Synthetic peptide conjugates—tailor-made probes for the biology of protein modification and protein processing

Peptide Thioester Formation via an Intramolecular N to S Acyl Shift for Peptide Ligation

In chemical protein synthesis, peptide building blocks are prepared by solid-phase peptide synthesis (SPPS), and then connected by chemical ligation methods. The peptide thioester is one of key building blocks used in chemical protein synthesis, and improvements in the Fmoc SPPS procedure for preparing such thioesters would be highly desirable. In this review we focus on a method for peptide thioester synthesis based on the use of an intramolecular N to S acyl shift reaction as a key reaction. Amide and thioester forms at the thiol-containing residue are in equilibrium as a result of a reversible intramolecular acyl shift, which is detectable by 13C NMR. The amide form is favored under neutral conditions, while the thioester predominates under acidic conditions. Thiol auxiliaries can be employed to facilitate the formation of a thioester from an amide via an intramolecular N-S acyl shift, and the peptide thioester is formed after intermolecular transthioesterification in the presence of excess amounts of thiols. Even under neutral conditions, thiol auxiliary-containing peptides can be ligated with a cysteinyl peptide via an intramolecular N-S acyl shift, followed by native chemical ligation (NCL) in a one-pot reaction. These procedures can be applied to the chemical synthesis of proteins which are post-translationally modified.

Lipidated ras and rab peptides and proteins--synthesis, structure, and function

Chemical biology can be defined as the study of biological phenomena from a chemical approach. Based on the analysis of relevant biological phenomena and their structural foundation, unsolved problems are identified and tackled through a combination of chemistry and biology. Thus, new synthetic methods and strategies are developed and employed for the construction of compounds that are used to investigate biological procedures. Solid-phase synthesis has emerged as the preferred method for the synthesis of lipidated peptides, which can be chemoselectively ligated to proteins of the Ras superfamily. The generated peptides and proteins have solved biological questions in the field of the Ras-superfamily GTPases that are not amendable to chemical or biological techniques alone.

Aryldithioethyloxycarbonyl (Ardec): A New Family of Amine Protecting Groups Removable under Mild Reducing Conditions and Their Applications to Peptide Synthesis

The development of phenyldithioethyloxycarbonyl (Phdec) and 2-pyridyldithioethyloxycarbonyl (Pydec) protecting groups, which are thiol-labile urethanes, is described. These new disulfide-based protecting groups were introduced onto the epsilon-amino group of L-lysine; the resulting amino acid derivatives were easily converted into N alpha-Fmoc building blocks suitable for both solid- and solution-phase peptide synthesis. Model dipeptide(Ardec)s were prepared by using classical peptide couplings followed by standard deprotection protocols. They were used to optimize the conditions for complete thiolytic removal of the Ardec groups both in aqueous and organic media. Phdec and Pydec were found to be cleaved within 15 to 30 min under mild reducing conditions: i) by treatment with dithiothreitol or beta-mercaptoethanol in Tris.HCl buffer (pH 8.5-9.0) for deprotection in water and ii) by treatment with beta-mercaptoethanol and 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) in N-methylpyrrolidinone for deprotection in an organic medium. Successful solid-phase synthesis of hexapeptides Ac-Lys-Asp-Glu-Val-Asp-Lys(Ardec)-NH2 has clearly demonstrated the full orthogonality of these new amino protecting groups with Fmoc and Boc protections. The utility of the Ardec orthogonal deprotection strategy for site-specific chemical modification of peptides bearing several amino groups was illustrated firstly by the preparation of a fluorogenic substrate for caspase-3 protease containing the cyanine dyes Cy 3.0 and Cy 5.0 as FRET donor/acceptor pair, and by solid-phase synthesis of an hexapeptide bearing a single biotin reporter group.

