What are Proteins? Definition, Types, Functions and Sources

Proteins are biological macromolecules made up of amino acids, contain nitrogen (~16% by weight) and are linked together in polypeptide chains. They perform essential structural and functional roles in living organisms and are important for growth, repair, metabolism, and maintenance of the body.

Based on their composition, proteins are classified into three main types.

Proteins also possess different levels of structural organisation.

Defined by the sequence and folding of polypeptide chains, these polymers of amino acids have complex structures.

These structural levels are determined by the sequence, folding, and interactions of amino acid chains.

Proteins function as enzymes, antibodies, hormones, transport molecules, defence, movement, storage and structural components of tissues and organs. They are crucial for growth and repair and essential for normal biological activities. Protein deficiency and excess can both be harmful.

In this article, let us learn more in detail about the proteins, their structure, the main types of proteins and the key functions they perform in the body.

Table of Contents

• What are Proteins?
• What are Proteins Made of?
• Structure of Protein
• Different Types of Proteins
• Primary Functions of Proteins
• Different Levels of Protein Structure
• What are the Different Food Sources of Proteins?
• Proteins: Deficiency and Excess
• Biological Importance of Protein
• How Much Protein do I Need Daily?
• Interesting Facts About Protein and its Sources
• Conclusion
• Frequently Asked Questions

What are Proteins?

Proteins are essential macronutrients made of amino acids. They are the building blocks of life, forming the structural framework of cells, tissues, and organs. These macronutrients help our body build and repair tissues, produce enzymes and hormones, and support immunity. These macromolecules are present in every cell and are involved in growth, recovery, and everyday body functions.

Proteins are complex organic nitrogenous compounds of high molecular weight.

Proteins are essential biomolecules that help in:

• Transportation of nutrients
• Coordinate bodily functions
• Catalyse biochemical reactions 
• Defend the body against pathogens
• Maintain and regulate the body's tissues and functions.

Proteins are the most complex materials produced in nature. They consist of α-amino acids linked together by peptide linkages.

The name "Protein" is derived from the Greek word "Proteios", which refers to “of prime importance or the first kind”. This significance arises because proteins are important components of the cytoplasm and play various roles in cellular functions.

Proteins are the most abundant macromolecules in living cells and constitute 50% or more of their dry weight. The majority of receptors and enzymes are protein in nature; however, certain catalytic RNA molecules, called ribozymes, also function as enzymes.

Proteins are polymers of α-amino acids. They are synthesised from only 20 amino acids, known as the common amino acids.

Amino acids are formed mainly of carbon, hydrogen, oxygen and nitrogen.

Nitrogen is a characteristic component of proteins, forming about 16% of their weight (i.e., 100 g of protein contains 16 g of nitrogen)

Protein molecules contain 50–55% carbon, 6–7% hydrogen, 20–23% oxygen, 12–19% nitrogen, and 0.2–3.0% sulphur.

Why are Proteins Important for the Body?

Proteins are the macronutrients that provide certain essential nutrients required for our bodies to grow; hence, they are also referred to as the 'building blocks of life'.

Proteins are the most important organic substances in a living cell. They have a high molecular weight, ranging up to several millions. Proteins perform various functions, such as enzymes, hormones, etc.

Many hormones are protein-based molecules. Some hormones, such as insulin and glucagon, are made up of amino acids and play important roles in regulating body functions. Nucleoproteins are conjugated proteins (proteins + nucleic acids) found in cell nuclei.

Blood proteins include the plasma proteins and haemoglobin. Many hormones are peptides and proteins. Protein is an essential nutrient required for growth, tissue repair, enzyme activity, and immune function.

What are Proteins Made of?

Proteins are made up of chains of amino acids. When we digest protein from foods or drinks, it is broken down into amino acids. The body then builds all the different proteins it needs from these amino acids.

What are Amino Acids?

Amino Acids are the building blocks of proteins. Each amino acid contains an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain attached to a central carbon atom.

There are around 20 standard amino acids commonly occurring in proteins. These amino acids are linked together by peptide bonds to form polypeptide chains.

Peptides are the short chains of amino acids, and the longer and more complex polypeptide chains fold to form functional proteins.

Different Types of Proteins

Proteins can be classified according to their structural organisation. Here is a detailed explanation of the classification of proteins based on structure, function, nutritional value, and composition:

Primary Functions of Proteins

Proteins form the basis for the major structural components of animal and human tissue. They act as biological catalysts (enzymes), form structural parts of organisms, participate in different cellular reactions, act as molecules of immunity, and also provide fuel.

