Protein

Contents:
Introduction of Proteins: Amino Acids, Essential Amino Acids, Complete Protein,
Partially, Complete Protein, Incomplete Protein, Sources of Protein, and Functions of
Protein.

Short Important Question

1. List down four essential amino acid. (1 Mark, 2016 + 2019)
2. Define amino acid and give two examples. (1 Mark, 2025)
3. What do you understand by complete protein? (1 Mark, 2025)
4. What is complete protein? (1 Mark, 2022)
5. Define amino acid. Classify protein o the basis of quality. (5 Marks, 2022)
6. List down four food groups which are rich in protein. (1 Mark, 2023)

Long Important Question

1. Compare complete and incomplete protein. How can you improve the quality of dietary protein? Explain. (10 Marks, 2016)

Introduction Of Protein

Proteins are one of the three major macronutrients required by the human body, the other two being carbohydrates and fats. They are essential for growth, repair, and maintenance of body tissues and play a vital role in almost every biological process. Proteins are present in every cell of the body and are necessary for the formation of muscles, skin, hair, nails, blood, hormones, enzymes, and antibodies.

Chemically, proteins are complex organic compounds composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulphur (S) and phosphorus (P). They are made up of smaller units called amino acids, which are linked together by peptide bonds to form long chains known as polypeptides. The arrangement of these amino acids determines the structure and function of each protein.

Proteins are often referred to as body-building nutrients because they are primarily responsible for building new tissues and repairing damaged ones. They are particularly important during periods of rapid growth, such as infancy, childhood, adolescence, pregnancy, and recovery from illness or injury. Although proteins can provide energy (4 kcal per gram), their main function is not energy production. The body primarily uses carbohydrates and fats for energy, while proteins are reserved for growth and other essential physiological functions.

The quality of a protein depends on the type and amount of essential amino acids it contains. Foods of animal origin generally provide high-quality proteins because they contain all essential amino acids in adequate amounts, whereas many plant proteins may lack one or more essential amino acids. However, by combining different plant foods, such as cereals and pulses, it is possible to obtain a balanced intake of essential amino acids.

Simple Definition: Protein is a body-building macronutrient made up of amino acids that is essential for the growth, repair, and maintenance of body tissues. Detailed Definition: Protein is an essential macronutrient composed of amino acids linked by peptide bonds, containing carbon, hydrogen, oxygen, nitrogen, and sometimes sulphur, which is required for the growth, repair, and maintenance of body tissues and for the formation of enzymes, hormones, antibodies, and other vital body structures, while also providing 4 kcal of energy per gram.

Basic Structure Of Protein

Proteins are those macro or biomolecules that serve as the structural units of our bodies and are composed of long chains of amino acids joined together by peptide bonds. For the formation of a protein, amino acids from any of the 21 distinct types can be combined. Peptide bond refers to the connection between the two amino acids. And, the functional group that distinguishes one amino acid from another is called the R-group.

• One amino acid has
• 1 carboxyl group acid (COOH)
• 1 amino group or nitrogen group (NH2)
• 1 hydrogen (H)
• and, 1 functional group (R)

Properties of Protein (Effect of Protein on Heat)

1. Denaturation:
   Proteins’ structural changes happen when they are subjected to heat, light, and/or pH changes. Denaturation is the term for these modifications to protein structure. It causes proteins’ solubility to alter. If the conditions that lead to it are moderate, it might be reversible, but most changes are irreversible. Daily occurrences of denaturing include boiling or heating meat and eggs, as well as milk curdling.
2. Coagulation:
   When proteins are exposed to heat or a pH change, they coagulate. Heat is used to first denature the proteins before coagulating them.
   Coagulation typically takes place within a temperature range of 65 to 90 degrees Celsius. Coagulation, in this context, refers to the transformation of a liquid protein solution into a solid or semi-solid mass. This process is characterized by curdling, bulk formation, congealing, or solidifying of the proteins involved.
   For example, when raw egg, which is initially in a liquid state, is subjected to heat through methods like boiling or scrambling, the egg proteins coagulate. This coagulation results in the transformation of the fluid egg into a solid mass with a different texture and structure.
   In another instance, when milk is incubated with curd, the lactic acid produced during the fermentation process coagulates the milk protein. This coagulation transforms the liquid milk into curd, a semi-solid product with a distinct texture and flavor.
3. Hydration of protein:
   The effect of heat on proteins is closely tied to the process of protein hydration. Protein hydration refers to the binding of water molecules to the surface of proteins. In the realm of food science, this connection is crucially important.
   
