, and pathogenesis of Alzheimer's disease. However, systematic analysis of its potential substrates in biological system is still lacking. Therefore, we performed a mixture‐based oriented peptide library screening to identify putative substrates of HtrA1. We identified [AEGR]‐[LAGR]‐[IAMLR]‐[TVIAL] as consensus residues for P1 to P4 sites. We identified several putative substrates of HtrA1 involved in the pathogenesis of various diseases. In this study, we report on the identification of tubulins as potential substrates of HtrA1, and validated tubulins as in vitro and intracellular substrates of HtrA1. These results provide initial insights into substrate identification and functional characterization of HtrA1 in pathogenesis of various diseases. J. Cell. Biochem. 107: 253–263, 2009. © 2009 Wiley‐Liss, Inc.     

Synthesis of a cyclic peptide library based on the somatostatin sequence using the backbone amide linker approach

Herein, we report on the synthesis of a library of cyclic peptides targeted at the somatostatin receptor using the backbone amide linker strategy. After optimising head-to-tail cyclisation and cleavage conditions, a library of discrete cyclic peptides was assembled in high purity and good overall yield.     

Synthesis of a cyclic peptide library based on the somatostatin sequence using the backbone amide linker approach

Summary Herein, we report on the synthesis of a library of cyclic peptides targeted at the somatostatin receptor using the backbone amide linker strategy. After optimising head-to-tail cyclisation and cleavage conditions, a library of discrete cyclic peptides was assembled in high purity and good overall yield.     

Synthesis of cyclic peptides and peptide libraries on a new disulfide linker.

A new cysteine-based disulfide linker for Fmoc solid phase peptide synthesis was developed (Fmoc-Cys(3-mercapto-3-methylbutanoic acid)OPp) that allows the on-resin assembly and side chain deprotection of cyclic peptides. Model peptides and a cyclic peptide library of the structure [a-a-x-x-a-a-c] composed of D-amino acids were assembled and the synthesis and cleavage conditions studied. The best cyclization results were obtained with PyBOP/HOAt/diisopropylethyl amine. Racemization rates of the cysteine in the analyzed model sequences were between 5.2 and 12.3%. Cleavage of the disulfide bond was best carried out with DTT in 50% 2-propanol/100 mM ammonium bicarbonate. The cleaved peptides can be used directly in biological assays.     

Design, synthesis and pharmacological evaluation of cyclic mimetics of the insulin-like peptide 3 (INSL3) B-chain.

Insulin-like peptide 3 (INSL3) is a peptide hormone belonging to the relaxin-insulin superfamily of peptides that plays important roles in testes descent, oocyte maturation and the control of male germ cell apoptosis. These actions are mediated via a specific G-protein coupled receptor, LGR8. Previous structure-activity studies have shown that the key binding site of INSL3 is situated within its B-chain. Recent studies in our laboratory have led to the identification of a cyclic peptide mimetic 2 of the INSL3 B-chain, which we have shown to compete with the binding of [33P]-relaxin to LGR8 expressed in HEK293T cells, and to inhibit cAMP-mediated signaling in these cells, i.e. it is an antagonist of INSL3. In order to further define the structure-activity relationships of cyclic analogues of the INSL3 B-chain, we used a structure-based approach to design a series of cyclic, disulfide-constrained INSL3 B-chain mimetics. To do this, we first created a model of the 3D structure of INSL3 using the crystal structure of human relaxin as a template. This model of INSL3 was then used as a template to design a series of disulfide-constrained mimetics of the INSL3 B-chain. The peptides were synthesized by solid-phase peptide synthesis using pseudoproline dipeptides to improve the synthesis outcome. Of the seven prepared INSL3 B-chain mimetics, three compounds were found to have partial displacement activity, while four were able to completely displace [33P]-relaxin from LGR8, including compounds that were markedly shorter than compound 2. The best of these, mimetic 6, showed significantly greater affinity for LGR8 than compound 2, but still displayed around 1000-fold less affinity for LGR8 than native INSL3. Analysis of selected mimetics for their alpha-helical content using circular dichroism (CD) spectroscopy revealed that, generally, the mimetics showed less than expected helicity. The inability of the compounds to display true native INSL3 structure is likely contributing to their reduced receptor binding affinity. We are currently examining alternative INSL3 B-chain mimetics that might better present key receptor binding residues in the native INSL3-like conformation.     

