Proteins: See Definition, Classification, Structure, Functions

Proteins are large, complex molecules that play many critical roles in the body. They are essential for the growth, maintenance, and repair of body cells and tissues and form an important component of our organs, muscles, skin, hair, and nails. Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. Proteins are the essential building blocks of our body and the vital nutrients in our diet.
Let us talk about Proteins definition, sources, classification and function in this article below.

What are Proteins?

Proteins are complex biomolecules made up of many small units called amino acids. The amino acids strands are twisted to form long chains. There are \(20\) amino acids common to most proteins. The specific order of these amino acids determines the structure and function of proteins. The long chains of amino acids then fold in peculiar ways, and a three-dimensional structure is formed, which becomes the functional protein. 

Thus, amino acids become the building blocks of proteins, and since many amino acids are joined together, a protein molecule becomes quite large, hence called a macromolecule. 

Structure of Proteins

Protein is made up of amino acids. Each amino acid is an organic molecule consisting of a central carbon atom attached to an amino group, carboxyl group, hydrogen atom and a variable side chain (R group). There are \(20\) amino acids common to most proteins and fewer common ones. The general structure of an amino acid looks like this:

Each amino acid is linked with the next one through a covalent bond called a peptide bond. Many such amino acids join to form a linear chain. This linear chain is called a polypeptide. Biologically occurring polypeptides vary in size and consist of two to three thousand amino acids. The linear chain of amino acids, i.e., polypeptide, becomes the primary structure. It may consist of one or many polypeptides. 

When the amino acids join, the side chains or \(R\) groups acquire a particular spatial arrangement, called conformation. Such conformations decide the secondary and tertiary structures. Due to intermolecular attractions between the \(R\) groups, the polypeptide chains fold onto themselves in characteristic ways. The foldings form two patterns called \(\alpha  – \)helix and \(\beta  – \)sheets.
These stable foldings make the secondary structures of a protein. The three-dimensional arrangement of helices and sheets makes the tertiary structure. Most proteins contain multiple helices and sheets, and the entire aggregation of them with three-dimensional foldings makes the quaternary structure. Thus, we can see \(4\) levels of protein structure:

1. Primary structure
2. Secondary structure
3. Tertiary structure 
4. Quaternary structure

Sources of Proteins

1. Lean meat – Pork, beef and lamb
2. Poultry – Chicken and eggs
3. Fish and seafood – Prawns, crab, lobster, etc. 
4. Beans, pulses (also known as lentils) 
5. Mushrooms
6. Nuts and seeds
7. Dairy products – Milk, cheese, curd and yoghurt
8. Soy products

Functions of Proteins

Proteins carry out various functions in our body as below:

1. Proteins are required for the growth and repair of body tissues.
2. All our body muscles are made up of proteins. Roughly \(30\) per cent of our body is muscles.
3. Proteins provide the essential structure to the body. They provide elasticity, rigidity, and shape to many internal and external organs.
4. All enzymes in our body are made up of proteins. So, all biochemical reactions in the body are carried by proteins.
5. Every cell has a membrane composed of different proteins essential for all vital functions.
6. Many messengers of the body, i.e., hormones, are proteins. Thus, they act as messengers too.
7. Proteins contribute to our blood in a greater way:
   a. \(6-8\) per cent of blood plasma is proteins.
   b. The Hemoglobin of RBCs is protein.
   c. Many proteins present in the blood plasma like albumin, globulin maintain the osmotic pressure and \(pH\) of the blood and thus keep the body fluids of the entire body.
   d. Proteins boost our immunity by making up the immunoglobulins (antibodies) and complement proteins in our blood.
   e. They help in blood clotting.
8. It transports different types of substances across the body through blood.
9. Proteins also provide the energy required by our body for various functions.

Classification of Proteins

Proteins can be classified according to their composition, structure, and function. Let us look at them one by one.

