Synthesis of peptide thioesters via an N-S acyl shift reaction under mild acidic conditions on an N-4,5-dimethoxy-2-mercaptobenzyl auxiliary group

Dissecting the role of protein phosphorylation: a chemical biology toolbox

Protein phosphorylation is a crucial regulator of protein and cellular function, yet, despite identifying an enormous number of phosphorylation sites, the role of most is still unclear. Each phosphoform, the particular combination of phosphorylations, of a protein has distinct and diverse biological consequences. Aberrant phosphorylation is implicated in the development of many diseases. To investigate their function, access to defined protein phosphoforms is essential. Materials obtained from cells often are complex mixtures. Recombinant methods can provide access to defined phosphoforms if site-specifically acting kinases are known, but the methods fail to provide homogenous material when several amino acid side chains compete for phosphorylation. Chemical and chemoenzymatic synthesis has provided an invaluable toolbox to enable access to previously unreachable phosphoforms of proteins. In this review, we selected important tools that enable access to homogeneously phosphorylated protein and discuss examples that demonstrate how they can be applied. Firstly, we discuss the synthesis of phosphopeptides and proteins through chemical and enzymatic means and their advantages and limitations. Secondly, we showcase illustrative examples that applied these tools to answer biological questions pertaining to proteins involved in signal transduction, control of transcription, neurodegenerative diseases and aggregation, apoptosis and autophagy, and transmembrane proteins. We discuss the opportunities and challenges in the field.

Acyl donors for native chemical ligation

Native chemical ligation (NCL) has become one of the most important methods in chemical syntheses of proteins. Recently, in order to expand its scope, considerable effort has been devoted to tuning the C-terminal acyl donor thioesters used in NCL. This article reviews the recent advances in the design of C-terminal acyl donors, their precursors and surrogates, and highlights some noteworthy progress that may lead the future direction of protein chemical synthesis.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Advances in Fmoc solid-phase peptide synthesis

Today, Fmoc SPPS is the method of choice for peptide synthesis. Very-high-quality Fmoc building blocks are available at low cost because of the economies of scale arising from current multiton production of therapeutic peptides by Fmoc SPPS. Many modified derivatives are commercially available as Fmoc building blocks, making synthetic access to a broad range of peptide derivatives straightforward. The number of synthetic peptides entering clinical trials has grown continuously over the last decade, and recent advances in the Fmoc SPPS technology are a response to the growing demand from medicinal chemistry and pharmacology. Improvements are being continually reported for peptide quality, synthesis time and novel synthetic targets. Topical peptide research has contributed to a continuous improvement and expansion of Fmoc SPPS applications.

Chemical synthesis of proteins using N-sulfanylethylanilide peptides, based on N-S acyl transfer chemistry

Native chemical ligation (NCL), which features the use of peptide thioesters, is among the most reliable ligation protocols in chemical protein synthesis. Thioesters have conventionally been synthesized using tert-butyloxycarbonyl (Boc)-based solid-phase peptide synthesis (SPPS); however, the increasing use of 9-fluorenylmethyloxycarbonyl (Fmoc) SPPS requires an efficient preparative protocol for thioesters which is fully compatible with Fmoc chemistry. We have addressed this issue by mimicking the naturally occurring thioester-forming step seen in intein-mediated protein splicing of the intein-extein system, using an appropriate chemical device to induce N-S acyl transfer reaction, avoiding the problems associated with Fmoc strategies. We have developed N-sulfanylethylanilide (SEAlide) peptides, which can be synthesized by standard Fmoc SPPS and converted to the corresponding thioesters through treatment under acidic conditions. Extensive examination of SEAlide peptides showed that the amide-type SEAlide peptides can be directly and efficiently involved in NCL via thioester species in the presence of phosphate salts, even under neutral conditions. The presence or absence of phosphate salts provided kinetically controllable chemoselectivity in NCL for SEAlide peptides. This allowed SEAlide peptides to be used in both one-pot/N-to-C-directed sequential NCL under kinetically controlled conditions, and the convergent coupling of large peptide fragments, which facilitated the chemical synthesis of proteins over about 100 residues. The use of SEAlide peptides, enabling sequential NCL operated under kinetically controlled conditions, and the convergent coupling, were used for the total chemical synthesis of a 162-residue monoglycosylated GM2-activator protein (GM2AP) analog.

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.

A thioethylalkylamido (TEA) thioester surrogate in the synthesis of a cyclic peptide via a tandem acyl shift

The cyclic cystine-knot peptide, kalata B1, was synthesized by employing a novel Fmoc-compatible thioethylalkylamido (TEA) thioester surrogate via an N-S acyl shift followed by a thiol-thioester exchange reaction. TEA thioester surrogate is cost-effective, conveniently prepared in one-step with starting materials, readily available from commercial sources, and highly efficient in preparing peptide thioesters.

Chemical synthesis of circular proteins

Circular proteins, once thought to be rare, are now commonly found in plants. Their chemical synthesis, once thought to be difficult, is now readily achievable. The enabling methodology is largely due to the advances in entropic chemical ligation to overcome the entropy barrier in coupling the N- and C-terminal ends of large peptide segments for either intermolecular ligation or intramolecular ligation in end-to-end cyclization. Key elements of an entropic chemical ligation consist of a chemoselective capture step merging the N and C termini as a covalently linked O/S-ester intermediate to permit the subsequent step of an intramolecular O/S-N acyl shift to form an amide. Many ligation methods exploit the supernucleophilicity of a thiol side chain at the N terminus for the capture reaction, which makes cysteine-rich peptides ideal candidates for the entropy-driven macrocyclization. Advances in desulfurization and modification of the thiol-containing amino acids at the ligation sites to other amino acids add extra dimensions to the entropy-driven ligation methods. This minireview describes recent advances of entropy-driven ligation to prepare circular proteins with or without a cysteinyl side chain.

Peptide and protein thioester synthesis via N-->S acyl transfer

Peptide and protein thioesters are playing an increasingly prominent role in the chemical toolbox for protein assembly and modification through Native Chemical Ligation (NCL). In this Emerging Area we highlight recent developments in a somewhat surprising route to thioesters: selective disruption of amides, the more stable carboxylic acid derivatives.

Alternative chemistries for the synthesis of thrombospondin-1 type 1 repeats

Synthetic protein engineering has benefited from the development of diverse methods for the synthesis of functionalized peptide fragments and approaches for their subsequent assembly into full length polypeptides. Here we describe a series of synthetic approaches for the total chemical synthesis of the second type 1 repeat of thrombospondin-1 (TSR2) that vary in both the location of the ligation site and alpha-amine protecting group strategy (Boc/Fmoc) used for the synthesis of the associated peptide fragments. These syntheses illustrate that challenging peptide sequences can result from the protecting group strategy as well as from sequence-dependent factors. Importantly, we find that such challenges can be overcome by altering the chemistry used for solid phase peptide synthesis, the choice of ligation site, and the resin used as a solid support. From these studies, we have developed a robust synthetic route to the TSR2 polypeptide consisting of native chemical ligation between an N-terminal fragment synthesized by Boc-SPPS and a C-terminal fragment synthesized by Fmoc-SPPS. Finally, the folded TSR2 domain is obtained following an optimized oxidative folding protocol using an excess of oxidized glutathione. This optimized synthesis will enable the use of unnatural amino acids to probe the unusual structure and anti-angiogenic activity of this protein domain.