Examination of methodology for the synthesis of cyclic thioether peptide libraries derived from linear tripeptides.

Work was undertaken to examine methodology for the cyclization of linear tripeptides on the solid phase via intramolecular S-alkylation using the Multipin(trade mark) Solid-Phase Peptide Synthesis platform. While previous work had shown that this chemistry could be used to efficiently cyclize linear tetrapeptide libraries, application of this synthetic strategy to the model linear tripeptide sequence Leu-Ser-Lys resulted in significant cyclic dimer formation. Ultimately, it was found that the addition of a large excess of lithium in the form of LiCl to the cyclization solution, significantly reduced cyclic dimer formation affording highly pure crude cyclic monomer. The application of this modified cyclization protocol to the preparation of cyclic peptide libraries was successfully demonstrated with the synthesis of a 20-membered library 4{1-20} based on the linear tripeptide sequence Leu-Xxx-Lys in which the position Xxx was varied with the standard 20 proteogenic residues.     

Quantitative analysis of cyclic beta-turn models.

The beta-turn is a frequently found structural unit in the conformation of globular proteins. Although the circular dichroism (CD) spectra of the alpha-helix and beta-pleated sheet are well defined, there remains some ambiguity concerning the pure component CD spectra of the different types of beta-turns. Recently, it has been reported (Hollósi, M., Kövér, K.E., Holly, S., Radics, L., & Fasman, G.D., 1987, Biopolymers 26, 1527-1572; Perczel, A., Hollósi, M., Foxman, B.M., & Fasman, G.D., 1991a, J. Am. Chem. Soc. 113, 9772-9784) that some pseudohexapeptides (e.g., the cyclo[(delta)Ava-Gly-Pro-Aaa-Gly] where Aaa = Ser, Ser(OtBu), or Gly) in many solvents adopt a conformational mixture of type I and the type II beta-turns, although the X-ray-determined conformation was an ideal type I beta-turn. In addition to these pseudohexapeptides, conformational analysis was also carried out on three pseudotetrapeptides and three pseudooctapeptides. The target of the conformation analysis reported herein was to determine whether the ring stress of the above beta-turn models has an influence on their conformational properties. Quantitative nuclear Overhauser effect (NOE) measurements yielded interproton distances. The conformational average distances so obtained were interpreted utilizing molecular dynamics (MD) simulations to yield the conformational percentages. These conformational ratios were correlated with the conformational weights obtained by quantitative CD analysis of the same compounds. The pure component CD curves of type I and type II beta-turns were also obtained, using a recently developed algorithm (Perczel, A., Tusnády, G., Hollósi, M., & Fasman, G.D., 1991b, Protein Eng. 4(6), 669-679). For the first time the results of a CD deconvolution, based on the CD spectra of 14 beta-turn models, were assigned by quantitative NOE results. The NOE experiments confirmed the ratios of the component curves found for the two major beta-turns by CD analysis. These results can now be used to enhance the conformational determination of globular proteins on the basis of their CD spectra.     

Efficient methodology for the cyclization of linear peptide libraries via intramolecular S-alkylation using Multipin solid phase peptide synthesis.

Methodology is described here for the efficient parallel synthesis and cyclization of linear peptide libraries using intramolecular S-alkylation chemistry in combination with Multipin solid phase peptide synthesis (Multipin SPPS). The effective use of this methodology was demonstrated with the synthesis of a 72-member combinatorial library of cyclic thioether peptide derivatives of the conserved four-residue structural motif DD/EXK found in the active sites of the five crystallographically defined orthodox type II restriction endonucleases, EcoRV, EcoRI, PvuII, BamHI and BglI.     

