Protein- Definition, Classification, Structure, and Function


Protein is the most abundant organic molecules present in the body of living organism. Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are require for the structure, function, and regulation of the body’s tissues and organs. Chemically proteins are composed of amino acids, which are organic compounds made of carbon, hydrogen, nitrogen, oxygen, and sometimes sulfur. There are 20 different types of amino acids that can be combine to make a protein. 

Composition: Hydrogen, nitrogen, carbon, oxygen, and sometimes sulfur.

Structural unit: Protein are made up of small units called amino acid.

Amino acid: Amino acids are molecules that combine to form protein. Amino acids and proteins are the building blocks of life.

Classification of Protein

1- On the basis of their shape proteins can be classified into two types i.e.- Fibrous proteins and Globular proteins.

Fibrous proteins:-  Fibrous proteins are made up of polypeptide chain that are elongated and fibrous in nature or have a sheet like structure. They are generally insoluble in water but soluble in concentrated acids and alkalis. They are contractile.  The fibrous proteins are fundamental components of structure tissues such as tendons, bone, hair, horn, leather, claws, and feathers. e.g.- Keratin, collagen, elastin, fibrin. 

Globular Proteins:- Globular proteins are polypeptide chains are compactly folded to form globular or spherical shaped proteins. They are generally soluble in water, dilute acids, alkalis and salts. They are non contractile. e.g.- albumin, insulin, hemoglobin, myoglobin, amylase and immunoglobulins.

2-On the basis of their chemical composition protein can be classified into three types i.e.- Simple proteins, Conjugated proteins, and Derived proteins.

Simple proteins:- This proteins are made up of amino acids and their derivatives. Example- globulins, albumin, histones, collagen, ribonuclease enzyme.

Conjugated proteins:- This are the proteins which are combined with a non protein molecules. Example- hemoproteins, nucleoproteins, lipoproteins, glycoproteins opsin.

Derived Proteins:- They are derivatives or degraded products of a simple and conjugated proteins. Example- peptides, peptones.

Levels of protein structure

Due to different rearrangement of amino acids, four level of structure organization.

Primary structure:- Protein structure is a sequence of amino acid in polypeptide chain held together by peptide bond only.

Secondary structure:- Polypeptide chain is linked by peptide bond as well as hydrogen bond.

Tertiary structure:- The overall 3-Dimensional shape of a protein. It is generally stabilized by outside polar hydrophilic hydrogen and ionic bond interactions, and internal hydrophobic interaction between nonpolar amino acid side chains.   

Quaternary structure:- Aggregation of two or more polypeptide chain, into a single functional unit it does not apply to every protein.

Function of Proteins:- Protein has many roles in your body.

  • Proteins are the major source of energy that helps in movements in our body.
  • It helps in growth and maintenance.
  •  Antibody formation.
  • It build and repair all body tissues.
  • Acid-base regulation.
  • Skin and bone containing collagen, a fibrous protein.
  • Muscle contraction (Actin and Myosin).
  • Regulate fluid balance.
  • Regulate pH balance.
Example of proteins:- 
  • Fish.
  • Eggs.
  • Seafood.
  • Lean meat and poultry.
  • Tofu.
  • Nuts.
  • Beans.
  • Milk.
  • Cheese.


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