Polypeptides: Definition, Structure and Functions of These Molecules That Make Proteins

They are chains of amino acids that have various lengths.

A peptide contains two or more amino acids, and a polypeptide, on the other hand, includes ten or more amino acids.

Peptide bonds hold both peptides and polypeptides together. T cells in the body recognize peptides and polypeptides as tiny proteins.

In some pharmaceuticals, peptides, and polypeptides are used in vaccines to stimulate the production of antibodies.

Biochemists generally use polypeptide to describe medium-sized peptide chains consisting of 10 or more amino acids.

The term protein is nonspecific in biochemistry because it includes amino acid chains of any length.

However, polypeptides refer to proteins of a particular size. The term “polypeptide” is described as a general size of peptide chains.


For example, the pancreatic hormone insulin is an example of a polypeptide. Insulin’s job is to help the body use and store sugar.


Polypeptides function as structural elements of the body. Some of them are vital components of muscles and bones.

For example, the peptides actin and myosin are essential structural components of muscles.

Polypeptides also contribute to the shape and strength of bones. Bones and muscles not only provide structure to the body but also participate in movement and provide protection to internal organs.

Polypeptides function as structural elements of the body. Some of them are vital components of muscles and bones.

Peptides and polypeptides are chains of amino acids, and the endocrine system secretes peptides and polypeptides.

After they are secreted, the blood is responsible for distributing the peptides and polypeptides to end organs such as the heart, kidneys, and liver.

The endocrine organs involved in this secretion process include the thyroid gland, hypothalamus, pituitary gland, adrenal glands, endocrine pancreas, adipose tissues, and ovaries.

The order and arrangement of these amino acids in a polypeptide chain will produce different chemical structures.

The structure of a protein is essential to its function. Therefore, several amino acids can be joined to form a particular polypeptide chain with a specific part.

Similarly, the alphabet letters can be rearranged to form different words, each with its meaning.

In an image of a simple representation of a polypeptide chain made up of several amino acids, each pattern represents a different amino acid.

Thus we can say that a polypeptide chain consists of many different amino acids linked together.

Peptide bonds link the amino acid chains. Each end of the polypeptide is called the amino-terminal or the N terminal, which has a free amino group.

The other end of the polypeptide has a free carboxyl group called the C-terminal or the carboxyl-terminal. Polypeptides play a vital role in proteins in cells.

Proteins consist of one or more polypeptide molecules. Proteins are essential as they help cells in several ways. Half of the cell’s mass is made up of proteins.

It is also compatible with the cell structure and stores essential substances; it also controls metabolic functions and improves cells’ immune response.

Polypeptides make proteins by linking various amino acids together. Two or more polypeptides are joined and folded in a specific way to form a particular protein.

The other name for a polypeptide is an amino acid polymer. They are chains of monomers and subunits joined by a chemical bond.

A single polypeptide chain is called a single protein; two examples of polypeptides are insulin and growth hormone.

Polypeptide structure levels

Polypeptides have four levels of structure and are as follows:

Primary structure

The primary level of structure in a protein is the most linear sequence of amino acids formed by a condensation reaction (and therefore the extraction of water) in protein synthesis.

Each amino acid residue is linked through peptide bonds. Other covalent bonds are also included in the primary structure, such as disulfide bonds.

The peptide bond has a partial double bond character, and therefore all the atoms in the peptide bond lie in the same rigid plane, called the amide plane.

This restriction is essential to define the folding of a protein in three dimensions.

The only possible rotation of the main chain is over the alpha carbon-carbon bond (designated by the Greek letter psi) and the alpha carbon-nitrogen bond (represented by the Greek letter phi).

Therefore, the amide planes define the polypeptide backbone.

Secondary structure

The secondary level of structure in a protein is the regular folding of the polypeptide chain regions. The two most common types are the alpha helix and the beta-pleated sheet.

  • α-helix: This is a good spiral in which each peptide bond is in the trans conformation.
  • Β-pleated sheet: This has an extended polypeptide chain with a nearby chain that runs antiparallel. Each folded sheet β is trans and flat. Hydrogen bonding can occur between nearby polypeptide chains.

It refers to the ordered arrangement of amino acids at the localized location of the polypeptide. The folding pattern is stabilized with the help of hydrogen bonds.

Both myoglobin and hemoglobin are composed primarily of alpha-helices.

The amino acids are arranged in a regular helical conformation in a rod-like alpha-helix.

The carbonyl oxygen in each peptide bond is hydrogen-bonded to the hydrogen at the amino group of the fourth amino acid away, with the hydrogen bonds running nearly parallel to the axis of the helix.

In an alpha helix, there are 3.6 amino acids per turn of the helix that cover a distance of 0.54 nm, and each amino acid residue represents a 0.15 nm advance along the helix axis.

The side chains of the amino acids are all arranged along the side of the cylindrical helix.

Tertiary structure

Tertiary structure refers to the spatial arrangement of amino acids that are widely separated in a linear sequence, as well as adjacent residues.

Again, it is the amino acid sequence that determines the three-dimensional structure.

In water-soluble globular proteins, such as myoglobin, the main driving force behind the polypeptide chain folding is the energetic requirement to bury the non-polar amino acids in the hydrophobic interior away from the surrounding aqueous medium.

Once folded, the three-dimensional, biologically active (native) molecule is maintained not only by hydrophobic interactions but also by electrostatic forces, hydrogen bonds, and, if present, disulfide bonds.

Quaternary structure

It is a term used to describe proteins and consists of multiple polypeptide molecules. Each polypeptide molecule is called a monomer.

Proteins that contain more than one polypeptide chain, such as hemoglobin, exhibit the fourth level of protein structure called the quaternary structure.

This level of structure refers to the spatial arrangement of the polypeptide subunits and the nature of their interactions.

In the case of hemoglobin, the four subunits are held together by weak van der Waals forces.

Hemoglobin has four monomers, two α chains, each containing 141 amino acids, and two β chains, each containing 146 amino acids.

Polypeptide functions

Polypeptides have several vital functions, and their position depends on their size and amino acid sequence.

There are 20 naturally occurring amino acids, and each one is a little different from one another. All amino acids have the same basic structure, yet something called the R group distinguishes one amino acid from another.

Peptides are found in all cells and tissues in the body and are an integral part of most biological processes. It is necessary to maintain adequate levels of peptide concentration and activity to achieve homeostasis and maintain nutritional health.

Peptides are molecules more commonly known as proteins. All peptides are composed of chemically linked amino acids.

Peptides range in size from two amino acids, dipeptides, to thousands of amino acids or polypeptides. The function of a peptide is determined by its scope and amino acid sequence.

Polypeptides can function as:


All cells have a protective membrane that prevents most substances from quickly entering the cell.

Some polypeptides act as transporters that selectively allow specific substances to pass through the permeable cell membrane into the cell.

For example, glucose transporters are necessary for glucose to travel from the blood to other cells.

Glucose moves to muscles or some other cell so it can be used for energy. Similarly, cell waste products can also exit the cell via specific peptide transporters.


Enzymes are biological catalysts that accelerate the body’s metabolic reactions.

Most enzymes are polypeptides. Hundreds of enzymes are distributed throughout the body to speed up the reactions involved in many processes.

These processes include the digestion of food, the production of energy, and the synthesis of cellular components.


Hormones act as biological messengers that carry information from tissue and move through the blood to a distant tissue.

Two standard classes of hormones are peptide hormones and steroids. Examples of peptide hormones include those involved in regulating blood glucose, such as insulin and glucagon, and those that hold appetites, such as ghrelin and leptin.