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 are joined in a chain, they can be called a polypeptide. Every bond between amino acids, where the attraction of energy holds the building 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 referred to as peptides, and very long ones, with more than about 100 amino acids, are referred to as proteins. All proteins belong to the group of polypeptides, but some polypeptides do not meet 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 the alpha-carboxyl group and another end called the alpha-amino group. These two groups have different chemical properties.
An alpha-carboxyl tends to bond to an alpha-amino and vice versa. Therefore, a single amino acid tends to bind to another amino acid in a specific way, such as one person holding hands with another. One uses the right hand and the other uses the left hand to complete the link. This form of bond is a peptide bond, and when the two molecules come together, they produce one molecule.
The joining of two amino acids requires energy. The body supplies this energy when it needs to produce new polypeptides for biological use. Once the two amino acids are linked together, the bond is quite stable and is not easily broken.
Polypeptides contain many of these amino acids linked together in a straight line, like a row of people holding hands. Usually, a polypeptide chain also has chains that stick out to the sides at certain points. A polypeptide can contain up to 2,000 individual amino acids.
In the body, polypeptides can perform functions as they are. They may also need to associate with others to form a new protein with a biological function. Sometimes a single polypeptide is created as a large cellular product and then the cell uses an enzyme to break it down into functional pieces.
Polypeptides are first produced when a cell reads its genetic instructions and translates that information into amino acid sequence. Each gene codes for 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.