What are Protein ? | Definition, Structure, & Classification

What are Protein ?—

• Protein is essential for survival, therefore, it is the most important element of all. That is why protein is called as ‘body-building unit’. Protein is derived from Greek word ‘proteios’, meaning “primary”, “in the lead”, or “standing in front ‘. A Dutch chemist Gerrit Mulder is credited with coining the term “protein” in 1838. The 1/5″ part of human body’s weight or 20% of it is made up of protein.

Chemical Formation of Protein —

• protein is an organic compound. Carbon, hydrogen, oxygen, nitrogen elements are present in it. The main part of protein, about 16%, is made up of nitrogen. Amino acid is the smallest unit of protein. Many amino acids together make a protein, so an amino acid is called as base unit.
• Amino group (-NH₂ ) is basic in nature and provides the basic character to the protein. Carboxyl group (-COOH) is acidic in nature and provides acidic character to the protein. Therefore, presence of both these groups makes amino acid neutral in nature.
• The chemical formula of amino acid is—

• Emil Fischer and Hoff Mischer in 1920 reported the linking of amino acids through peptide bonds.
• The quality of protein depends on types of amino acids present, their quantity and their way of linking. We know about 22 types of amino acids present in our body and food. Out of these, 10 amino acids are very important for the growth and development of our body. These are known as essential amino acids.

Essential and non-essential amino acids—

Essential amino acids are drawn from the food that we cat because in the absence of essential amino acids growth and development of our body is disrupted. Following are the essential amino acids—

1. Histidine
2. Leucine
3. Isoleucine
4. Lysine
5. Arginine
6. Methionine
7. Threonine
8. Phenyl alanine
9. Valine
10. Tryptophan

Non-essential amino acids are formed in the body itself in the presence of nitrogen. Following are the major non-essential amino acids—

1. Alanine
2. Hydroxyproline
3. Proline
4. Aspartic acid

Classification of protein—

Protein is classified as given in the table— 13.3

1. On the basis of quality—

Protein is classified on the basis of amino acids present in them because quality of any protein depends on the presence of quantity, type and quality of amino acids.

• Superior or complete protein— In these high biological valued proteins all the essential acids are present in adequate quantity and proper ratio. These carry out the function of overall growth and development of the body and perform formative functions. This type of protein is obtained from animal sources such as milk, curd, meat, fish, eggs, liver and from plat sources like dry fruits, soya bean, etc.
• Medium or partially complete protein— In these some essential amino acids are present but one or two essential amino acids are absent. These proteins maintain life but are not useful for physical growth and development. These proteins do not form new nerves and cells. The protein obtained from plant sources such as lentils, cereals, soya bean are medium quality proteins. Cereals lack lysine while lentils lack ‘methionine.
• Inferior or incomplete protein— In these proteins, essential amino acids are totally absent. And so these types of protein are useless in physical growth and development. Incomplete protein is present in stem-root vegetables, fruits and maize. Gelatin in fruits is also an incomplete protein.

2. On the basis of source—

• Plant protein— Protein obtained from plant sources is of medium or of inferior quality. Such protein is present in lentils, soya bean, cashew, almonds, groundnuts, dry fruits, etc. This protein can be used as a complete protein in mixed form. For example, lentils lack methionine while in wheat lysine is absent. Lentils and wheat together make a complete protein. Similarly lentils and wheat can be combined with milk or milk products to make complete proteins, for example in dahi bada, kheer, kedgeree, khichari etc.
• Animal protein— Protein obtained from animal sources is of superior quality. These are complete proteins because the essential amino acids important for body’s growth and development are present in it. For example, milk, curd, eggs, meat, fish, liver, cottage cheese, buttermilk, etc.

3. On the basis of chemical structure—

proteins are divided into 3 categories on the basis of physical characters and solubility.

Simple protein— These proteins are made up only amino acids. On hydrolysis they give simple units of amino acids. For example—

• The protein in yellow part of egg— albumin
• Protein in wheat— glutenins, gliadins
• Protein in milk— lactoglobulin
• Protein in maize— zein

Conjugated protein— A conjugated protein is a protein that functions in interaction with other chemical groups.

• Simple protein + other chemical groups = conjugated protein
• These proteins are named on the basis of chemical groups present—
• Glycoprotein— simple protein + carbohydrate
• Nucleoprotein— simple protein + nucleic acid
• Lipoprotein— simple protein + lipid
• Haemoglobin — simple protein (globin) + heme (iron)
• Phosphoprotein — simple protein + phosphorous

Derived protein— Derived proteins are formed by partial fragmentation of protein because of physical activities, temperature, somatic stress, digestion. For example

• In milk— casein
• In blood clot— fibrin
• In boiled eggs— albumin
• In digestive juices— peptones, proteoses, peptides

Functions of proteins—

1. Growth and development of body—

• Protein is essential for growth and development. Cells are made up of protein. The repair of damaged cells is also carried out by cells. During infancy, childhood and adolescence protein is important because these stages are of rapid development. Other than this for repair work protein is needed till old age.

