10.2.3. Structure of Proteins

Proteins are the most abundant biomolecules of the living system. Chief sources of proteins are milk, cheese, pulses, peanuts, fish, meat, etc. They occur in every part of the body and form the fundamental basis of structure and functions of life. They are also required for growth and maintenance of body. The word protein is derived from Greek word, “proteios” which means primary or of prime importance.

Proteins are essential molecules that perform a wide variety of functions in living organisms. They exist in many forms and serve many roles such as building tissues, transporting molecules, and acting as enzymes to speed up biochemical reactions. The focus on proteins as a key component of life is highlighted by their prevalence and importance in our diet and bodily functions.

Think of proteins as the building blocks of a house. Just like bricks are crucial for constructing a sturdy house, proteins are vital for building and maintaining the body's structure, such as muscles and organs.

All proteins are polymers of a-amino acids.

Proteins are made up of smaller units called amino acids. These amino acids link together in specific sequences to form long chains called polypeptides. There are 20 different amino acids that combine in various ways to create a vast array of proteins, each with unique functions in the body.

Imagine amino acids as the letters of the alphabet. Just as different combinations of letters form various words, different sequences of amino acids form different proteins, each serving its own purpose in the body.

Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only α-amino acids are obtained on hydrolysis of proteins.

Amino acids are organic compounds characterized by the presence of both an amino group and a carboxyl group. They can be classified based on the structure of their side chains and their positions relative to the functional groups. When proteins are broken down (or hydrolyzed), they yield primarily α-amino acids, which are the main building blocks of proteins.

Consider amino acids like a collection of different tools in a toolbox. Each tool (amino acid) has a specific shape and purpose, and they must be arranged in the right way to build (form) something functional like a piece of furniture (protein).

Proteins are connected to each other by peptide bond or peptide linkage. Chemically, peptide linkage is an amide formed between –COOH group and –NH group.

Peptide bonds are the connections that hold amino acids together in a protein chain. This bond is formed through a chemical reaction where the amino group of one amino acid reacts with the carboxyl group of another, releasing water in the process. This results in a covalent bond that is strong and stable, allowing for the formation of long chains of amino acids.

Think of peptide bonds like the links in a chain. Each link represents a peptide bond between amino acids, and together they create a strong and flexible structure that can bend and twist without breaking.

Protein structure can be classified into four levels — primary, secondary, tertiary and quaternary, each level being more complex than the previous one.

The structure of proteins is organized into four levels:

1. Primary structure: This is the linear sequence of amino acids in the polypeptide chain.
2. Secondary structure: This involves the folding of the chain into alpha-helices and beta-pleated sheets due to hydrogen bonding.
3. Tertiary structure: This represents the overall 3D shape of the protein due to interactions between R groups.
4. Quaternary structure: This occurs when multiple polypeptide chains come together to form a functional protein.

Think of the different structures of a protein like the layers of a cake. The primary structure is the base (ingredients), the secondary is the spongy texture (layers), the tertiary is the final shape of the cake, and the quaternary structure involves adding more cakes (layers) together to create a tiered cake.

When a protein in its native form is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix gets uncoiled and protein loses its biological activity.

Denaturation is a process where proteins lose their three-dimensional structure due to external factors like heat or changes in pH. This unfolding causes the protein to become inactive and unable to perform its biological functions, which is crucial for cellular processes.

Imagine denaturation like cooking an egg. When you boil an egg, the clear liquid egg white becomes solid and white; it has undergone denaturation and cannot return to its original form, just like proteins lose their function after denaturation.

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