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"Proteins" - Definition, Structure, & Classification

news-details Image Source Oct 15, 2020 11:37 IST , Updated: Sep 04, 2021 04:37 IST · 6 min read

Basically, proteins are the fundamental building blocks and most abundant molecules present in living system, they forms about 60% of the dry weight of cells. Our hair and nails, most cells, enzymes, regulatory and structural parts of the body are made up of proteins.

Proteins are required for growth and maintenance of the body, they also provide strength to the body in producing hormones, enzymes, and other metabolic chemicals.

Food like eggs, pulses, milk and other milk products form the major high protein foods for the body.

Proteins play multiple functions in the body: Digestion (proteinaceous digestive enzymes), Movement (Myosin Protein), Structure (Keratin Protein), Cellular communication (receptors are made of proteins), Messenger (communication between the cells, tissues, and organs)etc.

Protein structure

Protein structure is the three-dimensional arrangement of atoms in an amino acid-chain molecule. Proteins are polymers - specifically polypeptides - formed from sequences of amino acids, the monomers of the polymer.

The structure and position of amino acids gives particular properties to the proteins. Only α-amino acids are obtained on hydrolysis of proteins.

Amino acids are organic compounds (substituted methanes) containing an amino group [NH2] and an acidic group [COOH] as substituents on the same carbon i.e., the a-carbon.

Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral, more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic.

Amino acids are usually colorless, crystalline solids, these are water-soluble, high melting and behave like salts rather than carboxylic acids.

Fibrous proteins

When the polypeptide chains run parallel and are held together by hydrogen and disulphide bonds, then an elongated fibre-like structure is formed.

Such proteins are generally insoluble in water, some common examples are keratin (present in hair, wool, silk) and myosin (present in muscles), etc.

Globular proteins

When the chains of polypeptides coil around to give a spherical shape, then a compact structure is formed.

Such proteins are usually soluble in water, insulin and albumins are the common examples of globular proteins.

Protein Levels

Fibrous and Globular proteins may comprise of one of four types of protein structures:

1) Primary structure: The primary structure of a protein is the linear polypeptide chain formed by the amino acids in a particular sequence. Changing the position of even a single amino acid will result in a different chain and hence a different protein.

2) Secondary structure: The secondary structure of a protein is formed by hydrogen bonding in the polypeptide chain. These bonds cause the chain to fold and coil in two different conformations known as the α-helix or β-pleated sheets.

3) Tertiary structure: The tertiary structure is the final 3-dimensional shape acquired by the polypeptide chains under the attractive and repulsive forces of the different R-groups of each amino acid, this coiled structure is very necessary for protein functions.

4) Quaternary Structure It is the arrangement of multiple folded protein subunits in a multi-subunit complex.

Protein Synthesis

Protein bio-synthesis is a biological process, occurring inside cells, balancing the loss of cellular proteins through the production of new proteins. This process is actually consists of two processes - transcription and translation.

During transcription, DNA is used as a template to make a molecule of messenger RNA (mRNA). The molecule of mRNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs.

During translation, the genetic code in mRNA is read and used to make a protein. DNA > RNA > Protein.

Protein Test

To test a food item for protein, make a paste of it or powder it. Put some of this in a clean test tube, add 10 drops of water to it and shake the test tube. Now, using a dropper, add two drops of solution of copper sulphate and ten drops of solution of caustic soda to the test tube.

Did the contents of the test tube turn violet - A violet colour indicates presence of proteins in the food item.

Protein Facts

Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein in the whole of the biosphere.

Collagen is the most abundant protein in the human body, found in the bones, muscles, skin, and tendons (attaches muscle to bone). It is the substance that holds the body together. Collagen forms a scaffold to provide strength and structure.

Trypsin is an enzyme that helps us digest protein. In the small intestine, trypsin breaks down proteins, continuing the process of digestion that began in the mouth. It may also be referred to as a proteolytic enzyme, or proteinase. Trypsin is produced by the pancreas in an inactive form called trypsinogen.

Insulin is a peptide hormone produced by beta cells of the pancreatic islets, it is considered to be the main anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of glucose from the blood into liver, fat and skeletal muscle cells.

Keratin is the type of protein that makes up your hair, skin, and nails. Keratin can also be found in your internal organs and glands. Keratin is a protective protein, less prone to scratching or tearing than other types of cells your body produces.

Myosins are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility (move) processes in eukaryotes.

Tubulin is the protein that polymerizes into long chains or filaments that form microtubules, hollow fibers which serve as a skeletal system for living cells. Tubulin-binding drugs kill cancerous cells by inhibiting microtubule dynamics, which are required for DNA segregation and therefore cell division.

Myoglobin is an iron- and oxygen-binding protein found in the skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin.

Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils.

Eukaryote, any cell or organism that possesses a clearly defined nucleus. The eukaryotic cell has a nuclear membrane that surrounds the nucleus, in which the well-defined chromosomes (bodies containing the hereditary material) are located. Eukaryotic cells are found in plants, animals, fungi, and protists.


Enzymes are proteins that act as biological catalysts (biocatalysts) to accelerate chemical reactions.

The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products.

Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life.

Enzymes are known to catalyze more than 5,000 biochemical reaction types.

Other biocatalysts are catalytic RNA molecules, called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures.

Some enzymes are used commercially, for example, in the synthesis of antibiotics.

Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, and enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew.

Related Questions
  1. Which of the following substances is not a protein ?

    A) Cotton

    B) Hair

    C) Spider Silk

    D) Hoof

    Show answer

    UPPSC 2019

  2. In human body digestion of Protein begin in which of the following organs ?

    A) Liver

    B) Mouth

    C) Small Intestine

    D) Stomach

    Show answer

    UPPSC 2020

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