The peptide bond is a covalent bond that is established between two molecules, when the carboxyl group of one reacts with the amino group of the other through a condensation reaction (or dehydration, which leads - that is - to the elimination of a water molecule ).
Usually, the peptide bond is formed between two amino acids, originating a dipeptide.
Since in its molecule a dipeptide still comprises an amino group and a carboxylic one, it can form a peptide bond with a third amino acid originating a tripeptide, and so on. When the number of amino acids is relatively small it is called oligopeptide, while if the number of amino acids increases it is called polypeptide or protein.
In reality, in spite of the figure, the peptide bond is not simple, but presents for 60% a single bonding nature and for 40% a double bond nature.
In living organisms, the formation of the peptide bond is catalyzed by an enzyme - called peptidyl transferase - present in the major subunit of ribosomes.
Even the proteins introduced into the diet are made up of chains of amino acids joined by peptide bonds. During digestion these bonds are broken by particular enzymes (peptidases) present in the gastric and pancreatic juices. The single amino acids, once absorbed by the intestine, pass from the blood and are picked up by the cells - above all by the hepatic ones - which unite them through new peptide bonds to form the proteins they need (not only structural but also hormonal, enzymatic, etc. .). In nature, in fact, there are a large number of proteins, with different physico-chemical characteristics, which derive from the different properties of the 20 ordinary amino acids and from how these are combined in the polypeptide chain. Suffice it to say that a protein of 100 amino acids, therefore relatively small, can consist of 20100 = 1.27 x 10130 possible polypeptide chains. The instructions for building the correct amino acid chain are enclosed in the genome of the individual.