   Proteins have a number of functional groups with unshared electrons that can bind and attract the hydrogen atoms in water molecules. As a result, these protein groups start to bind with water molecules. As a result, these protein groups start to bind with water molecules. As a result of this initial bonding, aggregates of water molecules establish around each polar group on the protein molecule as a result of a cascade effect in which one bound water molecule attracts another bound water molecules.
   
   The concentration of the protein, the pH of the solution, the temperature, and the presence of other compounds that may interact with water all have an impact on how much the protein is hydrated.
   
   Gelatin gives a good example of how proteins are hydrated. Cold water causes gelatin to swell as a result of hydration. However, it breaks down into the water when heated. It solidifies once more when it cools. This characteristic enables the network of raised gelatin particles to retain water. Due to its capacity to create gels, serve as a whipping agent in the production of foam, and act as a clarifying agent in fruit juices—all made possible by its special hydration properties—gelatin is used in numerous food applications.
4. Gelation:
   Gelation is the ability of proteins to form gels when they undergo a specific temperature treatment, typically heating followed by cooling. This process is essential in foods like jellies, puddings, and yogurts. When proteins denature and bond together, they create a 3D network that traps water and other ingredients, resulting in a firm and cohesive structure in these products.
   
   When proteins are heated, they go through a process called denaturation, which causes them to separate from their native structure. This process is how gelation happens. These denatured proteins then have an a tendency to interact with one another, resulting in the formation of a three-dimensional network or matrix. This network not only interacts with one another but also draws the proteins closer together. As the network grows, it successfully captures water molecules and other elements within of its framework.
   
   The outcome is a gel with a solid and consistent structure. With the stability, thickness, and form that consumers need, this gelation process gives a variety of foods the desired texture and consistency.
5. Emulsification:
   Emulsification is a vital property of proteins when subjected to heat. This ability allows proteins to stabilize oil-water mixtures, which is crucial in various food products like mayonnaise, salad dressings, and ice cream.
   During emulsification, proteins create a protective layer around oil droplets, effectively preventing them from coalescing and separating from the surrounding water. This protective layer ensures the mixture remains stable and maintains a smooth and consistent texture. This process plays a key role in the quality and texture of many beloved food items.
6. Foaming: This unique ability allows proteins to entrap air, giving rise to stable foams that adorn a variety of delectable dishes such as meringues, soufflés, and cakes. Under the influence of heat, proteins gracefully unfold, weaving an intricate network that captures and cradles air bubbles, creating a delicate, airy matrix.
   It results light and airy desserts that delight both the palate and the eye, making foaming a crucial process in culinary creativity.
7. Viscosity :
   Viscosity is the capacity of protein to thicken and give food items body. In many food products, including sauces, gravies, and soups, this is crucial. When proteins connect with one another and unfold, a network is formed that traps water and other components. Its texture becomes smooth and velvety as this network thickens.

Amino Acids

Amino acids are the basic building blocks of proteins. Just as bricks are used to construct a building, amino acids combine to form proteins that are essential for the growth, repair, and maintenance of the human body. When proteins are digested, they are broken down into amino acids, which are then absorbed into the bloodstream and used to synthesize new proteins according to the body’s needs.

Chemically, every amino acid contains an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a unique side chain (R-group) attached to a central carbon atom. The different side chains determine the structure and function of each amino acid. Amino acids are joined together by peptide bonds to form peptides and proteins.

There are 20 common amino acids required by the human body to synthesize proteins. Based on the body’s ability to produce them, amino acids are classified into essential, non-essential, and conditionally essential amino acids. Essential amino acids cannot be synthesized in sufficient amounts by the body and therefore must be obtained through the diet, whereas non-essential amino acids can be produced by the body.

Essential and Non-Essential Amino Acids

Based on nutritional requirements amino acids are grouped in two classes:
I. Essential Amino Acids
II. Non – Essential Amino Acids

1. Essential Amino Acids
   Those amino acids which can’t be synthesized in sufficient amounts by the body and must be provided by the diet are called essential amino acids. Out of 21 amino acids, there are 10 essential amino acids. Among those 9 are for adults with it 1 for children.
   • Histidine
   • Methionine
   • Threonine
   • Tryptophan
   • valine
   • Phenylalanine
   • Isoleucine
   • Leucine
   • Lysine
   • Arginine (Additional for child)

Functions of Essential Amino Acids:

