Classification of Proteins

What is Protein?

Before we shall go for our discussion about the classification of proteins we shall get some idea about the protein. Proteins are polymers of amino acids. Each protein is composed of a large number of amino acid. Naturally, they are essentially made up of C, H, O and N. In addition, most proteins contain S and some contain P. Other elements like I, Fe, Cu, Zn, etc., are also occasionally present.
In a protein, the amino acids are arranged in series through the acid-amide type of bond (CO-NH) called peptide linkage which connects the carboxyl (-COOH) group of one amino acid to the amino (-NH2) group of the next amino acid by removal of one molecule of water. Hydrolysis of proteins involves splitting, of peptide linkages. A chain of amino acids formed by peptide linkages is referred to as a peptide chain. Peptide chains are classified on the basis of the number of amino acids present in the molecule.

Classification of proteins

According to the following four matters the classification of proteins has been done:
1) Structure
2) Nutritional value
3) Physiological functions
4) Physical and chemical properties.
Now let us go for some details:

Classification of Proteins according to structure

According to structure, proteins are of two types —
i) Globular
ii) Fibrous.

Globular proteins

In globular proteins, the polypeptide chains remain compactly folded so that the molecule acquires a spherical shape. The tertiary structure of such proteins is very prominent. These proteins are more easily soluble in water. Examples of globular proteins are enzymes, antibodies, albumin etc.

Fibrous proteins

In fibrous classification of proteins, the polypeptide chains remain arranged parallely and cross-linked at intervals. They are insoluble in water. In such proteins, the secondary structures are high up and strong but tertiary structures are weak. Examples are: - keratin, fibrin, myosin, collagen etc.

Classification of Proteins According to their nutritional value

According to their nutritional value, dietary proteins are divided into two groups, those are: -
i) First class proteins
ii) Second class proteins

First class proteins

First class proteins are those that contain all the essential amino acids. Hence, these are also called complete proteins. Animal proteins i.e., the proteins found in meat, fish, egg, milk etc.,, belong to this category. However, the protein found in soybeans is also of this type.

Second class proteins

Second class proteins are also called incomplete proteins because they are deficient in one or more essential amino acid(s). Plant proteins are generally of this type.

Classification of proteins According to physiological functions

As per physiological functions, proteins may be classified into the following types:
i) Enzyme proteins (dehydrogenases, oxidases etc.).
ii) Structural proteins (collagen, elastin, keratin etc.).
iii) Contractile proteins (actin, myosin of muscles).
iv) Transport proteins (hemoglobin, hemocyanin etc.).
v) Genetic proteins (nucleoproteins like DNA protein & RNA protein).
vi) Hormone proteins (STH, ACTH. insulin etc.).
vii) Immune proteins (gamma globulins).

Classification of proteins According to physical and chemical properties

Most widely used classification of proteins is based on their physical and chemical properties such as solubility, heat coagulability, precipitation, composition, etc. According to this system, proteins are classified into three main groups—simple proteins, conjugated proteins and derived proteins; each of these groups is subdivided into a number of classes.
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