Here are the primary functions of proteins in living systems:

Different Levels of Protein Structure

Proteins have four levels of structural organisation. Each level contributes to the protein’s shape and biological functions:

What are Different Food Sources of Protein?

Proteins can be obtained from various sources and are commonly found in both animal and plant food products.

Plant-Based Protein Sources

Some food sources of dietary protein include:

• Legumes and beans – all beans, lentils, chickpeas, split peas, and tofu
• Nuts and seeds – almonds, pine nuts, walnuts, macadamias, hazelnuts, cashews, pumpkin seeds, sesame seeds, sunflower seeds
• Some grains and cereal-based products are also sources of protein, but they are generally not as high in protein as meat and meat-alternative products.

Animal-Based Protein Sources

Some food sources of dietary protein include:

• Lean meats – beef, lamb, veal, pork, kangaroo
• Poultry – chicken, turkey, duck, emu, goose, and more
• Fish and seafood – fish, prawns, crab, lobster, mussels, oysters, scallops, clams
• Eggs
• Dairy products – milk, yoghurt (especially Greek yoghurt), cheese (especially cottage cheese)

Protein Deficiency and Excess

Both too much protein intake and a lesser quantity of protein can be dangerous, as they can result in deficiency and disorders caused by excessive secretion.

Protein Deficiency

According to a 2017 survey conducted by the Observer Research Foundation (ORF)

• 73 per cent of Indians are protein deficient.
• 90 per cent are unaware of their daily protein requirements.

According to the major survey conducted by the Indian Council of Medical Research (ICMR)

• Protein consumption in India is much lower than recommended levels. 
• Nearly 60 per cent of urban Indians do not consume protein-rich foods daily.

PEM:

The term 'protein-energy malnutrition' (PEM) is one of the most common forms of malnutrition, particularly among children with inadequate protein and calorie intake.

Marasmus:

• Weight less than 60% of expected weight and no oedema (swelling caused by excess fluid trapped in body tissues, commonly affecting legs, feet, and hands). 

Kwashiorkor:

• Weight between 60% and 80% of expected weight and oedema (swelling caused by excess fluid trapped in body tissues, commonly affecting legs, feet, and hands)

Protein Toxicity

Protein toxicity refers to health complications that may arise when excessive protein intake places stress on the body’s metabolic and excretory systems, particularly in individuals with impaired kidney or liver function.

Excessive protein intake can increase the production of nitrogen-containing waste products formed during protein metabolism. In healthy individuals, the kidneys usually remove these wastes efficiently. However, people with impaired kidney function may have difficulty eliminating them effectively, which can place additional stress on the kidneys.

In severe cases associated with kidney dysfunction, symptoms may include nausea, vomiting, loss of appetite, fatigue, and an ammonia-like odour on the breath. These symptoms require medical attention and should not be considered a result of high protein intake alone.

Possible symptoms associated with severe kidney dysfunction may include:

• Nausea and vomiting
• Loss of appetite
• Fatigue and weakness
• Dehydration
• Ammonia-like odour in the breath

A balanced protein intake is important for maintaining overall health and preventing metabolic stress.

Also Read: Indian Pumpkin: Why is it Regarded as a National Vegetable?

Biological Importance of Protein 

Proteins are biomolecules and the most important organic substances in a living cell. It serves as a fundamental constituent of the cytoplasm of the cell.

• All antibodies (immunoglobulins) are proteins in nature
• Haemoglobin is a protein that carries oxygen (O₂) in blood
• Blood proteins include the plasma proteins and haemoglobin
• Some hormones are proteins in nature (e.g., insulin and growth hormone)
• Some proteins are supportive (e.g. Collagen; the most abundant protein in animals)
• Some proteins are protective (e.g. Keratins (skin, hair and nails) make the skin resistant to chemicals)
• Actin and myosin are contractile proteins found in muscle cells and responsible for muscular contraction
• Plasma membrane proteins (channel, carrier, pump proteins) regulate the transfer of many substances across the cell membrane
• Many hormones are peptides and proteins. They play important role in regulation of metabolic activities
• Membrane proteins: 
• Proteins and lipids form the major components of cell membranes. Many enzymes are associated with the membranes

How Much Protein Do I Need Daily?

