NEET Chemistry Notes Biomolecules – Proteins

NEET Chemistry Notes Biomolecules – Proteins

Proteins

Proteins

Proteins are polymers of amino acids (the compounds which have both the acid and amino group). The total number of amino acids that have been found in proteins are twenty.
On the basis of their synthesis, amino acids are divided into two classes :

1. Essential Amino Acids are the amino acids which cannot be synthesised in the body and must be obtained through diet, e.g. valine, leucine, lysine, isoleucine, arginine etc. ’
2. Non-essential Amino Acids are the amino acids which can be synthesised in the body, e.g. glycine, alanine, glutamic acid, aspartic acid etc.

•  Amino acids behave like salts rather than simple amines or carboxylic acids. This is due to the presence of both acidic and basic groups in the same molecule.
  
•  At a certain pH of the medium, called the isoelectric point of an amino acid, the structure behaves as a dipolar ion and does not migrate to any electrode on passing current.
• Proteins give biuret test, Millon’s test, ninhydrin test. Those proteins give Molisch’s test, which contain a carbohydrate group. Carbohydrates also gives Molisch’s test.

Peptide Bond

Amino acids may be joined together by an amide linkage called peptide linkage (—CO—NH—).

The molecule derived from two amino acids containing a single peptide linkage is called a dipeptide, that derived from three amino acids is termed as a tripeptide.
The peptides having 2-10 amino acid residues are called oligopeptides while those with greater than 10 amino acid residues are called polypeptides.
Polypeptide with molecular weight greater than 10,000 u is termed as a protein. Proteins generally have more than 70 amino acid residues, but a polypeptide with fewer α – Amino acids may also be called a protein if it has a well defined conformation characteristic of a protein such as insulin

Classification of Proteins

Proteins can be classified into two types on the basis of their molecular shape.

1. Fibrous proteins Polypeptide chains form fibre like structure, e.g. keratin and myosin etc.
2. Globular proteins This structure results when the chains of polypeptides coil around to give a spherical shape. These are usually soluble in water, e.g. insulin and albumins.

Structure of Proteins

N-terminal of one amino acid combines with C-terminal of other amino acid and forms peptide bond (—NHCO—).
In the same way, many amino acids combine to each other and form polypeptide bonds. Polypeptides with fewer amino acids are likely to be called proteins. Structure and shape of proteins may be discussed at four different levels.

1. Primary Structure
   Proteins may have one or more polypeptide chains. Specific sequence of amino acids in a chain gives the primary structure of that protein. Any change in this structure gives a different protein.
2. Secondary Structure
   The polypeptide chains are linked by hydrogen bonds. They are found to exist in two different types of structures α – helix and β pleated sheet structure.
3.  Tertiary Structure
   It has polypeptide bonds, hydrogen bonds, disulphide linkages, van der Waals’ forces and electrostatic forces of attraction. It gives rise to two major molecular shapes viz. fibrous and globular.
4.  Quarternary Structure
   Some of the proteins are composed of two or more polypeptide chains referred to as subunits. The spatial arrangement of these subunits with respect to each other is known as quarternary structure.

Denaturation of Proteins

Disturbance of hydrogen bonds either by acids or alcohols or heat, results in unfolding of globules. Thus, helix get uncoiled and protein loses its biological activity due to change in temperature or pH. This is called denaturation of proteins. During denaturation, secondary and tertiary structures of proteins are destroyed while primary structures remains intact.

Enzymes

Enzymes are globular protein bodies, which are biological catalysts. Enzyme inhibitors reduce the activity of a particular enzyme. These are mostly inorganic ions or complex organic molecules.
Congenital and albinism diseases are caused by the deficiency of the enzymes phenyl ketone urea and tryosinase respectively.

Vitamins

Organic compounds required in the diet in small amounts to perform specific biological functions for normal maintenance of optimum growth and health of the organism, are termed as vitamins.

Types of Vitamins

• Fat or oil soluble vitamins, e.g. A D, E and K.
• Water soluble vitamins, e.g. B group vitamins and vitamin C.

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