Peptide
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Peptides
A peptide is a compound formed by linking amino acids through amide bonds, also called peptide bonds. The carboxyl of one amino acid reacts with the amino groups of another amino acid to form an intermolecular peptide bond with the elimination of water molecule.
The resulting compound is a peptide. Two amino acids linked through a peptide bond form a dipetide , e.g., glycylglycine. Three amino acids form a tripeptide, four a tetrapeptide and so on. If a peptide is made up of not more than 10 amino acid residues , it is called an oligopeptide and beyond that it will be called polypeptide. Each peptide begins with an amino terminal and ends with a free carboxyl terminal. Smaller peptides are fewer in nature , but there are a few which are biologically important.
Peptide Synthesis
Synthesis of peptides is a complex process, but it has gained immense interest in protein chemistry. Three steps are followed to achieve peptide synthesis. First, in order to couple two amino acids through a peptides linkage, the amino and carboxyl groups which are not to participate in the reaction must be chemically blocked. It is necessary to block the side chains of amino acids so that they do not participate in reaction. Second, in order to link up amino and carboxyl functions a method should be used that does not cause alteration in the side chains. Thirdly, the blocking groups must be removed in such a way that there is no rearrangement or cleavage of peptide bonds.Naturally Occurring Peptides
A polypeptide with more than 100 amino acid residues is called a protein. So polypeptides are low molecular weight compounds as compared to proteins, but some of them are very potent substances and cannot be over looked.
1. Glutathione It is a natural and widely distributed tripeptide present in many plant and animal tissues. It consists of glutamic acid, cysteine and glycine and can easily be dehydrogenated and converted to disulphide form.
2. Carnosine and anserine These are dipetides of β-alanine and histidine and are found in the vertebrate muscle tissue.
3. Oxytocin and vasopressin These are nonapeptides and function as hormones, secreted from the mammalian neurohypophysis (pituitary). The contraction of smooth muscles and ejection of milk in females is controlled by oxytocin. Vasopressin has a pressor influence on the blood vessels and it cause a rise in blood pressure. It is also supposed to have antidiuretic effects. Oxytocin and vasopressin are structurally related except for the residuce sin positions 3 and 8.
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4. Bradykinin It is a nonapeptide, a kinin which is hypotensive agent. It has the following structure.
Arg – Pro – Pro – Gly – Phe – Ser – Pro – Phe – Arg
5. Corticotropin This is adrenocorticotropic hormone (ACTH) released from the anterior lobe of hypophysis of 39 amino acids (MW ≈4500). It stimulates growth and metabolic activity of adrenal cortex.
6. Insulin Insulin is a polypeptide, a hormone secreted by pancreas. It is composed of 51 amino acids(MW ≈ 6000) whose sequence was established by sanger in 1953. The hormone lower the blood suger level.
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