• Histidine: This amino acid is necessary for the development of histamine, a neurotransmitter involved in a number of physiological activities, as well as for the growth and repair of tissues.
• Isoleucine: Isoleucine is essential for the formation of hemoglobin, immune system activity, and muscle metabolism. Additionally, it helps keep blood sugar levels stable.
• Leucine: Leucine is an essential component of muscle protein synthesis and plays a crucial role in both muscle development and recovery.
• Lysine: Lysine is necessary for the development of collagen, the assimilation of calcium, and the synthesis of hormones, enzymes, and antibodies. Additionally, it aids in tissue healing.
• Methionine: Methionine is an essential amino acid for metabolism and detoxification activities and a precursor to other significant compounds like cysteine and glutathione.
• Phenylalanine: Phenylalanine is required for the synthesis of a number of neurotransmitters, such as norepinephrine, epinephrine, and dopamine. Furthermore, it functions as a precursor to the amino acid tyrosine.
• Threonine: Threonine is important for preserving a healthy protein balance in the body and aids in the production of collagen and elastin, both of which are necessary for the health of the skin and connective tissues.
• Tryptophan: A neurotransmitter that controls mood and sleep, serotonin, is made from tryptophan. Niacin (vitamin B3) synthesis depends on it as well.
• Valine: Valine is particularly vital for stamina and muscle strength. It is also necessary for tissue healing and muscle coordination.

2. Non-Essential Amino Acids

These amino acids can be synthesized in our body from other amino acids, so they are not dependent on food sources to be obtained. Altogether, there are 11 non-essential amino acids.
Namely: Cysteine, Serine, Tyrosine, Glutamic acid, Glutamine, Glycine, Proline, Alanine, Arginine, Asparagine, Aspartic acid.

Note: Non-essential amino acids are primarily produce in human liver.

Functions of Non-Essential Amino Acids:

1. Alanine: Alanine is necessary for the metabolism of glucose and provides energy during prolonged activity.
2. Asparagine: Asparagine is involved in the production of other amino acids and is necessary for preserving the body’s nitrogen balance.
3. Aspartic Acid: Aspartic acid plays a crucial role in the urea cycle, which gets rid of ammonia from the body.
4. Glutamic Acid: Glutamic acid is a neurotransmitter in the central nervous system that is important for cognitive function.
5. Serine: Serine participates in the metabolism of fats and fatty acids and serves as a precursor for the production of other amino acids.
6. Glycine: Glycine contributes to the synthesis of creatine, which is necessary for proper neuron and muscle function.
7. Proline: Proline is a part of collagen, which is essential for the health of the skin, joints, and connective tissues.
8. Cysteine: Cysteine is essential for the production of proteins, enzymes, and the antioxidant glutathione.
9. Tyrosine: Tyrosine serves as an important building block for the production of neurotransmitters like dopamine, adrenaline, and norepinephrine.
10. Arginine: Arginine participates in a number of metabolic processes, including the urea cycle and nitric oxide synthesis, which controls blood vessel function.
11. Glutamine: Glutamine is a main source of energy for the cells lining the digestive track and is necessary for the immune system.

While the body may produce non-essential amino acids, they can also be acquired through diet. But these amino acids are typically present in sufficient amounts in a balanced diet. Additionally, when the body’s need for some non-essential amino acids, such as glutamine, surpasses its capacity to produce them because of sickness or stress, they might turn conditionally essential.

S. N	Characteristics	Essential Amino Acids	Non-Essential Amino Acids
1.	Definition	It can’t be synthesized by human body.	It is synthesizable by the human body.
2.	Requirement	It must be obtained from the daily diet.	It is not strictly required.
3.	Other names	It is also known as indispensable amino acids.	It is also known as dispensable amino acids.
4.	Synthesize	Adults can’t synthesize 9 amino acids.	Adult can synthetize 11 amino acids.
5.	Variety	There are total 10 essential amino acids.	There are total 11 no-essential amino acids.
6.	Examples	Example: Tryptophan, Lysine, Arginine, Leucine, etc.	Example: Proline, Glycine, Cysteine, Alanine, etc.
7	Health Impact	Deficiency can cause health issues and needs to be treated with eating habits.	Although deficiencies are less common, they can emerge under certain circumstances, such as illness or stress, necessitating the use of supplements.
8	Conditionally Essential	No, they are always essential	Can become conditionally essential under specific circumstances

Classification of protein

On the basis of nutritive value of proteins, it can be classified as:-

• Complete Protein
• Incomplete Protein
• Partially Complete Protein

1. Complete protein (High Quality Protein)
   All of the necessary essential amino acids are present in these proteins in appropriate amounts and ratios to meet the body’s requirements. Even when served as the only protein supply, they continue to support life. They are regarded as high-quality proteins due to their nutritional completeness.
   For example: milk, egg, meat, poultry products, fish and dairy products (like milk, cheese & yogurt) are all examples of proteins that are derived from animals.
2. Incomplete Protein
   These proteins cannot sustain life on their own because they lack one or more essential amino acids.
   Vegetables, fruits, beans, legumes, tofu, grains, pulses, nuts, and oilseeds are all plant sources of protein, and they all contain varied amounts of protein deficient. The protein that results from combining two sources of incomplete proteins may be of higher quality. For vegetarians, incomplete protein is preferable to complete protein. as in Khichdi and Kheer.
   
