Proteins are made up of building blocks called amino acids. When two or more amino acids stick together in a chain, they can be called a polypeptide. Each link between the amino acids, where energy attraction holds the blocks together, is a peptide bond. Polypeptides perform many functions in the body.
Amino acids are small molecules that are essential building blocks in biology. Many biological functions rely on the action of a protein or polypeptide. Generally, very short polypeptides are usually called peptides, and very long ones, with more than about 100 amino acids, are called proteins. All proteins fall into the polypeptide group, but some polypeptides do not fit the criteria to be a protein.
The peptide portion of the name originates from the type of bond between two amino acid building blocks. Each amino acid has one end called an alpha-carboxyl group and another end called an alpha-amino group. These two groups have different chemical properties.
An alpha-carboxyl tends to bond to an alpha-amino and vice versa. A single amino acid, therefore, tends to become attached to another amino acid in a specific way, kind of like one person holding hands with another person. One uses his right hand and the other uses her left hand to complete the bond. This form of bond is a peptide bond, and when the two molecules stick together, they produce one molecule.
Sticking together two amino acids requires energy. The body furnishes this energy when it needs to make new polypeptides for biological use. After the two amino acids are stuck together, the bond is quite stable and does not break down easily.
Polypeptides contain many of these amino acids stuck together in a straight line, in the same manner as a line of people holding hands between them. Typically, a polypeptide chain also has chains sticking off to the side at certain points. One polypeptide can contain as many as 2,000 individual amino acids.
In the body, polypeptides can perform functions as they are. They may also need to join up with another to form a new protein with a biological function. Sometimes, a single polypeptide is created as a large cell product, and then the cell uses an enzyme to chop it up into functional portions.
Polypeptides are first made when a cell reads its genetic instructions and translates that information into the sequence of amino acids. Each gene codes for a particular product, and the necessary amino acids are collected and stuck together in the correct order. The sequence is essential for the polypeptide to function properly, as otherwise, it cannot interact properly with its targets.