Peptide Bonds Peptide Bonds A peptide bond (amide bond) is a covalent chemical bond formed between two molecules when the carboxyl group of one molecule reacts with the amino group of the other molecule, causing the release of a molecule of water (H2O), hence the process is a dehydration synthesis reaction (also known as a condensation reaction), and usually occurs between amino acids. The resulting C(O)NH bond is called a peptide bond, and the resulting molecule is an amide. The four-atom functional group -C(=O)NH- is called a peptide link. Polypeptides and proteins are chains of amino acids held together by peptide bonds, as is the backbone of PNA. A peptide bond can only be broken by amide hydrolysis (the adding of water). The peptide bonds in proteins are metastable, meaning that in the presence of water they will break spontaneously, releasing 8–16 kilojoule/mol (2–4 kcal/mol) of free energy, but this process is extremely slow. In living organisms, the process is facilitated by enzymes. Living organisms also employ enzymes to form peptide bonds; this process requires free energy. The wavelength of absorbance for a peptide bond is 190–230 nm[2] (which makes it particularly susceptible to X-ray radiation).
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Peptide Bond Formation The amide linkage (-C(=O)NH-) between Îą-amino acid is called as peptide bond. For example, when carboxyl group (-COOH) group of glycine react with the amino group (-NH2) group of Alanine, it forms a dipeptide, glycylalanine (Gly-Ala). Similarly the peptide linkage between two glycine (CH2(NH2)COOH) amino acid to form glycylglycine (Gly-gly). The configuration of peptide depends upon the configuration of amino acids involve in peptide bond formation. If the amino group (-NH2) bonded at chiral carbon atom lies on the left side as in Lglyceraldehyde, the amino acid is said to belong to L-series and the location of amino group at right side belongs to D-series. Similarly amino acids bonded in a chain to form a long chain to form polymer, called as polypeptide. In each polypeptide chain more than thousand amino acids liked together through peptide linkage through condensation reaction. PeptideBond Mechanism :- The peptide bond between is a simple amide linkage between amino group and carboxyl group of same of different amino acids. However it's a biochemical reaction, takes place under certain temperature and in the presence of some other biochemical compounds. Reaction required some amount of energy which provided by adenosine triphosphate (ATP) which is an energy center for biochemical reactions. The peptide bond formation completed in three steps.
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1. Carboxylate Activation 2. Nucleophilic Addition and Formation of the tetrahedral intermediate 3. Decomposition of the tetrahedral intermediate to form product 1. Carboxylate Activation Since amino acids exits as a dipolar ion also known as zwitterion in which carboxyl group (COOH) exists as carboxylate ion (-COO-) and amino group (-NH2) as -NH3+ ion. The lone pair on oxygen atom of carboxylate ion reacts with phosphate group of ATP molecule through SN2 mechanism and form a transition state which proceeds by the formation of ADP molecule and removal of hydroxyl group from carboxylate ion with the formation of a new phosphate ester. 2. Nucleophilic Addition and Formation of the tetrahedral intermediate In second step, the lone pair of nitrogen atom of the amino-terminus of second amino acid reacts with the carbonyl carbon of the phosphate ester formed in previous step and form a tetrahedral intermediate which has four substituents on the carbonyl carbon. Finally, the amine nucleophile gets deprotonated and converts in to a neutral state.
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