Peptide bond

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 chain. 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 chain. It can also be called an eupeptide bond to separate 'it' from an isopeptide bond, a different type of amide bond between two amino acids. When two amino acids form a dipeptide through a peptide bond it is a type of condensation reaction. 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. One loses a hydrogen and oxygen from its carboxyl group (COOH) and the other loses a hydrogen from its amino group (NH2). This reaction produces a molecule of water (H2O) and two amino acids joined by a peptide bond (-CO-NH-). The two joined amino acids are called a dipeptide. The amide bond is 'synthesized' when the carboxyl group of one amino acid molecule reacts with the amino group of the other amino acid molecule, causing the release of a molecule of water (H2O), hence the process is a dehydration synthesis reaction. The formation of the peptide bond consumes energy, which, in organisms, is derived from ATP. Peptides and proteins are chains of amino acids held together by peptide bonds (and sometimes by a few isopeptide bonds). Organisms use enzymes to produce nonribosomal peptides, and ribosomes to produce proteins via reactions that differ in details from dehydration synthesis. Some peptides, like alpha-amanitin, are called ribosomal peptides as they are made by ribosomes, but many are nonribosomal peptides as they are synthesized by specialized enzymes rather than ribosomes. For example, the tripeptide glutathione is synthesized in two steps from free amino acids, by two enzymes: glutamate–cysteine ligase (forms an isopeptide bond, which is not a peptide bond) and glutathione synthetase (forms a peptide bond). A peptide bond can be broken by hydrolysis (the addition of water). In the presence of water they will break down and release 8–16 kilojoule/mol (2–4 kcal/mol) of Gibbs energy. This process is extremely slow, with the half life at 25 °C of between 350 and 600 years per bond. In living organisms, the process is normally catalyzed by enzymes known as peptidases or proteases, although there are reports of peptide bond hydrolysis caused by conformational strain as the peptide/protein folds into the native structure. This non-enzymatic process is thus not accelerated by transition state stabilization, but rather by ground state destabilization. The wavelength of absorption A for a peptide bond is 190–230 nm (which makes it particularly susceptible to UV radiation).