   The only incomplete protein in animal products is gelatin. In addition to having little leucine, it is deficient in three essential amino acids, including tryptophan, valine, and isoleucine.
3. Partially complete protein
   Regarding the distribution of amino acids, partially complete proteins fall between full and incomplete proteins.
   While they may not include all essential amino acids in sufficient quantities, they are protein sources that offer a more balanced profile of essential amino acids than normal incomplete proteins. Soy and quinoa are two typical examples of its origins.

Sources of Protein

Proteins are obtained from a variety of foods of both animal and plant origin. The quality and quantity of protein vary among different food sources. In general, animal sources provide complete proteins because they contain all the essential amino acids in adequate amounts, whereas most plant sources are incomplete or partially complete proteins. However, combining different plant foods, such as cereals and pulses, can provide all the essential amino acids required by the body.

Protein-rich foods should be included regularly in the diet to support growth, repair body tissues, maintain muscle mass, and ensure the normal functioning of the body.

1. Animal Source of Protein
   Animal foods are considered high-quality proteins because they contain all the essential amino acids in the right proportions. They are easily digested and efficiently utilized by the body.
   
   Characteristics of animal protein:
   a. Rich in complete protein.
   b. High biological value (HBV).
   c. Easily digestible.
   d. Also provide vitamins and minerals such as calcium, iron, zinc, and vitamin B₁₂
   
   Example:
   • Milk
   • Yogurt (Curd)
   • Cheese
   • Paneer
   • Eggs
   • Chicken
   • Mutton
   • Buffalo meat
   • Pork
   • Fish
   • Seafood
2. Plant Source of Protein
   Plant foods are valuable sources of protein, particularly for vegetarians. Although many plant proteins lack one or more essential amino acids, combining different plant foods improves their protein quality.
   
   Characteristics:
   a. Generally contain incomplete or partially complete proteins.
   b. Rich in dietary fibre, vitamins, minerals, and antioxidants.
   c. Usually lower in saturated fat than animal sources.
   d. Combining cereals and pulses improves protein quality.
   
   Example:
   • Lentils (Dal)
   • Chickpeas
   • Kidney beans
   • Black gram
   • Soybeans
   • Peas
   • Peanuts
   • Almonds
   • Cashew nuts
   • Sesame seeds
   • Sunflower seeds
   • Oats
   • Quinoa

Functions of Protein

Proteins perform numerous essential functions in the human body. They are not only responsible for building and repairing body tissues but also play important roles in regulating body processes, supporting immunity, transporting substances, and maintaining overall health. Although proteins can provide energy, their primary role is to support the structure and normal functioning of the body.

The major functions of proteins are as follows:

1. Growth and Development
   Proteins are essential for the growth of the body. They help in the formation of new cells and tissues, making them particularly important during infancy, childhood, adolescence, pregnancy, and lactation.
2. Repair and Maintenance of Body Tissues
   Proteins repair worn-out, damaged, or injured tissues and replace old cells with new ones. They are necessary for wound healing and recovery after illness or surgery.
3. Formation of Enzymes and Hormones
   Many enzymes and hormones are proteins or are made from proteins. Enzymes speed up chemical reactions in the body, while hormones regulate important body functions such as growth, metabolism, and reproduction.
4. Production of Antibodies
   Proteins are required for the production of antibodies, which protect the body against bacteria, viruses, and other disease-causing microorganisms. Therefore, adequate protein intake helps strengthen the immune system.
5. Maintenance of Muscle Mass
   Proteins help build and maintain muscles, making them especially important for physically active individuals, athletes, and older adults who need to prevent muscle loss.
6. Transportation of Nutrients
   Certain proteins transport oxygen, nutrients, hormones, and other substances throughout the body. For example, haemoglobin, a protein found in red blood cells, carries oxygen from the lungs to body tissues.
7. Maintenance of Fluid and Acid–Base Balance
   Proteins help maintain the normal balance of body fluids by preventing excessive fluid loss from blood vessels into surrounding tissues. They also help regulate the body’s acid-base (pH) balance, which is essential for normal cellular function.
8. Source of Energy
   Although carbohydrates and fats are the body’s preferred energy sources, proteins can also provide energy when needed. One gram of protein provides approximately 4 kilocalories (kcal) of energy.
9. Formation of Body Structures
   Proteins are major structural components of muscles, skin, hair, nails, connective tissues, and internal organs, helping maintain their strength and integrity.