Influence of serine O-glycosylation or O-phosphorylation close to the vJun nuclear localisation sequence on nuclear import

Nuclear import triggered by the nuclear-localisation sequence (NLS) of the viral Jun (vJun) protein is mediated by phosphorylation of a serine close to the NLS. Since phosphorylation and glycosylation of serine residues are often in a reciprocal "yin-yang" relationship, we investigated whether glycosylation of this serine with O-linked N-acetylglucosamine (O-GlcNAc) would also regulate nuclear import via the vJun NLS. Peptides containing the vJun NLS with an adjacent O-phosphorylated, O-GlcNAc-functionalised or unmodified serine, and equipped with an N-terminal biotin or a 7-nitrobenz-2-oxa-1,3-diazolyl (NBD) fluorescent label, were synthesised on the solid phase by means of an Fmoc/Boc strategy and a Pd0-sensitive HYCRON linker. Fluorescence-polarisation measurements on the NBD-labelled peptides indicated that modification with phosphate or O-GlcNAc leads to a decrease in affinity to the import-mediating adapter protein, importin alpha, of about one order of magnitude compared to the unmodified NLS. Microinjection of biotinylated NLS peptide conjugated with fluorescently labelled avidin into NIH/3T3 and MDCK cells, revealed that avidin-unmodified-NLS peptide was rapidly imported into the nucleus. However, either phosphate or O-GlcNAc next to the NLS caused almost complete exclusion of the protein conjugate from nuclear import. These findings indicate that nuclear import by the vJun NLS might not be regulated by a "yin-yang" modification of an adjacent serine with phosphate or O-GlcNAc. Rather, negative regulation of binding between the polybasic NLS and importin by a negatively charged or a bulky, uncharged residue appears likely.

Aziridine-2-carboxylic Acid-Containing Peptides: Application to Solution- and Solid-Phase Convergent Site-Selective Peptide Modification

The development of a method for site- and stereoselective peptide modification using aziridine-2-carboxylic acid-containing peptides is described. A solid-phase peptide synthesis methodology that allows for the rapid generation of peptides incorporating the aziridine residue has been developed. The unique electrophilic nature of this nonproteinogenic amino acid allows for site-selective conjugation with various thiol nucleophiles, such as anomeric carbohydrate thiols, farnesyl thiol, and biochemical tags, both in solution and on solid support. This strategy, combined with native chemical ligation, provides convergent and rapid access to complex thioglycoconjugates.

One-Pot Synthesis of Sialo-Containing Glycosyl Amino Acids by Use of anN-Trichloroethoxycarbonyl-?-thiophenyl Sialoside

We describe an efficient synthesis of 2,6- and 2,3-sialyl T antigens linked to serine in a one-pot glycosylation. We first investigated the glycosidation of thiosialosides by varying the N-protecting group. Modification of the C-5 amino group of beta-thiosialosides into the N-9-fluorenylmethoxycarbonyl, N-2,2,2-trichloroethoxycarbonyl (N-Troc), and N-trichloroacetyl derivatives enhanced the reactivity of these compounds towards glycosidation. Addition of a minimum amount of 3 A molecular sieves was also effective in improving the yield of alpha-linked sialosides. Next, we conducted one-pot syntheses of the glycosyl amino acids by using the N-Troc sialyl donor. The N-Troc derivative can be converted into the N-acetyl derivative without racemization of the amino acids. Branched-type one-pot glycosylation, initiated by regioselective glycosylation of the 3,6-dihydroxy galactoside with the N-Troc-beta-thiophenyl sialoside, provided the protected 2,6-sialyl T antigen in good yield. Linear-type one-pot glycosylation, initiated by chemoselective glycosylation of galactosyl fluoride with the N-Troc-beta-thiophenyl sialoside, afforded the protected 2,3-sialyl T antigen in excellent yield. Both protected glycosyl amino acids were converted into the fully deprotected 2,6- and 2,3-sialyl T antigens linked to serine in good yields.

Site-Selective Conjugation of Thiols with Aziridine-2-Carboxylic Acid-Containing Peptides

The synthesis and convergent site-selective conjugation of aziridine-2-carboxylic acid-containing peptides with thiols, both in solution and on solid support, are described. The synthesis and use of FmocAzyOH in this capacity demonstrate both the efficient incorporation and tolerance of the Azy moiety in multistep Fmoc solid-phase peptide synthesis (SPPS), as well as the competence of solution and on-bead ligation through a highly regioselective base-promoted aziridine ring-opening process.