Computational aided mechanistic understanding of Camellia sinensis bioactive compounds against co-chaperone p23 as potential anticancer agent

Co-chaperon p23 has been well established as molecular chaperon for the heat shock protein 90 (Hsp90) that further leads to immorality in cancer cells by providing defense against Hsp90 inhibitors, and as stimulating agent for generating overexpressed antiapoptotic proteins, that is, Hsp70 and Hsp27. The natural compounds such as catechins from Camellia sinensis (green tea) are also well known for inhibition activity against various cancer. However, molecular interaction profile and potential lead bioactive compounds against co-chaperon p23 from green tea are not yet reported. To this context, we study the various secondary metabolites of green tea against co-chaperon p23 using structure-based virtual screening from Traditional Chinese Medicine (TCM) database. Following 26 compounds were obtained from TCM database and further studied for extra precision molecular docking that showed binding score between −10.221 and −2.276 kcal/mol with co-chaperon p23. However, relative docking score to known inhibitors, that is, ailanthone (−4.54 kcal/mol) and gedunin ( 3.60 kcal/mol) along with ADME profile analysis concluded epicatechin (−7.013 kcal/mol) and cis-theaspirone (−4.495 kcal/mol) as potential lead inhibitors from green tea against co-chaperone p23. Furthermore, molecular dynamics simulation and molecular mechanics generalized born surface area calculations validated that epicatechin and cis-theaspirone have significantly occupied the active region of co-chaperone p23 by hydrogen and hydrophobic interactions with various residues including most substantial amino acids, that is, Thr90, Ala94, and Lys95. Hence, these results supported the fact that green tea contained potential compounds with an ability to inhibit the cancer by disrupting the co-chaperon p23 activity.     

Computational screening,ensemble docking and pharmacophore analysis of potential gefitinib analogues against epidermal growth factor receptor

The observable mutated isoforms of epidermal growth factor receptor (EGFR) are important considerable therapeutic benchmarks in moderating the non-small cell lung cancer (NSCLC). Recently, quinazoline-based ATP competitive inhibitors have been developed against the EGFR; however, these imply the mutation probabilities, which contribute to the discovery of high probable novel inhibitors for EGFR mutants. Therefore, SAR-based bioactivity analysis, molecular docking and computational toxicogenomics approaches were performed to identify and evaluate new analogs of gefitinib against the ligand-binding domain of the EGFR double-mutated model. From the diverse groups of molecular clustering and molecular screening strategies, top high-binding gefitinib-analogues were identified and studied against EGFR core cavity through three-phase ensemble docking approach. Resulted high possible leads showed good binding orientations than gefitinib (positive control) thus they were subjected to pharmacophore analysis that possesses possible molecular assets to tight binding with EGFR domain. Residues Ser720, Arg841 and Trp880 were observed as novel hot spots and involved in H-bonds, pi-stacking and π-cation interactions that contribute additional electrostatic potency to sustain stability and complexity of protein-ligand complexes, thus they have ability to profoundly adopted by pharmacophoric features. Furthermore, lead molecules have an inhibition percent probability, anticancer potency, toxic impacts, flexible pharmacokinetics, potential gene-chemical interactions towards EGFR were revealed by computational systems biology tools. Our multiple screening strategies confirmed that the druggable sub-pocket was crucial to strong EGFR-ligand binding. The essential pharmacophoric features of ligands provided viewpoints for new inhibitors envisaging, and predicted scaffolds could used as anticancer agents against selected EGFR mutated isoforms.     

Screening of commercial cyclic peptide as inhibitor NS5 methyltransferase of Dengue virus through Molecular Docking and Molecular Dynamics Simulation

Dengue has become a major global health threat, especially in tropical and subtropical regions. The development of antiviral agent targeting viral replication is really needed at this time. NS5 methyltransferase presents as a novel antiviral target. This enzyme plays an important role in the methylation of 5''-cap mRNA. Inhibition of the NS5 methyltransferase could inhibit dengue virus replication. In this research, two sites of NS5 methyltransferase (S-Adenosyl methionine/SAM binding site and RNA-cap site) were used as targets for inhibition. As much as 300 commercial cyclic peptides were screened to these target sites by means of molecular docking. Analysis of ligand-enzyme binding free energy and pharmacological prediction revealed two best ligands, namely [Tyr123] Prepro Endothelin (110-130), amide, human and Urotensin II, human. According to molecular dynamic simulation, both ligands maintain a stable complex conformation between enzyme and ligand at temperature 310 K and 312 K. Hence, Urotensin II, human is more reactive at 312 K than at 310 K. However, both ligands can be used as potential inhibitor candidates against NS5 methyltransferase of dengue virus with Urotensin II, human exposes more promising activity at 312 K.     

