Proteins [Peptide Linkage, Structure, Classification, Functions of Proteins]


Proteins are the most abundant organic compounds to be found in cells and comprise over 50% of their total dry weight. They are present in all types of cells and in all parts of the cell. Proteins are the macromolecules responsible for the biological processes in the cell. Proteins are polymers of amino acids, the compounds consisting of carbon, nitrogen, oxygen, and hydrogen.

The number of amino acids differs from a few to 3000 or perhaps more in different proteins. About 170 types of amino acids have been discovered to take place in cells and tissues. Of these, about 25 are constituents of proteins. Most of the proteins are nevertheless, made from 20 kinds of amino acids.

Peptide Linkage

Amino acids are linked together to form polypeptides proteins. The amino group of one amino acid may react with the carboxyl group of another releasing a molecule of water. The linkage between the hydroxyl group of the carboxyl group of one amino acid and the hydrogen of the amino group of another amino acid release H2O and C – N link, to form a bond called a peptide bond.


The resultant compound has 2 amino acid subunits and is a dipeptide. A dipeptide has an amino group at one end and a carboxyl group at the other end of the molecule. So, both reactive parts are again offered for additional peptide bonds to produce tripeptides, tetrapeptides, and pentapeptides, etc., causing polypeptide chains.

Structure of Proteins

Due to various rearrangement of amino acids, the structure of proteins divides into four types:

  • Primary: the covalent linkages of the proteins
  • Secondary: the linear peptide chains fold either into an alpha-helical structure(coiled) or a beta-pleated structure(sheets) which contain hydrogen bonds.
  • Tertiary: The arrangement and interconnection of proteins into specific loops and bends forms the tertiary structures. This structure consists of hydrogen, ionic, and disulfide bonds.
  • Quaternary: this structure is proteins consisting of more than one peptide chain.
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Classification of Proteins

Classification of proteins is done on the basis of the following:

On the basis of the Constitution

Simple proteins:

They are composed of just amino acid residue. On hydrolysis, these proteins yield just constituent amino acids.

Conjugated proteins:

They are integrated with non-protein parts. E.g., Nucleoprotein, Phosphoprotein, Lipoprotein, Metalloprotein.

Derived proteins:

They are derivatives or degraded products of simple and conjugated proteins. They may be:

Primary derived protein: Proteans, Metaproteins, Coagulated proteins

Secondary derived protein: Proteosesn or albunose, peptones, peptides.

On the basis of shape

Fibrous proteins:

They include molecules having one or more polypeptide chains in the form of fibrils. The secondary structure is most important in them. They are insoluble in liquid media. They are non-crystalline and are elastic in nature. They perform structural roles in cells and organisms.

Examples are silk fiber (from silkworm, and spiders’ web) myosin (in muscle cells), fibrin (of a blood clot), and keratin (of nails and hair).

Globular proteins:

These are spherical or ellipsoidal due to multiple folding of polypeptide chains. Tertiary structure is crucial in them. They are soluble in liquid media such as salt solution, solution of acids or bases, or aqueous alcohol. They can be crystallized. They disorganize with changes in the physical and physiological environment.

Examples are enzymes, antibodies, hormones, and hemoglobin.

On the basis of nature of Molecules

Acidic proteins: They exist as anion and consist of acidic amino acids. e.g., blood groups.

Basic proteins: They exist as cations and are abundant in basic amino acids e.g., lysine, arginine, etc.

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Functions of Proteins

Proteins are vital for the development and growth, and their functions are unlimited. They also have an enormous variety of biological functions and are the most important end products of the pathways.

  • Proteins, which are made up of amino acids, serve in many functions in the body (e.g., as enzymes, structural parts, hormones, and antibodies).
  • They function as structural elements such as keratin of hair and nail, the collagen of bone, and so on.
  • Proteins are the molecular instruments through which genetic details are expressed.
  • They execute their activities in the transport of oxygen and CO2 by hemoglobin and unique enzymes in the red blood cells.
  • They function in the hemostatic control of the volume of the flowing blood and that of the interstitial fluids through the plasma proteins.
  • They are involved in blood clots through thrombin, fibrinogen, and other protein factors.
  • They act as the defense against infections by means of protein antibodies.
  • They perform hereditary transmission by nucleoproteins of the cell nucleus.
  • Ovalbumin, glutelin, and so on are storage proteins.
  • Actin, myosin serves as a contractile protein crucial for contraction.

Proteins are polymers of amino acids. The major constituents of proteins are Carbon, hydrogen, oxygen, nitrogen and sometimes sulphur. Proteins have physiological importance.

They are structural materials. Proteins act as chemical messengers (hormones). They act as receptors and carriers on cell membrane. Almost all enzymes are protein and have vital role in metabolic activities.

Each type of protein contains specific number and kind of amino acids which are arranged in a specific order that any minor change in the sequence can lead to abnormality or disorder.

The linear arrangement of amino acids in polypeptide chain is primary structure. The secondary structure is formed by repeating patterns of bonds. Tertiary structure is formed by folding of helix. In quaternary structure, double helix of proteins are folded and many chains join to form a larger protein.