Induction of carbohydrate-specific antibodies in HLA-DR transgenic mice by a synthetic glycopeptide: a potential anti cancer vaccine for human use

Over the last few years, anticancer immunotherapy has emerged as a new exciting area for controlling tumors. In particular, vaccination using synthetic tumor-associated antigens (TAA), such as carbohydrate antigens hold promise for generating a specific antitumor response by targeting the immune system to cancer cells. However, development of synthetic vaccines for human use is hampered by the extreme polymorphism of human leukocyte-associated antigens (HLA). In order to stimulate a T-cell dependent anticarbohydrate response, and to bypass the HLA polymorphism of the human population, we designed and synthesized a glycopeptide vaccine containing a cluster of a carbohydrate TAA B-cell epitope (Tn antigen: alpha-GalNAc-Ser) covalently linked to peptides corresponding to the Pan DR 'universal' T-helper epitope (PADRE) and to a cytotoxic T lymphocyte (CTL) epitope from the carcinoembryonic antigen (CEA). The immunogenicity of the construct was evaluated in outbred mice as well as in HLA transgenic mice (HLA-DR1, and HLA-DR4). A strong T-cell dependent antibody response specific for the Tn antigen was elicited in both outbred and HLA transgenic mice. The antibodies induced by the glycopeptide construct efficiently recognized a human tumor cell line underlying the biological relevance of the response. The rational design and synthesis of the glycopeptide construct presented herein, together with its efficacy to induce antibodies specific for native tumor carbohydrate antigens, demonstrate the potential of a such synthetic molecule as an anticancer vaccine candidate for human use.

Phenylhydrazide as an enzyme-labile protecting group in peptide synthesis

The enzymatic cleavage of amino acid phenylhydrazides with the enzyme tyrosinase (EC 1.14.18.1) offers a new, mild, and selective method for C-terminal deprotection of peptides. The advantages of the described methodology are the very mild oxidative removal of the protecting group at room temperature and pH 7, a high chemo- and regioselectivity, and the availability of the biocatalyst. Even in oxygen-saturated solution, the oxidation of sensitive methionine residues was not observed. These features make the methodology suitable for the synthesis of sensitive peptide conjugates. Mechanistic data suggest that the hydrolysis of the oxidized adducts proceeds by a free-radical mechanism.

Novel Tn antigen-containing neoglycopeptides: synthesis and evaluation as anti tumor vaccines

The fully unprotected alpha-C-glycosyl analogue of N-acetylgalactosamine 9 was conjugated by a non-natural oxime bond to the segment peptides (328--340)OVA and (327--339)OVA, affording neoglycopeptides 1--2 and 3, having one or two sugar units, respectively. The three neoglycopeptides were tested in vitro in an antigen presentation assay as antitumor vaccines. Neoglycopeptides 1--3 could be presented to and recognized by the T cell receptor; neoglycopeptide 3, bearing two B-epitopes, was presented to the TCR with higher efficiency, compared to neoglycopeptide 2, having only one B-epitope.

Sensitive and rapid analysis of protein palmitoylation with a synthetic cell-permeable mimic of SRC oncoproteins

The localization of oncogenic Src and Ras proteins to cellular plasma membranes is critical for the proliferation of specific cancers. In addition to other lipid modifications, these proteins require posttranslational palmitoylation of specific cysteine residues by the enzyme palmitoyl acyltransferase (PAT) in order to be stably anchored at plasma membranes. Hence, the identification of inhibitors of protein palmitoylation has significant potential to define a new class of antitumor agents. However, studies of protein palmitoylation have been hindered by the dynamic and reversible nature of cysteine acylation and the lack of sensitive and convenient assays of PAT activity. To facilitate the rapid identification of compounds that affect protein palmitoylation, we report the solid-phase synthesis of a fluorescent cell-permeable palmitoyl acyltransferase substrate that mimics the N-terminus of Src family proteins. Metabolic radiolabeling and epifluorescence microscopy of Jurkat lymphocytes treated with this Src-mimetic lipopeptide revealed that this compound is palmitoylated intracellularly, which confers localization at cellular plasma membranes. Addition of the palmitoylation inhibitor 2-bromopalmitic acid to substrate-treated cells blocked palmitoylation and diminished substrate-mediated plasma membrane fluorescence. Analysis of inhibition of palmitoylation by flow cytometry revealed that this fluorescent lipopeptide substrate represents a highly sensitive molecular probe of palmitoyl acyltransferase activity that enables unprecedented high-throughput assays of protein palmitoylation.