Peptides

Peptides are chains of amino acids  in which two or more amino acids are joined by a characristics bond termed as a peptide bond.When two amino acids are covalently joined through a substituted amide linkage , termed a peptide bond, to yield a dipeptide. Such a linkage is formed by a removal of the elements of water from the α-carboxylic group of one amino acid and the α- amino group of another . The peptide bond between two amino acids  are formed by a condensation reaction.

Three amino acids can be joined by two peptide bonds to form a tripeptide ; similarly amino acids can be linked to form tetrapeptides, pentapeptides, and so forth. When few amino acids are joined in this fasion , the structure is called an oligopeptide.When many amino acods are joined, the product is called a polypeptide. Proteins are also a polymer of amino acids. Although the terms protein and polypeptide are sometimes used interchangeably, molecules reffered to as polypeptides generally have molecular weights below 10000 and those  called proteins have higher molecular weights.The number of amino acids is less than about 50 these molecules are named peptides while larger sequences are referred to as proteins.

In a peptide , the amino acid residue at the end with free α- amino group is the amino-terminal(N-terminal) residue : the residue at the other end, which has a free carboxyl group, is the carboxy terminal (C-terminal) residue .

                                                          
                                                              Peptide Bond

                                                            Peptides

Peptide classes  : Depending on synthesis of  peptides are classified into some major classes including

Milk peptide : This type of peptide are produced by enzymatic breakdown of milk protein  or  by the proteinases formed by lactobacilli during the fermentation of milk . Several milk peptide show antihypertensive effects in animal and in clincal studies.

Ribosomal peptide : Ribosomal peptides are synthesized by translation of mRNA.They are often subjected to proteolysis to generate the mature form. These act as hormones and  signaling molecules.

Nonribosomal Peptides : These peptides are assembled by enzymes that are specific to each peptide, rather than by the ribosome. The most common non-ribosomal peptide is glutathione, which is a component of the antioxidant defenses of most aerobic organisms.

Peptone : Peptones are derived from animal milk or meat digested by proteolytic digestion.Peptone is used in nutrient media for growing bacteria and fung.

Biologically active peptides and polypeptides 

Peptides  are present in every living cell and possess a variety of biochemical activities.Naturally occuring peptides range in length from two to many thousands of amino acids residues. Even the smallest peptides can have biologically important effects. Many small peptides exert their effects at very low concentrations. A number of vertebrate hormones are small peptides include oxytocin(nine amino acid residue) ,which is secreted by the posterior pituitary and stimulates uterine contractions ; brddykinin (nine residues) ,which inhibits inflammation of tissues ; and thyrotropine realising factor (three residues), which formed in the hypothalamus and stimulates the release of another hormone ,thyrotropin ,from the anterior pituitary gland. Some toxic mushrooms poisons such as amanitin, are also small peptides, as are many antibiotics .

Slightly larger are small polypeptides and oligopeptides such as the pancreatic hormone insulin which contains two plypeptide chains onehaving 30 amino acids residues and the other 21.Glucagon ,another pancreatic hormone,has 29 residues; it opposes the action of insulin.Corticotropin is a 39 residue hormone of the anterior pituitary gland that stimulates the adrenal cortex.

Synthesis of peptides :

Peptides are synthesized by coupling the carboxyl group or C-terminus of one amino acid to the amino group or N-terminus of another. There are two strategies for peptide synthesis: liquid-phase peptide synthesis and solid-phase peptide synthesis (SPPS).

                                          Figure : Solid -phase peptide synthesis




Peptides in Molecular Biology  :

Peptides have recently received prominence in molecular biology for several reasons. The  peptides allow the creation of peptide antibodies in animals without the need to purify the protein of interest.This involves synthesizing antigenic peptides of sections of the protein of interest.

Another reason is that peptides have become instrumental in mass spectrometry, allowing the identification of proteins of interest based on peptide masses and sequence. In this case the peptides are most often generated by in-gel digestion after electrophoretic separation of the proteins.

Peptides have recently been used in the study of protein structure and function. For example, synthetic peptides can be used as probes to see where protein-peptide interactions occur.

Inhibitory peptides are also used in clinical research to examine the effects of peptides on the inhibition of cancer proteins and other diseases.