Proteins are among the most important biomolecules found in living organisms. They are polymers of α-amino acids linked together through peptide bonds. Proteins are essential for growth, repair, and maintenance of body tissues. They also perform various biological functions such as catalysis, transport, protection, and regulation.
Amino Acids: Building Blocks of Proteins
Proteins are composed of smaller units known as amino acids. Every amino acid contains an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a side chain (R group) attached to the α-carbon atom. The side chain differs from one amino acid to another and determines its properties.
Examples:
- Glycine – Simplest amino acid.
- Alanine – Contains a methyl group as the side chain.
Classification of Amino Acids
Amino acids are classified on the basis of whether they can be synthesized by the human body or not.
1. Essential Amino Acids: These amino acids cannot be synthesized by the body in sufficient amounts and must be obtained through food. They are necessary for growth, tissue repair, and proper functioning of the body.
Examples: Valine , Leucine , Lysine , Methionine ,Tryptophan
2. Non-Essential Amino Acids: These amino acids can be synthesized by the body. They support normal metabolic activities and protein synthesis.
Examples: Glycine , Alanine , Serine , Aspartic acid
Peptide Bond Formation
Proteins are formed when amino acids join together through peptide bonds.
- A peptide bond is formed between the carboxyl group of one amino acid and the amino group of another amino acid with the elimination of a water molecule.
- This reaction is known as a condensation reaction.
- The linkage formed (–CO–NH–) is called a peptide bond.
Example: Glycine + Alanine ⟶ Glycylalanine + Water
Dipeptides, Tripeptides and Polypeptides
Depending on the number of amino acids linked together, different peptide molecules are formed.
1. Dipeptides: A dipeptide is formed when two amino acids are joined together by a single peptide bond. During this process, the carboxyl group of one amino acid reacts with the amino group of another amino acid, resulting in the elimination of one molecule of water.
Example: Glycylalanine
2. Tripeptides: A tripeptide is formed when three amino acids are linked together through two peptide bonds. The amino acids are arranged in a specific sequence, which influences the properties of the peptide.
Example: Glycylalanylglycine
3. Polypeptides: A polypeptide is a long chain formed when many amino acids are linked together through numerous peptide bonds. The sequence and number of amino acids determine the structure and function of the resulting protein.
Example: Protein chains present in haemoglobin and insulin.
Classification of Proteins
Proteins are classified according to their shape and biological functions. They are broadly divided into two main categories fibrous proteins and globular proteins. These two types differ in their structure, solubility, and biological functions.
1. Fibrous Proteins
Fibrous proteins consist of long, thread-like polypeptide chains arranged parallel to one another.
- These proteins are usually insoluble in water and are mainly responsible for providing mechanical support and strength to tissues.
- Due to their elongated structure, they serve structural functions rather than metabolic functions.
- They have Long and fibre-like structure.
- Proteins are generally insoluble in water.
- They provide strength and support.
Examples:
- Keratin – Found in hair, nails, horns, and wool.
- Collagen – Found in skin, tendons, and connective tissues.
2. Globular Proteins
Globular proteins have polypeptide chains folded into compact, spherical shapes.
- These proteins are generally soluble in water and perform a variety of biological functions such as transport, catalysis, regulation, and immunity.
- These are compact and spherical structure.
- They are also soluble in water.
- These proteins are biologically active.
- They are involved in metabolic processes.
Examples: Hemoglobin , Insulin , Albumin , Most enzymes
Structure of Proteins
The biological properties and functions of proteins depend upon their structure. Proteins are not simply long chains of amino acids; they possess specific levels of structural organization.
1. Primary Structure
The primary structure of a protein refers to the specific sequence in which amino acids are arranged in a polypeptide chain.
- Amino acids are linked together through peptide bonds to form the chain.
- Each protein has a unique amino acid sequence, which determines its properties and biological function.
- The primary structure serves as the foundation for all higher levels of protein structure.
Example: Insulin possesses a specific amino acid sequence necessary for its function.