Synthesis of cyclic herpes simplex virus peptides containing 281–284 epitope of glycoprotein D‐1 in endo‐ or exo‐position

We have prepared two types of cyclopeptides containing the ²⁸¹DPVG²⁸⁴ sequence from the 276–284 region of glycoprotein gD‐1 of the Herpes simplex virus (HSV). The syntheses were performed by solid phase methodology using MBHA or BHA resin and orthogonal protection schemes. Head‐to‐side‐chain cyclization included the N‐terminal part of the epitope, while side‐chain‐to‐side‐chain lactam bridge formation resulted in a peptide containing a C‐terminal cycle. Peptides elongated by Cys at the N‐terminal of the sequence were also prepared. Boc chemistry using Fmoc and OFm orthogonal protection was applied for on‐resin cyclization. Based on the orthogonality of Bzl and cHex esters under a 1 m TMSOTf‐thioanisole/TFA cleavage condition, a new approach for the cyclization on BHA‐resin has also been developed. Preliminary studies on solution conformation of the cyclic peptides by CD spectroscopy indicated the importance of the location and the size of the cycle within the epitope sequence. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.     

A 4.2 kDa synthetic peptide as a potential probe to evaluate the antibacterial activity of coumarin drugs.

The coumarin antibiotics are potent inhibitors of DNA replication whose target is the enzyme DNA gyrase, an ATP-dependent bacterial type II topoisomerase. The coumarin drugs inhibit gyrase action by competitive binding to the ATP-binding site of DNA gyrase B protein. The production of new biologically active products has stimulated additional studies on coumarin-gyrase interactions. In this regard, a 4.2 kDa peptide mimic of DNA gyrase B protein from Escherichia coli has been designed and synthesized. The peptide sequence includes the natural fragment 131-146 (coumarin resistance-determining region) and a segment containing the gyrase-DNA interaction region (positions 753-770). The peptide mimic binds to novobiocin (Ka = 1.4+/-0.3 x 10(5) M(-1)), plasmid (Ka = 1.6+/-0.5 x 10(6) M(-1)) and ATP (Ka = 1.9+/-50.4 x 10(3) M(-1)), results previously found with the intact B protein. On the other hand, the binding to novobiocin was reduced when a mutation of Arg-136 to Leu-136 was introduced, a change previously found in the DNA gyrase B protein from several coumarin-resistant clinical isolates of Escherichia coli In contrast, the binding to plasmid and to ATP was not altered. These results suggest that synthetic peptides designed in a similar way to that described here could be used as mimics of DNA gyrase in studies which seek a better understanding of the ATP, as well as coumarin, binding to the gyrase and also the mechanism of action of this class of antibacterial drugs.     

Molecular docking based virtual screening of natural compounds as potential BACE1 inhibitors: 3D QSAR pharmacophore mapping and molecular dynamics analysis

Beta-site APP cleaving enzyme1 (BACE1) catalyzes the rate determining step in the generation of Aβ peptide and is widely considered as a potential therapeutic drug target for Alzheimer’s disease (AD). Active site of BACE1 contains catalytic aspartic (Asp) dyad and flap. Asp dyad cleaves the substrate amyloid precursor protein with the help of flap. Currently, there are no marketed drugs available against BACE1 and existing inhibitors are mostly pseudopeptide or synthetic derivatives. There is a need to search for a potent inhibitor with natural scaffold interacting with flap and Asp dyad. This study screens the natural database InterBioScreen, followed by three-dimensional (3D) QSAR pharmacophore modeling, mapping, in silico ADME/T predictions to find the potential BACE1 inhibitors. Further, molecular dynamics of selected inhibitors were performed to observe the dynamic structure of protein after ligand binding. All conformations and the residues of binding region were stable but the flap adopted a closed conformation after binding with the ligand. Bond oligosaccharide interacted with the flap as well as catalytic dyad via hydrogen bond throughout the simulation. This led to stabilize the flap in closed conformation and restricted the entry of substrate. Carbohydrates have been earlier used in the treatment of AD because of their low toxicity, high efficiency, good biocompatibility, and easy permeability through the blood–brain barrier. Our finding will be helpful in identify the potential leads to design novel BACE1 inhibitors for AD therapy.