Protein chemical synthesis by native peptide

ligation Ligation may refer to: * Ligation (molecular biology), the covalent linking of two ends of DNA or RNA molecules * Chemical ligation, the chemoselective condensation of unprotected peptides * In medicine, the making of a ligature (tie) * Tubal liga ...

of unprotected

peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...

segments is an interesting complement and potential alternative to the use of living systems for producing proteins. The synthesis of proteins requires efficient native peptide ligation methods, which enable the

chemoselective Chemoselectivity is the preferential reaction of a chemical reagent with one of two or more different functional groups. In a chemoselective system, a reagent in the presence of an aldehyde and an ester would mostly target the aldehyde, even if it ...

formation of a native

peptide bond In organic chemistry, a peptide bond is an amide type of covalent chemical bond linking two consecutive alpha-amino acids from C1 (carbon number one) of one alpha-amino acid and N2 (nitrogen number two) of another, along a peptide or protein cha ...

in aqueous solution between unprotected peptide segments. The most frequently used technique for synthesizing proteins is

Native chemical ligation Native Chemical Ligation (NCL) is an important extension of the chemical ligation concept for constructing a larger polypeptide chain by the covalent condensation of two or more unprotected peptides segments. Native chemical ligation is the most ...

(NCL). However, alternatives are emerging, one of which is SEA Native Peptide Ligation.

Overview

The SEA group belongs to the ''N,S''-acyl shift systems because its reactivity is dictated by the intramolecular

nucleophilic In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they a ...

addition of one SEA

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

group on the

C-terminal The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, carboxy tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When t ...

carbonyl group In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such as aldehydes ...

of the peptide segment. This results in the migration of the peptide chain from the nitrogen to the sulfur. The overall process of SEA native peptide ligation involves first an ''N,S''-acyl shift for in ''in situ'' formation of a peptide

thioester In organic chemistry, thioesters are organosulfur compounds with the molecular structure . They are analogous to carboxylate esters () with the sulfur in the thioester replacing oxygen in the carboxylate ester, as implied by the thio- prefix ...

, and later on, after thiol-thioester exchange, an ''S,N''-acyl shift for formation of the peptide bond.

Description of the reaction

SEA is an abbreviation of ''bis''(2-sulfanylethyl)amido (Scheme 1). SEA ligation involves the reaction of a peptide featuring a C-terminal ''bis''(2-sulfanylethyl)amido group with a Cys peptide. This reaction proceeds probably through the formation of a transient thioester intermediate, obtained by intramolecular attack of one SEA thiol on the peptide C-terminal carbonyl group as shown in Scheme 1. Then, the thioester undergoes a series of thiol-thioester exchanges, including with exogeneous thiols present in the ligation mixture such as mercaptophenyl acetic acid (MPAA). Exchange with the

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

thiol group of the second peptide segment results in a transient thioester intermediate, which as for Native Chemical Ligation, rearranges by intramolecular ''S,N''-acyl shift migration into a native peptide bond.

Publication

The first peer reviewed publication describing SEA native peptide ligation was published in ''Organic Letters'' by Melnyk, O. ''et al.'' (Ollivier, N.; Dheur, J.; Mhidia, R.; Blanpain, A.; Melnyk, O., Bis(2-sulfanylethyl)amino native peptide ligation. Org. Lett. 2010, 12, (22), 5238–41; Publication Date (Web): October 21, 2010. A few weeks later, the same reaction was published in the same journal by Liu, C. F (Hou, W.; Zhang, X.; Li, F.; Liu, C. F., Peptidyl N,N-Bis(2-mercaptoethyl)-amides as Thioester Precursors for Native Chemical Ligation. Org. Lett. 2011, 13, 386–389; Publication Date (Web): December 22, 2010).

SEA on/off concept

SEA ^on/off concept exploits the redox properties of SEA group. Oxidation of SEA ^on results in a cyclic disulfide called SEA ^off, which is a self-protected form of SEA ^on. SEA ^off and SEA ^on can be easily interconverted by reduction/oxidation as shown in Scheme 2.

References

{{reflist Peptides Chemical reactions