2. Secondary Structure
The secondary structure refers to the regular folding of the polypeptide chain due to hydrogen bonding between peptide groups. This folding gives the protein a more stable arrangement.
Main Types of Secondary Structure:
a) α-Helix Structure: In this structure, the polypeptide chain coils into a right-handed spiral resembling a spring. Hydrogen bonds between different parts of the chain help maintain the helical shape.
Example: Keratin found in hair and wool.
b) β-Pleated Sheet Structure: In this structure, polypeptide chains are arranged side by side, forming sheet-like structures. Hydrogen bonds between adjacent chains provide stability.
Example: Silk fibroin.
3. Tertiary Structure
The tertiary structure refers to the overall three-dimensional folding of a polypeptide chain. The folded shape results from interactions among the side chains (R groups) of amino acids.
The structure is stabilized by:
- Hydrogen bonds
- Ionic interactions
- Disulfide bonds
- Van der Waals forces
Example: Many globular proteins such as enzymes possess tertiary structure.
4. Quaternary Structure
The quaternary structure is found in proteins that consist of more than one polypeptide chain. It allows proteins to perform complex biological functions.
- It describes the arrangement and interaction of these individual chains to form a functional protein.
- Each polypeptide chain is called a subunit.
- These subunits associate in a specific manner to produce a biologically active protein.
Example: Haemoglobin contains four polypeptide chains.
Denaturation of Proteins
Denaturation is the process in which a protein loses its native (natural) conformation and biological activity due to external factors such as heat, changes in pH, or the action of certain chemicals.
Causes of Denaturation
Proteins may undergo denaturation due to various physical and chemical agents.
1. Heat: High temperatures increase the kinetic energy of protein molecules and disrupt the weak forces responsible for maintaining their three-dimensional structure.
2. Change in pH: Strong acids or bases can alter the ionic charges present in proteins, leading to disruption of their structure.
3. Chemicals and Organic Solvents: Certain chemicals such as alcohol, acetone, and urea can interfere with the interactions that stabilize protein structures.
4. Heavy Metal Salts: Salts of heavy metals such as mercury and lead can react with proteins and cause denaturation.
Examples of Denaturation
Few examples of denaturation are:
1. Coagulation of Egg White: Egg white contains the protein albumin. On heating, albumin loses its natural structure and changes from a transparent liquid to a white solid mass.
2. Curdling of Milk: When milk becomes acidic, the protein casein denatures and coagulates, resulting in the formation of curd.
3. Loss of Enzyme Activity: Enzymes are proteins. Excessive heat or unsuitable pH can denature enzymes, causing them to lose their catalytic activity.
Functions of Proteins
Proteins perform a wide variety of functions in living organisms. Their specific functions depend on their structure and the sequence of amino acids present in them. They are involved in almost every biological process, making them essential for life.
1. Structural Function
Some proteins provide strength, support, and shape to cells and tissues. They form important structural components of the body and help maintain the integrity of organs and tissues.
Examples: Keratin which is found in hair, nails, wool, and horns.
2. Transport Function
Certain proteins transport substances from one part of the body to another. They help in the movement of oxygen, nutrients, hormones, and other molecules required for normal body functions.
Examples: Haemoglobin which transports oxygen from the lungs to body tissues and carries carbon dioxide back to the lungs. .
3. Catalytic Function
Many proteins act as enzymes, which are biological catalysts. These proteins increase the rate of biochemical reactions without being consumed in the process.
Examples: Amylase breaks down starch into simpler sugars , Pepsin helps digest proteins in the stomach.
4. Hormonal Function
Some proteins function as hormones that regulate various physiological activities of the body. They act as chemical messengers and coordinate activities between different organs.
Examples: Insulin helps regulates blood glucose levels.
5. Protective Function
Proteins play an important role in protecting the body against infections and diseases. Certain proteins identify and neutralize harmful microorganisms and foreign substances.
Examples: Antibodies (Immunoglobulins) , Defensive proteins present in the immune system.