2. For formation and repair of tissues—

• Tissues get damaged due to continuous working and different physical activities. Their regeneration and repair becomes important and this is done by protein. The envelope of alimentary canal, production of red blood cells, blood clotting requires protein.

3. Regulation of different activities of body—

• Protein regulates acids and bases in our body, acts as buffer.
• For production of hormones— Different hormones are made up protein molecules and these hormones regulate and control different activities. In the absence of protein, proper production and secretion of hormones does not take place.
• Contraction of muscles— Myosin and actin are important for contraction and relaxation of muscles. Myosin and actin are made up proteins.
• Production of enzymes— Proteins make enzymes. Enzymes carry out various activities in the body such as digestion of food, oxygenation, metabolism, etc.
• Formation of vitamins— Some amino acids act as precursors for vitamin formation. Forexample, Colin of vitamin B group needs methionine, tryptophan for niacin, etc.

6. Helpful in normal vision—

• cones and rods, present in retina of eyes, form special products in the presence of protein and helps in seeing colours and dyes in dim light.

7. Providing energy—

• in the absence of carbohydrates and fat, protein performs the function of providing energy. One gram of protein gives 4 kilocalories of energy.

8. Maintaining water balance—

• protein present in plasma generates osmotic pressure in the body which maintains the water balance of the body.

Effects of protein deficiency—

• The reason for deficiency of protein in the body is lack of protein-rich food items in diet or consumption of incomplete or partially incomplete protein-rich foods for long time. When protein is not present in adequate quantities in the food, it disrupts growth and development of the body. The effect of protein deficiency is known as protein energy malnutrition because protein deficiency leads to deficiency of protein as well as energy. Deficiency of proteins leads to following diseases— (1) Kwashiorkor (2) Marasmus (3) Marasmic Kwashiorkor

(1) Kwashiorkor—

it occurs in children of 1-4 years of age. It is caused by lack of protein in diet. This occurs in children whose diet includes carbohydrates but lack protein. Cicely Williams in 1935 reported first about Kwashiorkor. It is an African word which means “disease of a baby deposed from the breast when the next one is born”.

• Growth failure— both growth and developmentof children get arrested. Also weight and height get stunted.
• Edema—body swells due to deficiency of protein. Water gets into the cells and tissues of the body. Because of swelling child appears to be healthy.
• Muscles begin to degenerate. Arms and legs weaken and grow thin.
• Behavior of child becomes irritating and disinterested. Child feels lethargic, lazy and tired.
• Hemoglobin cannot be produced due to lack of protein. This results in anemia. Mental development is arrested.
• Disease resistance capacity reduces and incidence of other diseases increase.

2. Marasmus—

Marasmus occurs in children of 6-12 months of age due to undernourishment. Protein as well as energy deficiency are the main cause. Marasmus is a Greek word meaning “to waste”. The main reason for Marasmus is weaning at a very young age and lack of proper and nutritious food. The symptoms of Marasmus are—

1. Physical growth and development stops, height does not increase, drop in body weight.

2. Inflammation and absence of fat from under the skin.

3. Frequent urination due to infection of alimentary canal.

4. Skin becomes dry, rough, dull, and lifeless.

5. Muscle deformation.

6. Limbs become thin and weak.

3. Marasmic Kwashiorkor—

• In underdeveloped and developing countries where protein- energy deficiency is prevalent, symptoms of both Marasmus and Kwashiorkor appear together. Proper treatment, nutritious food can result in better health of children suffering from Kwashiorkor. But treatment of Marasmus takes time to show tangible results. Deficiency of protein in pregnant mothers leads to improper development of the foetus. The baby born is slightly built and weak. Sometimes there is scarcity of milk in feeding mothers. Symptoms of deficiency of protein in adults are — loss of body weight, aanemia, loss of disease resistance capacity, presence of illness, etc.

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Proteins Questions and Answers :-

1. Which of the following is not a macronutrient?
a. Protein
b. Carbohydrates
c. Fat
d. Vitamins

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2. Which element is present in all proteins?
a. Nitrogen
b. Oxygen
c. Carbon
d. Hydrogen

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3. Which of the following is not a function of proteins in the body?
a. Transport of oxygen
b. Building and repairing tissues
c. Energy production
d. Enzyme catalysis

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4. How many different amino acids are used to make proteins in the body?
a. 20
b. 50
c. 100
d. 200

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5. Which of the following foods is not a good source of protein?
a. Chicken
b. Tofu
c. Lentils
d. White bread
Ans. ⇒ ????