What are Proteins? | Definition, Structures, Importance and Classification

Proteins are compounds, which are formed by the combination of amino acids. Amino acids are the compounds having both Amino and Carboxylic (COOH) groups attach to the same carbon atom. Proteins are considered as complex organic compounds.proteins

When amino acids are attached together, a “polypeptide” chain is formed. When many polypeptide chains are linked together a protein is produced.


The polymers of amino acids containing carbon, nitrogen, oxygen and hydrogen are called proteins. The number of amino acids is different in different proteins. It may be from a few to 3000 or more.

Structure of Proteins

Each protein has specific properties. These properties are determined by numbers and specific sequences of amino acids in protein molecules. These properties also depend on the shape of proteins. It gives final shape to the proteins. These are four levels of organization in proteins:

1.      Primary Structure of Proteins

The structure of protein which shows number and sequence of amino acids is called primary structure of proteins.


When amino acids are arranged in straight manner in a polypeptide chain, it is called primary structure of protein.

This protein also contains disulphide (S-S) bond. Insulin and Haemoglobin are examples of primary structure of protein.

a)     Sequence of Amino Acids in Proteins

·         Sequence of Insulin molecule
  1. Sanger determines the sequence of first protein molecule. Sanger worked on insulin for ten years. He found that insulin is composed of 51 amino acids. It has two chains of amino acids. One chain has 21 amino acids. The other chain has 30 amino acids. Both these chains are held together by disulphide bridges (bonds).
·         Sequence of Haemoglobin molecule

Haemoglobin is discovered by Ingram (scientist). Haemoglobin is composed of 574 amino acids. These amino acids form four chains of amino acids. These are two alpha and two beta chains. Each alpha chain has 141 amino acids and each beta chain has 146 amino acids.

primary structure of proteins

b)     Size of Protein molecule

The size of protein depends on the type of and number of amino acids present in a protein.

c)      Specific arrangement of Amino Acids

There are 1000 proteins in the human body. Each protein is composed of unique and specific arrangement of 20 types of amino acids. A sequence of nucleotides is present in the DNA. This sequence of nucleotides determines the sequence of amino acids in a protein molecule. The arrangement of amino acids is highly specific for the proper functioning of protein. If a single amino acid is not in its normal place, the protein fails to perform its normal functions. The best example is sickle cell Haemoglobin in human beings. In this case, one molecule of amino acid is replaced by some other amino acid. This Haemoglobin fails to carry sufficient oxygen. It may cause death.

2.      Secondary Structure of Protein

The structure of protein formed by coiling and folding of polypeptide chains is called secondary structure of protein. The protein molecules usually do not remain flat. They coil in many ways:

·         𝜶-helix

The spiral (like spring) coiling of the polypeptide chain is called 𝜶-helix. It is one of the most common secondary structures. The α-helix forms a uniform geometric structure. Each turn of the helix has 3.6 amino acids. Hydrogen bonds are formed among the amino acids of successive turn of the spiral. It keeps the helix intact.

·         𝜷-pleated sheet

The folding back of the polypeptide is called 𝛃-pleated sheet. It forms a folded sheet like structure.

beta-pleated sheet

3.      Tertiary Structure of Protein

The globular structure formed by the bending and folding of polypeptide chain upon itself is called tertiary structure. It forms tertiary conformation (shape) of proteins. Tertiary conformation of protein is maintained by three types of bonds: ionic, hydrogen and disulphide (S-S). Most stable tertiary conformation is made in an aqueous environment. The hydrophobic amino acids are buried inside the aqueous medium. While the hydrophilic (water loving) amino acids are present on the surface of the protein molecule.

4.      Quaternary Structure of Protein

The structure formed by the aggregation of tertiary polypeptide chains is called quaternary structures. These polypeptide chains are held together by hydrophobic interactions, hydrogen and ionic bonds. Haemoglobin carries oxygen of red blood cells.

structures of protein

Classification of Protein

Proteins have complex structures. Different proteins perform different functions. So, there is no single method of classification of protein. However, proteins are divided into following types on the basis of their structures:

1)     Fibrous proteins

  • Their molecules are composed of one or more polypeptide chain. These chains are present in the form of fibrils.
  • Secondary structure is most important in them.
  • These are insoluble in aqueous media.
  • They are non-crystalline and are elastic in nature.
  • These proteins perform structural role in cells and organisms.

Examples: Silk fibers (from silk worm and spider’s web), Myosin (in muscle cells), Fibrin (of blood clot), and Keratin (of nail and hair).

2)     Globular Proteins

  • They have multiple folding of polypeptide. So, their chains are spherical and ellipsoidal.
  • Tertiary structure in the most important in them.
  • These proteins are soluble in aqueous media such as salt solution, solution of acids or bases or aqueous alcohol.
  • They can be crystallized.
  • They disorganize with the change in the physical and physiological environment.

Examples: Enzymes, Antibodies, Hormones and Haemoglobin.

Importance of Proteins

Proteins are most abundant compound present in the cells. Proteins form over 50% of dry weight of the bodies of the organisms. They are present in all types of cells and in all parts of the cell. Proteins perform following functions:


  • They build many structures of the cells like cell membrane.
  • All Enzymes are protein in nature. These enzymes control the metabolism of cell.
  • Most Hormones are also protein in nature. Hormones regulate metabolic processes.
  • Some proteins (e.g. Haemoglobin) work as carrier. They transport specific substances such as oxygen, lipids, metal ions etc.
  • Some proteins are called Antibodies. Antibodies defend the body against pathogens.
  • Blood clotting proteins prevent the loss of blood from the body after an injury.
  • The proteins cause the movement of organism. They also cause the movement of chromosomes during anaphase of cell division.
  • They form plasma membrane of cell.
  • Proteins form seeds in plants.
  • On oxidation, it provides the energy.
  • Protein act as membrane receptors and transporters, allow passage of important substances.

Amino Acids

The compounds with an amino group and a carboxylic group (COOH), attached to the same carbon atom, known as Alpha Carbon are called “Amino Acids”. 170 types of amino acids are present in cells and tissues. Only 25 amino acids form proteins. However, most of the proteins contain 20 types of amino acids. They have general formula as:amino group


R may be a hydrogen atom as in glycine amino acid. Or it may be HCHH as in alanine amino acid. Or it may be any other group. Thus amino acids mainly differ due to the type or nature of R group.

peptide linkage

Formation of Protein from Amino Acids

Amino acids are linked to form polypeptide chain. The amino group of one amino acid reacts with the carboxyl group of another amino acid and C-N bond is formed. This C-N bond is called peptide bond. Water is released during peptide bond formation.

For example, glycine and alanine may combine to form glycylalanine. It has two amino acid units. Thus it is called dipeptide. A dipeptide has again an amino group at one end and a carboxyl group at the other end. So, both reactive parts are again available for further peptide bond formation. In this way, tripeptide, tetrapeptide and pentapeptide etc. are formed. They ultimately form a polypeptide chains.


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