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Peptide bond formation via dehydration reaction. When two amino acids form a dipeptide through a peptide bond, [1] it is a type of condensation reaction. [2] In this kind of condensation, two amino acids approach each other, with the non-side chain (C1) carboxylic acid moiety of one coming near the non-side chain (N2) amino moiety of the other.
Coupling of two amino acids in solution. The unprotected amine of one reacts with the unprotected carboxylic acid group of the other to form a peptide bond.In this example, the second reactive group (amine/acid) in each of the starting materials bears a protecting group.
The S−H bond in thiols is weak compared to the O−H bond in alcohols. For CH 3 X−H, the bond enthalpies are 365.07 ± 2.1 kcal/mol for X = S and 440.2 ± 3.0 kcal/mol for X = O. [ 21 ] Hydrogen-atom abstraction from a thiol gives a thiyl radical with the formula RS • , where R = alkyl or aryl.
Thioesters hydrolyze to thiols and the carboxylic acid: RC(O)SR' + H 2 O → RCO 2 H + RSH. The carbonyl center in thioesters is more reactive toward amine than oxygen nucleophiles, giving amides: This reaction is exploited in native chemical ligation, a protocol for peptide synthesis. [8]
The conjugate base of thioacetic acid, thioacetate, is a reagent used for installing thiol groups via the displacement of alkyl halides by a two-step process. The halide is displaced to give a thioester intermedate, which is then hydrolyzed: R−X + CH 3 COS − → R−SC(O)CH 3 + X − R−SC(O)CH 3 + H 2 O → R−SH + CH 3 CO 2 H
Typically, the hydroxyl group of a serine (rarely, threonine) or the thiol group of a cysteine residue will attack the carbonyl carbon of the preceding peptide bond, forming a tetrahedrally bonded intermediate [classified as a hydroxyoxazolidine (Ser/Thr) or hydroxythiazolidine (Cys) intermediate].
In native chemical ligation, the ionized thiol group of an N-terminal cysteine residue of an unprotected peptide attacks the C-terminal thioester of a second unprotected peptide, in an aqueous buffer at pH 7.0 and room temperature. This transthioesterification step is reversible in the presence of an aryl thiol catalyst, rendering the reaction ...
Reaction mechanism of the cysteine protease mediated cleavage of a peptide bond. See also: catalytic triad The first step in the reaction mechanism by which cysteine proteases catalyze the hydrolysis of peptide bonds is de protonation of a thiol in the enzyme 's active site by an adjacent amino acid with a basic side chain , usually a histidine ...