Classification and Function of Protein

Classification of protein

There is no universally accepted classification of protein because there is no hard and fast rule for protein classification. For the classification of protein different criteria are used which are described below:

The brief description of classification of proteins are as follows:

A) Solubility

B) Shape

C) Co-factor

D) Nutritional value

E) Function

A) Solubility:

Based on the solubility , proteins are classified into two groups:

1) Water soluble

2) Water insoluble

1) Water soluble proteins:

It includes the followings proteins;

a) Albumin:

They are widely distributed in nature but more abundant in seeds. They are soluble in water and the dilute solution of acids, bases, and salts.eg Serum, albumin, ova albumin(egg) ,Lactalbumin (milk)

b) Globulins: These are two types:

1. Pseudoglobulin
2. Euglobulin

The euglobulin is more widely distributed in nature than pseudo globulin.They are coagulated by heat.

eg: Pseudoglobulin: Globulin of milk whey

Euglobulin : Serum globulin from blood plasma

c) Protamines: These are basic proteins and occurs almost entirely in animals mainly in sperms cells .They are not coagulated by heat. eg: Salminen from Salmon sperm.

2) Water insoluble proteins:

a) Prolamines: These are isolated from plants seeds and are water insoluble but soluble in dilute acids and alkali .These are not coagulated by heat. eg: Gliadin (Wheat), Zein from maize.

b) Scleroproteins: They are also called fibrous proteins. These occur entirely in animals and therefore commonly called as scleroprotein. eg: collagens of bones,keratins of hair and nail ,elastin in ligaments.

B) Based on shape:

On the basis of shape of protein molecules , the protein have been divided into two main categories:

a) Globular protein:

These proteins have an axial ratio (length: width ) less than 10 and hence contain a relatively spherical or ovoid shape. They are usually soluble in water and in aqueous media containing acid , base, salt and alcohol. The globular proteins are more complex in configuration than fibrous proteins. They have the greater variety of function , nearly all enzymes are globular . Examples: Haemoglobin.

b) Fibrous proteins:

These proteins have the axial ratio greater than 10 . These are fibre like in shape and insoluble in water and resistance to digestion. These are mainly of animal origin. Most fibrous proteins serve as structural components of our body eg; Keratins, elastins, collagens. The collagens and elastins are found as a component of skin , connective tissues and blood vessels wall whereas keratin is found in skin and hair. The important fibrous proteins are:

1. Collagens
2. Elastins
3. Keratins
4. Fibroin

1. Collagens:It is the most abundant protein in our body and it is the major protein of connective tissues ( tendons and cartilage). The 3 polypeptide chains are held together for the formation of collagens.
2. Elastins: It is the major constituent of yellow elastins tissues (blood vessels and ligaments) .It is rubber like proteins
3. Keratins: It is a tough fibre protein and mainly found in skin ,hairs, feather , horns, and nails.
4. Fibrions: It is a principle constituent of the fibre of skin and composed of main glycine, alanine,and serine units.

C) Based on co- factor:

On the basis of presence or absence of cofactor in the protein molecule, they are classified as either simple or conjugated proteins.

1) Simple proteins:

The proteins which contain only amino acids eg: different enzymes

2) conjugated proteins:

The proteins which contain non-amino acid part. The conjugated proteins are further divided or classified as:

a) Metalloproteins: The proteins which contain metallic ions eg: Siderophilin which contains iron.

b) Chromoprotein: The protein which contains coloured pigments. eg; haemoglobin and cytochrome.

c) Glycoproteins: The protein which contains carbohydrate part as prosthetic group eg : egg albumin.

d) Phosphoproteins: Proteins which contains a phosphoric group eg: Casein in milk and Vitelline in egg yolk.

e) Lipoprotein : The proteins which consist of lipids as co factor eg: membrane lipoprotein, serum lipoprotein.

f) Nucleoprotein:The proteins which are conjugated with nucleic acids eg: histone proteins.

D) Based on nutritional value:

According to this classification proteins are classified as:

a) First class proteins: Meat protein

b) Second class proteins: Vegetable protein

E) Based on functions :

Based on their functions proteins can be classified as;

a) storage proteins

b) Enzymatic proteins

c) Structural proteins

d) Transport proteins

e) Regulatory proteins

f) Contractile proteins

g) Repressor proteins

Functions of proteins:

protein is a versatile molecule so it contains the diverse function which is given below;

1) Enzymatic functions:

Most of the enzymes are proteins is nature and they are involved in thousands of different biochemical reactions. Example; enzyme involve in glycolysis.

2) Structural function:

Many proteins serve as supporting filaments of seeds to give biological structure strength and protection. The major components of tendons and cartilage are the fibrous proteins, collagens, ligaments contain elastin in hair, fingers nail and feathers contains the insoluble protein keratins.

3) Transportation function:

Different proteins are involved for transportation of molecules eg; haemoglobin of erythrocytes binds with oxygen and transport to peripheral tissue.

4) Storage and nutritional functions:

The seeds of many plants store different nutrients proteins required for further growth of germinating seedlings. The protein present in milk ,meat ,egg, etc serves as nutritional proteins.

5) Defence proteins ( immunoglobulin / defence function):

Many proteins are involved to defence organism against invasion (entry) by the other species and protect them from injury. The immunoglobulins are involved for the neutralisation of invading bacteria , viruses,and foreign proteins from another species.

6) Contraction and motile function:

Some proteins are involved in the contraction and relaxation of the muscles , Myosin present in skeleton muscles. The flagellin protein of flagella helps in motility.

7) Regulatory function:

Some proteins are involved in the regulation of cellular or physiological activities eg; many hormones ( insulin) which regulate the sugar metabolism.

8) Generation and transmission of nerve impulse :

The response of nerve cell to specific stimuli is mediated by receptor proteins .eg: Rhodopsin present in a retinal nerve cell.

9) Control growth and differentiations:

Controlled sequential expression of genetic information is essential for the ordinary growth and differentiation of cells . Only a small fraction genome of the cell is expressed at any one time. In bacteria, repressor protein is important to control element that controls the specific segments of the DNA of a cell.

Structural organisation of protein:

Protein structure is the three-dimensional arrangement of atoms in protein molecules. Proteins are polymer specially polypeptides formed from sequences of monomer amino acid . By convention, a chain of 40 amino acids is often identified as a peptide, rather than a protein. To ba able to perform their biological function , proteins fold into one or more specific spatial conformations driven by a number of non-covalent interaction such as hydrogen bonding , ionic interactions, Vander Waals forces and hydrophobic packing . To understand the function of proteins at a molecular level, it is often necessary to determine their three-dimensional structure . This is the topic of the scientific field of structural biology, which employs techniques such as x-ray crystallography, NMR spectroscopy, and dual polarisation interferometry to determine the structure of proteins.

Protein structure range in size from tens to several thousand amino acids. By physical size , proteins are classified as nanoparticles, between 1-100nm . Very large aggregates can be formed from protein sub units. for eg many thousands of actin molecules assemble into microfilaments.

A protein may undergo reversible structural changes in performing its biological function. The alternative structures of the same protein are referred to as different conformations, and transitions between them are called conformational changes.

References

Cassida, L.E Jr. Industrial microbiology. New age into publishers, 1996.

I, Stever. Biochemistry. new york: Wall freeman company, 1995.

JE, Smith. Biotechnology. Sinaeur Association, 2000.

Nelson, D L and M M Cox. Lihininger Principle of Biochemistry. Fifth. Freeman publication, 2004.