PROTEIN

Protein

·         Protein name is derived from Greek word proteios means pre-eminent or first.

·         This name was suggested by a Swedish chemist Berzelius.

·         Proteins are macromolecule which make up half the total weight of  biomolecule in a cell.

·         Proteins are also known as building blocks of life which are composed of some smaller units known as amino acid.

 

        Structure of protein   

Proteins are made up of monomers called amino acid. There are 20 different kinds of amino acid found in nature and 21st amino acid is Selenocysteine. Amino acids are basically composed of four components:

 i.            A central carbon with one hydrogen

 ii.            One carboxyl group

   iii.            One amino group

  iv.            One R group

 

Carbon needs four bonds in which R group is anonymous i.e. R group may be hydrogen or a hydrocarbon chain. R group can be polar, non- polar or even charged.

Amino acids bind to each other by dehydration process in which water is removed as a byproduct where OH group of one amino acid(COOH) combines with H of second amino acid(NH2) resulting in the formation of covalent(peptide) bond.

When many amino acids bind together they form a polypeptide chain.

 

                 H2N-CH-C-OOH+H-NH-CH-C-OOH

                               R                             R

                                                                               H2O

                                H2N-CH-CO-NH-CH-C-OH

                                                                                         PEPTIDE BOND

Proteins have four level of structure. Such are as follows:

         i.            Primary structure

       ii.            Secondary structure

      iii.            Tertiary structure

     iv.            Quaternary structure

 

         i.  Primary structure:  In primary structure a long string or a linear sequence of amino acid are found.

                                             Eg:  Gly-Ile-Ala-Val-Lys

       ii. Secondary structure: It is arise when intermolecular forces especially H-bonds between amine and                                                       carboxyl group of amino acid cause primary sequence to bend or twist.                                                               Secondary structure also characterized into two types:

a)      Alpha helix: When secondary structure is found in the form of twists, it is called alpha helix.

b)      Beta-Pleated sheets: When a bend is found in the structure, it is called β-pleated sheets.

 

 

      iii. Tertiary structure: When primary and secondary bend and fold upon themselves, they give rise to                                                     tertiary structure. This is caused by the intermolecular forces (hydrophobic                                                           interaction or H-bonding), ionic bonding and covalent disulphide bridge.

 

     iv. Quaternary structure: Some proteins join together in order to carry out their function. They results once                                                all the interacting subunits of proteins have clumped together. Here multiple                                                          polypeptides interact with each  other to fold.

                                                Eg: Collagen, Haemoglobin

 

 

       Characterstics of Protein:

   i.   Proteins are organic substance made up of carbon, hydrogen, oxygen, nitrogen.

   ii.  Proteins are the fundamental constituent of cytoplasm of cell.

   iii.  They are structural elements of body tissues.

   iv. Proteins are polymers which are made up of monomer amino acid.

    v. Their molecular weight is 5-300KD.

Classification of protein

Proteins are classified according to their shape:

a) Globular or corpuscular protein: They are compactly folded protein possess relatively spherical shape.

Eg: Insulin, Albumin, Globulin

b) Fibrous protein: When proteins resemble ribbons or fibres in shape, they are called fibrous protein.

Eg: Collagen, Keratin

 

Function of protein:

  i.  Proteins are enzymes. They function as a catalyst for example Ribonuclease which accelerate       specific chemical reaction upto many times faster than they would spontaneously occur.

 ii.  They also function as structural materials like Keratin protein found in hairs and nails and               Collagen found in connective tissue.

 iii.  Proteins are antibodies which have specific binding that specifically bind to foreign substance       which fight against diseases and protect our body.

 iv. They also serve as a specific carrier including membrane transport protein that move                     substance across cell membranes. Blood protein Haemoglobin(Hb) carry oxygen and                 transport it to blood.

 v.   Proteins also aid in contraction (actin,myosin) fibres that interact in muscle tissue.

 vi.  Proteins also involve in signaling like insulin which regulate blood glucose level.

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