What is a polypeptide? (with photo)

People holding hands can be seen as a metaphor for a peptide bond.

Proteins are made up of building blocks called amino acids. When two or more amino acids join together in a chain, they can be called a polypeptide. Each bond between amino acids, where the attraction of energy 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 depend on the action of a protein or polypeptide. Generally, very short polypeptides are commonly 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 meet the criteria for being 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 bind to another amino acid in a specific way, like one person holding hands with another. One uses the right hand and the other uses the left hand to complete the bond. This form of bond is a peptide bond, and when the two molecules come together, they produce one molecule.

The union of two amino acids requires energy. The body supplies this energy when it needs to make new polypeptides for biological use. Once the two amino acids are stuck together, the bond is quite stable and doesn’t break easily.

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Polypeptides contain many of these amino acids joined in a straight line, much like a line of people holding hands between them. Typically, a polypeptide chain also has chains that stick out to the side at certain points. A polypeptide can contain up to 2000 individual amino acids.

In the body, polypeptides can perform functions as they are. They may also need to join with others 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 break it down into functional portions.

Polypeptides are first produced when a cell reads its genetic instructions and translates that information into the amino acid sequence. Each gene encodes a specific product and the necessary amino acids are collected and grouped in the correct order. The sequence is essential for the polypeptide to function properly, otherwise it cannot properly interact with its targets.

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