10.2. 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 large molecules that play critical roles in our bodies. They are composed of smaller units called amino acids. These biomolecules are crucial for various physiological functions, serving as building blocks for tissues, facilitating biochemical reactions, and supporting immune function. They are essential for growth and repair in the body, indicating their importance in our nutrition.

Think of proteins as the bricks and mortar of a house. Just as bricks are essential for building the walls and structure of a house, proteins are necessary for building up the body's tissues and ensuring its structural integrity.

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. They may contain other functional groups also.

Amino acids are the building blocks of proteins, each consisting of an amino group (–NH2), a carboxyl group (–COOH), and a unique side chain represented by 'R'. The classification (α, β, γ...) refers to the position of the amino group relative to the carboxyl group. For instance, α-amino acids have the amino group attached to the first carbon next to the carboxylic acid group, which is the most common type of amino acid found in proteins.

Think of amino acids as different types of nuts that can be mixed to make a variety of dishes. Each nut (or amino acid) has its unique flavor (R group), and when you combine them in different ways, you can create countless recipes (proteins) that satisfy various nutritional needs.

Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic. The amino acids, which can be synthesised in the body, are known as non-essential amino acids. On the other hand, those which cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids.

Amino acids can be categorized based on the number of their amino and carboxyl groups. If they have equal numbers, they are neutral; if there are more amino groups, they are basic; and if there are more carboxyl groups, they are acidic. Essential amino acids are those that our body cannot synthesize and must be obtained from our diet, while non-essential ones can be made by our body. This classification helps in understanding dietary needs for balanced nutrition.

Consider amino acids like cars in a parking lot. Some cars (essential amino acids) need a special key (diet) to start, while others (non-essential amino acids) can start with a regular key (the body can create them). Just like how different vehicles serve different purposes, essential and non-essential amino acids serve specific roles in our body.

You have already read that proteins are the polymers of α-amino acids and they are connected to each other by peptide bond or peptide linkage. Chemically, peptide linkage is an amide formed between –COOH group and –NH2 group. The reaction between two molecules of similar or different amino acids proceeds through the combination of the amino group of one molecule with the carboxyl group of the other.

Proteins are formed through the linkage of amino acids by peptide bonds. This bonding process occurs when the amino group of one amino acid reacts with the carboxyl group of another, resulting in the release of water and the formation of a peptide bond. This structure can lead to various formations such as dipeptides (two amino acids linked) or polypeptides (many amino acids linked together), which build up more complex proteins.

Imagine making a chain of paper clips. Each clip represents an amino acid, and connecting them forms a longer chain, just like how peptide bonds link amino acids to create proteins. Each unique arrangement of clips can create different shapes and lengths, reflecting the vast variety of proteins that exist.

Proteins can be classified into two types on the basis of their molecular shape: Fibrous proteins and Globular proteins. (a) Fibrous proteins are generally insoluble in water, e.g., keratin (present in hair, wool) and myosin (present in muscles). (b) Globular proteins, such as insulin and albumins, are usually soluble in water.

Fibrous proteins have elongated structures and tend to be insoluble in water, providing structural support, such as in hair and muscle, whereas globular proteins have a more compact and rounded shape, making them soluble and functional, such as hormones like insulin. This classification reflects their functions and roles in biological systems.

Think of fibrous proteins as the firm, tough ropes (like those used in construction) that provide structure and support, while globular proteins are like soft, squishy balloons; flexible and full of air, suited for carrying messages or performing tasks in the body.

Structure and shape of proteins can be studied at four different levels, i.e., primary, secondary, tertiary, and quaternary, each level being more complex than the previous one. (i) Primary structure refers to the specific sequence of amino acids. (ii) Secondary structure can exist in forms like α-helix and β-pleated sheet, arising from hydrogen bonding. (iii) Tertiary structure refers to the overall 3D folding. (iv) Quaternary structure describes the arrangement of subunits.

Proteins have four structural levels: The primary structure is the sequence of amino acids, which determines the protein's function. The secondary structure results from hydrogen bonds, forming either α-helices or β-pleated sheets. The tertiary structure is the overall 3D shape caused by various types of bonds and interactions, and finally, the quaternary structure describes how multiple polypeptide chains combine to form a functional protein complex. Each level is crucial for the protein's functionality.

Consider building a complex sculpture. The primary structure is the choice of materials (like clay or metal), secondary structure refers to the techniques used (carving or molding), tertiary structure is the overall shape you create (how the sculpture looks as a whole), and the quaternary structure is how different pieces fit together (like combining different sculptures into a gallery).

Protein found in a biological system with a unique three-dimensional structure and biological activity is called a native protein. 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. This leads to unfolding and loss of biological activity, called denaturation.

Denaturation is a process where proteins lose their functional shape due to external stressors such as heat or changes in pH. This unfolding disrupts the specific interactions that maintain the protein's structure, causing it to lose its original function or activity. For example, when egg whites are boiled, the proteins denature, changing from a liquid to a solid state, demonstrating the structural change.

Think of a knitted sweater. If you put it in hot water, it will lose its shape and stretch out—this represents denaturation. Just like the sweater can’t function as a cozy garment anymore, denatured proteins can no longer perform their specific biological tasks.

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