Proteins -  Definition, Types, Examples

Proteins are highly complex nitrogenous compound found in all living organisms. They form the structural and functional basis of life. Proteins are necessary for the growth and repair of bodily tissues. They also offer energy when calories are low. Protein is composed of carbon, hydrogen, nitrogen, and oxygen. Proteins are incredibly large molecules composed of monomers known as amino acids. Amino acids are the building blocks of proteins. There are twenty standard amino acids. Protein-rich foods include milk, fish, meat, eggs, nuts and dal. Soyabean seeds posses highest percentage of proteins (30-35%). Protein deficiency causes kwashiorkar - skin cracks and become scaly, abdomen swells, hair become reddish. Deficiency of protein and carbohydrate results in Marasmus.

Types of Proteins

There are seven types of proteins. They are antibodies, contractile proteins, enzymatic proteins, hormonal proteins, structural proteins, storage proteins and transport proteins

1. Antibodies

A protein produced by animal plasma cells as a result of the presence of an antigen. Antibody production is maintained by contact with the antigen; subsequent exposure to the antigen produces a greater antibody responses which provides the basis of immunity. Antibodies contribute to the body's resistance to infection and are responsible for the rejection of foreign tissue or organ transplants. Antibodies are an example of proteins that attach to one specific type of molecule. 

2. Contractile Proteins 

Contractile Proteins are responsible for the function of muscle contraction and movement. Cell cytoplasm is a colloidal network made up of contractile proteins.

3. Enzymatic Proteins

The most important proteins are the enzymes. Protein present in hair, skin and nails is keratin. Protein present in silk is fibroin. The first pure enzyme to be crystallized was Jackbean urease, prepared by James Batcheller Sumner (US) in 1926. Sumner's work proved that enzymes are proteins and that proteins can act catalytically. Enzymatic proteins are enzymes that speed up chemical reactions in our body.

4. Hormonal Proteins

Hormonal Proteins are messenger proteins that assist coordinate various body functions. Growth factors, Testosterone and Cortisol are examples of Hormonal Proteins.

5. Structural Proteins

Structural Proteins provide support in our body. They maintains and protects the structure of the our body. For example, the proteins in our connective tissues, such as collagen and elastin.

6. Storage Proteins

Storage Proteins store metal ions such as iron in our body. Casein is a storage protein found in egg whites. Examples of other storage proteins include Ferritin, which is a storage protein that stores iron.

7. Transport Proteins

Transport Proteins are carrier proteins that carry molecules from one place to another in the body. Hemoglobin and Cytochromes are examples of Transport proteins. The respiratory protein hemoglobin transports oxygen from the lung to body organs and tissues. Cytochromes functions as an electron carrier protein.