Proteins: Class 12 Chemistry Notes, Definition, Types, Benefits, Properties & Examples

As per the NCERT definition, "When the number of such amino acids is more than ten, then the products are called polypeptides. A polypeptide with more than hundred amino acid residues, having a molecular mass higher than 10,000u is called a protein".

For example: Consider a jewellery. The bead in the jewellery resembles an amino acid, and the thread holding the bead together is a peptide. The complete chain formation is called a polypeptide. When the number of amino acids is higher, the mass becomes higher, and a long chain form is called a protein. Proteins are of various types. They are classified based on their shape and composition.

Proteins are of various types. They are classified based on their shape and composition.

Based on shape

Fibrous Proteins

They have long, thread-like structures and remain insoluble in water. Usually, we know these are classified into collagen, keratin, and myosin.

Globular Proteins

Globular proteins are spherical in shape. Characterised as soluble in water, they have several different functions. They are responsible for catalyzing reactions and also transporting molecules from one place to another. Most of the biological processes are regulated because of these types of proteins. We can see them as enzymes, haemoglobin, antibodies, and hormones as examples.

Based on Composition

Simple proteins 

The simple proteins are made only of amino acids. The example of simple proteins includes globulin, histones, and albumin. 

Conjugated Proteins

Conjugated proteins are formed by the combination of amino acids and non-protein components. They belong to a prosthetic group. The examples of conjugated proteins include chromoproteins, lipoproteins, glycoproteins and nucleoproteins.

Proteins are biomolecules which is made up of a long chain of amino acids. The shape of the protein is folded and twisted. There are four hierarchical levels of protein structure.

Primary Structure

In the primary structure of proteins, amino acids are arranged in sequence. This order of amino acids is decided by the genetic code in our body. The properties of proteins can be affected by a single change in the sequence of amino acids.

Secondary Structure

The secondary structure of a protein is formed when the amino acid chain folds into repeated patterns. This shape is stabilized by hydrogen bonds present in amino acids. There are two main types of secondary structure:

• Alpha helix (𝛼-helix)
  • An alpha helix is like a spiral shape. It is formed by hydrogen bonding between amino acids.
• Beta Sheet (𝛽-pleated Sheet): 
  • This structure is formed by hydrogen binding between two or more adjacent polypeptide chains.

Tertiary Structure

The entire structure of protein folds to form a 3D shape because of the R-group (side chains) of amino acids. These amino acids are hydrogen bonds, ionic bonds, hydrophobic interactions, disulfide bridges and Van der Waals forces.

Quaternary Structure

Quaternary structure is found in proteins that have multiple polypeptide chains (subunits). This structure describes how subunits are arranged and interact. The example of quaternary structure is haemoglobin.

 

 

 

Here we have discussed the physical and chemical properties of proteins.

Physical Properties

• Proteins are large biomolecules with a high molecular weight. There are two types of protein, including fibrous and globular. Fibrous proteins are insoluble in water, while globular proteins are water-soluble.
• Proteins form colloidal solutions in water. Also, they contain both acid and basic groups. This means proteins can act as buffers.

Chemical Properties

• In the hydrolysis process, proteins break down into peptides and amino acids.
• When you react protein with copper sulfate in an alkaline solution, you will see a violet colour.
• When aromatic amino proteins are mixed with concentrated nitric acid, they give a yellow colour.
• Tyrosine proteins, when mixed with million's reagents, will give a red colour.

Proteins are the most essential biomolecules in living organisms. The benefits of proteins are mentioned below.

• Building and repairing tissues
• Maintaining the overall function of the body
• Transporting nutrients and oxygen
• Maintaining fluid balance
• Producing enzymes and hormones
• Providing energy
• Maintain the proper pH in the body

Proteins are an important biomolecule that are found in various food sources. Some examples of proteins are

Proteins	Sources
Animal-based proteins	Dairy products - milk, cheese, yogurt Meats - lamb, pork, beef,   Egg -  Fish - tuna, salmon Poultry - chick, duck, turkey
Plant based protein	Nuts and seeds - almonds, peanuts, sunflower seeds, walnuts Legumes - lentils, chickpeas, peas and beans Soy Products - Tofu, tempeh, and edamame