Peptides are chemicals that consist of several amino acids with a covalent bond link between them. They tend to form long chains, so they are often classified as polymers. Peptides are indispensable for organic life and every animal on the plant needs them as a building block. They are the precursors of proteins, which form when a peptide chain grows very long. Both peptides and proteins are very complex and many varieties exist, so they have been studied by scientists for a long time in order to understand the mechanisms inside the human body.

Chemists classify peptides as shorter polymers made from amino acids. They are shorter in length than proteins, but otherwise the amino acids are linked by the same peptide bonds. As a general rule, compounds with over 50 amino acids are considered to be proteins, while shorter ones are named peptides. The typical peptide bonds are of a covalent type, between atoms that share electrons. They link is formed as a result of a reaction between an amino group (NH3) of a molecule with two carboxyl groups (-COOH) of the second molecule.

The simplest type of peptide is the di-peptide, which consists of only two amino acid molecules linked by one peptide bond. Larger compounds, such as the tri-peptides, tetra-peptides, penta-peptides and so on, follow the same pattern. At one end of every peptide there is a carboxyl molecule, while an amino is found at the other end. Polypeptides are longer linear chains of amino acids, with peptide bonds between them. Proteins are even larger and have at least one polypeptide in their composition. They can also include chemical groups known as cofactors or prosthetic groups, which are not peptides. The structure of the proteins responds to functional needs and plays a role in biology. Many different types of peptides and proteins are critical for living organisms and some of them have been the focus of extensive research.

The study of peptides includes many complex scientific terms that must be briefly explained for a proper understanding of these compounds. When two or more atoms have shared electrons, the result is a covalent bond. Peptides are linked by a specific type of covalent bond named a peptide bond, or alternatively an amide bond. The most important molecules involved in these links are the carboxyl ones that form attachments. Every carboxyl group consists of atoms or oxygen, hydrogen and carbon.

The term "polymers" that is used to define peptides might also be confusing to people who lack solid knowledge of chemistry. Polymers are often considered to be a type of plastic. However, chemistry classifies any long chain compound linked by covalent bonds as a polymer. They can become extremely long and complex at the same time, despite their repetitive structure.

The biological role of peptides depends on the amino acids in their composition, which have various functions. They can perform a variety of tasks; some for example are part of the immune system while others regulate hormones. Human metabolism is programmed to source these compounds from food and reuse them. For example, the proteins found in meat are separated into peptides by the enzymes in the intestinal tract, which break the amide bonds. If the body needs these peptides, they are digested; otherwise they can be eliminated through excretion.

There is no clear boundary between peptides and proteins, from a chemical point of view. Proteins are longer and have a more complex structure, which is needed to link all of the amino acids in their composition. Scientists usually classify chains with up to 50 amino acids as peptides. Larger compounds of the same type are known as proteins, but there are also exceptions from this rule.

What are the major classifications of peptides?

Several major types of peptides exist, some of the most common are the ones found in milk. During milk fermentation, lactobacilli break proteins into smaller peptides. The same effect also happens due to the action of digestive enzymes. Some clinical tests and animal studies have discovered that milk peptides can reduce high blood pressure.

The translation of mRNA produces compounds known as ribosomal peptides. They are needed as signal molecules and hormones by most advanced life forms. They can also have an antibiotic role, in some cases. They consist only of the amino acids found in the initial ribosome, since they are synthetized through translation.

Some peptides are produced by enzymes that are specific to them, rather than the ribosome. As a result, they are known as non-ribosomal peptides. Glutathione is the most common and best known of these compounds, since almost all aerobic life forms use it as an antioxidant. Non ribosomal peptide synthesis is the name given to the process that produces this type of peptides in plants, fungi and even the most basic forms of unicellular organisms.

During the proteolytic digestion of meat and animal milk, the body produces a specific variety of peptides classified as peptones. They are actually an organic mixture of various compounds, not only small peptides but also vitamins, fats, metals, salts and many others. Peptone is great for growing simple organisms like fungi or bacteria, as a nutrient.

Proteins can be identified or classified by smaller parts of them, which are known in chemistry as peptide fragments. They can be found in many places and are used for example in forensic research, in order to study samples that have been decomposed by exposure to weather. Peptide fragments can be produced in labs by degrading a sample using enzymes.

Main roles of peptides

Peptides are bioactive compounds that consist of a chain of amino acids. The chain is linked by a series of covalent bonds, while the number of amino acids in the chain varies between 2 and 50. Peptides can also form bonds between them, which produce proteins. The amino acids in their composition determine the biological function of a particular peptide. Peptides are very important for the human body and are involved in many chemical reactions, so they are usually classified based on their biological role.


Hormones are probably the most important type of peptides, tasked with the transmission of signals from glands to cells. They regulate many of the body's functions and can for example balance blood sugar levels or initiate sleep. Hormones are produced by the endocrine glands, as well as by internal organs like the liver, kidneys, stomach or intestine. The most common peptide hormones are the growth one, insulin, gastrins, bradykinins and oxytocins.


Neuropeptides are located in neural tissues and the brain is responsible with their production. They relay signals generated in the brain and their role is to extend the function of the neurological system through the entire body. The best known peptides from this group are the endorphins.


Alkaloids are the peptides produced by some organisms as part of their mechanism of defence. They are found usually in plants and fungi, as well as shellfish and other simple animals. Alkaloids include compounds like pandamine, dynorphin or ergotamine.


Some peptides have antibiotic effects and can eliminate harmful pathogens from the body. Modern medicine uses them to prevent infection by killing the bacteria that causes it.

Structural support

Peptides with very long chains are classified as proteins. These compounds are very important for organic life, being the building blocks of human and animal tissues. One example of a structural protein is keratin, which is found in the human nails and hair as well as the claws, hooves, horns, feathers or scales of animals. Peptides also form collagen, which provides the structure of skin, tendons, ligaments, bone and teeth.

Skin care

Peptides are some of the most common ingredients of cosmetics, besides antioxidants. They are found in every anti-aging product for the skin.

Peptides have an active role in the human body and relay signals to cells. They are similar in structure to smaller proteins and can be formed through their decomposition. For example, collagen forms peptides when it breaks down, which send a signal to cells that the skin has to be repaired through the production of new collagen. This is the action mechanism of cosmetic products, which use peptides to trick skin cells to generate collagen.

The peptides used in anti-aging cosmetics are now synthetically produced in labs. They send a variety of signals and can have many roles. Some cosmetic ingredients, such as Matrixyl or Argireline, are actually trademarked combinations of several peptides.