Protein is an essential nutrient required for growth, tissue repair, enzyme activity, and immune function.

Protein requirements vary depending on age, lifestyle, and health conditions. A balanced diet combining different protein sources helps meet nutritional needs effectively.

Animal-based proteins generally contain all essential amino acids, while many plant-based proteins may lack one or more essential amino acids.

Generally speaking, a healthy human body comprises about 16% proteins, which are as important as water in our bodies.

Different foods contain different amounts of essential amino acids. Generally:

Animal products contain large amounts of all the essential amino acids and are known as 'high-quality protein'. This sources include:

• Fish 
• Beef
• Chicken
• Dairy products

Many plant-based protein sources may lack one or more essential amino acids. However, combining some foods can help provide a balanced amino acid profile. These sources include:

• Nuts
• Seeds 
• Grains
• Quinoa 
• Legumes
• Soy products

All these plant sources contain all of the essential amino acids. However, the amounts of one or two of these amino acids may be low.

Proteins are essential nutrients that the body uses for growth, repair, and maintenance of tissues. While the body cannot store protein in the same way it stores fats or carbohydrates, it can synthesise proteins from amino acids obtained through the diet.

Excess protein is not entirely stored; instead, it may be broken down and used for energy or converted into other compounds. Therefore, consuming protein in balanced amounts throughout the day helps the body utilise it more efficiently.

One of the most effective ways to meet daily protein needs is to include moderate portions of protein at each meal.

Both animal and plant sources can help meet daily protein needs. The required intake depends on age, gender, and body weight.

Accordingly, it is always recommended to have any food source in a limited amount. As the saying goes, "Too much is too bad," and having too much protein is dangerous for the body. Excess of protein in our body leads to weight gain, liver and kidney disorders, etc.

Interesting Facts About Protein and its Sources

Protein is an important part of a healthy diet. The nutritional value of a protein is measured by the quantity of essential amino acids it contains.

Here are some of the interesting facts about protein and its sources

Fact - 1

There are nearly 100,000 different types of proteins in our body, as they are very much essential for the cells and tissues to grow, to heal and repair all the wounds, and to carry each and every chemical reaction in the body.

Fact - 2

A good quality of protein is found in some food products such as milk, cheese, yogurt, paneer, peas, soybeans, pulses, nuts, potatoes, yams mushrooms, eggs, meat, etc.

Fact - 3

Proteins play a vital role in building immunity and during the production of both hormones and enzymes.

Fact – 4

Different protein-rich foods provide varying amounts of protein. Approximately 1 ounce (28–30 grams) of protein-rich food can be obtained from sources such as 1 large egg, ¼ cup of tofu, or ½ cup of cooked beans or lentils.

Fact – 5

Some proteins, such as antibodies, play an important role in protecting the body against infections and supporting immune responses.

Fact – 6

The human outer skin, nails, and hair are made up of an insoluble, fibrous structural protein that is highly resistant to digestion and breakdown called 'keratin'. Hence, it is always advised to eat enough protein to have glowing skin and long and healthy hair.

Fact – 7

Proteins also help prevent wear and tear on our muscles, build muscle mass, keep our brain healthy, and improve brain function by producing protein-based chemical messengers.

Fact – 8

Low-fat dairy is also an excellent source of protein. Whole grains contain more protein than refined or "white" products.

Fact – 9

There could be no life without protein, and about 18%–20% of the body is composed of proteins. It also plays an important role in the movement, structural support, storage, digestion, transportation and communication between cells.

Fact – 10

In addition to muscle growth, glowing skin, and healthy hair, proteins also help reduce body weight by regulating the range of blood sugar levels, which leads to improved metabolism.

Consuming a protein-rich diet helps in feeling full and prevents consuming extra food.

Conclusion

Proteins are essential biological molecules that perform a wide range of structural and functional roles in living organisms. They are major components of cells and tissues and are required for growth, repair, and normal body functions.

Proteins are composed of amino acids linked together in chains and serve important functions in the body. Proteins can be classified based on their composition, function, shape or nature. They play key roles such as structure, movement, signalling, catalysis and immunity.

Their importance includes serving as enzymes, hormones, and structural components, as well as being involved in processes such as DNA expression, oxygen transport, homeostasis, and immunity.

For more information about the proteins, their structure, function types and other related articles, read our blogs.