Proteins are macromolecules composed of one or more polypeptides (chains of amino acids).

Chemically a protein is made of carbon, hydrogen, oxygen, nitrogen, and sulfur.

The nature of protein is determined by a sequence of 20 different amino acids. Amino acids are a group of organic compounds containing two functional groups, a basic amino group (-NH,) and acidic carboxyl group (-COOH).


Structure of Proteins

Every protein has a three-dimensional structure that can have up to four levels of organisation:

(i) The primary structure is the basic linear sequence of amino acids in a polypeptide chain.

(ii) The secondary structure is the development of new stearic relationships of amino acids present in the linear sequence inside
the polypeptides. These are of three types: a-helix, B-pleated and collagen helix. These are accomplished through
hydrogen bonding between amino acids.

(iii) Tertiary structure is the bending and folding of a secondary strand of the polypeptide to various types of structures. It is stabilized by several types of bonds - hydrogen bonds, ionic bonds, van der Waals interactions, disulfide, hydrophobic bonds, etc.

(iv) Quaternary structure is found in multimeric proteins. Each polypeptide develops its own tertiary structure and functions as subunits of protein, e.g., hemoglobin.

On the basis of the constitution, proteins are of three types :

(i) Simple proteins are made up of amino acids only. Additional non-amino groups are absent, e.g., histones, keratin.

(ii) Conjugated proteins have non-amino prosthetic groups like metals or ions.

(iii) Derived proteins are obtained from proteins through denaturation, coagulation and breakdown, e.g., fibrin.

On the basis of shape, proteins are divided into two categories fibrous and globular proteins. Fibrous proteins are elongated,thread-like structures that have no tertiary structure. Globular proteins are rounded or oval in outline obtained by tertiary and quaternary foldings of polypeptides.

Functions of Proteins

Proteins are necessary in building and repairing body tissues.

They transport materials throughout the body


Post By : Preeti Rai 27 Dec, 2019 2186 views Biology