Based on the composition

proteins are classified as simple, conjugated, and derived proteins:

1. Simple proteins are made up of amino acids only, and no other chemical group is attached to them. Their structures are relatively simple.  Few examples are albumin, globulin, histones, protamines, etc.
2. Complex or Conjugate proteins have the non-protein moiety (prosthetic group) attached to them to become functional. Thus, they have both protein and non-protein components in them. Based on the nature of the prosthetic group, attached proteins can further be classified as:
   a. Metalloproteins: Prosthetic group is a metal ion and essential for the functioning of the protein.
   Examples: Cytochrome c oxidase, peroxidase, nitrate reductase, urease, carbonic anhydrase etc.
   b. Chromoproteins: When the prosthetic group is a pigment or chrome (colouring substance), protein is called chromoprotein.
   Examples: Haemoglobin (Fe gives red colour) and chlorophyll (tetrapyrrole rings give the green colour).
   c. Glycoproteins: Also known as mucoproteins, these have carbohydrates as a prosthetic group. 
   Examples: Antibodies, Heparin, Hyaluronic acid.
   d. Lipoproteins: The prosthetic group of these proteins is lipid. 
   Examples: Lipovitelline, chylomicrons, etc.
   e. Phosphoproteins: The prosthetic group is a phosphate molecule. Examples: Casein (milk protein) and ovo-vitelline (egg yolk protein).
   f. Nucleoproteins: Here, the prosthetic group is a nucleic acid.
   Examples: Proteins in chromosomes and ribosomes.
   g. Flavoproteins: The prosthetic group is FAD (Flavin Adenine Dinucleotide). Examples are proteins of the electron transport chain.
3. Derived proteins are those which are either derived from simple proteins or complex proteins. Heat, enzyme action or chemical actions may produce derived proteins. Examples: Proteins, peptides, and some artificially produced proteins.

According to their Structure

Based on the structure, proteins can be classified as:

1. Fibrous Proteins have long fibre-like structures. Their length to breadth ratio (axial ratio) is more than \(10.\) Fibrous proteins have more structural roles.
   Examples: Keratin, collagen, elastin, fibroin, etc.
2. Globular Proteins have spherical or rounded structures, as their name suggests, and their axial ratio is always less than \(10.\) Globular proteins have more functional roles.
   Examples: Albumin, globulin, histones, etc.
3. Intermediate Proteins have their structure intermediate between fibrous and globular proteins. They are short and more or less linear-shaped. An example is a fibrinogen.

According to the Functions

1. Catalytic Proteins: These carry out all biochemical and catalytic functions and a typical example includes all enzymes and coenzymes.
2. Structural Proteins: These make up the structural components of connective tissue, bones, cartilage, tendons, ligaments, skin, hair, feathers, nails and horns. Example: Collagen, elastin, keratin, etc.
3. Nutrient Proteins: These have nutritional values and provide nutrition. Example:  Albumin, casein, etc.
4. Regulatory Proteins: These regulate metabolic and cellular activities in the body.
   Examples: Hormones, cell signaling molecules and membrane proteins.
5. Defense Proteins: These provide immunity to the body against pathogens. Example: Antibodies and complement proteins
6. Transport Proteins: Helps transport various substances across the body. They transport nutrients, vitamins and even respiratory gases like oxygen and carbon dioxide. Haemoglobin is a classic example of transport protein that transports oxygen to every cell of the body. Other examples include globulins, albumin, etc.
7. Storage Proteins: They act as a storehouse of many ions and molecules. 
   Example: Ferritin (which stores ions), albumin, etc.
8. Mobile or Contractile Proteins: Nutrients that help in the movement or locomotion of body parts and can be called the force generators. 
   Example: Actin, myosin and tubulin
9. Toxic Proteins: Some are toxic to body tissues.
   Example: Snake venom and bacterial endotoxins 

Deficiency of Proteins

Deficiency of proteins occurs when one does not get enough proteins from the diet to meet the body’s requirements. Protein deficiency may lead to serious health complications. A severe form of protein deficiency leads to a disease called Kwashiorkor. It is common in Central Africa and South Asia, and 30% of children suffer. The general symptoms of kwashiorkor are:

1. Edema
2. Fatty liver
3. Fatigue
4. Diarrhea
5. Loss of muscle mass
6. Stunted growth
7. Irritability
8. Change in colour and texture of skin and nails
9. Flashy skin.
10. Increased severity of infections

Kwashiorkor can be treated with increased intake of proteins and calories overall, especially if treatment is started early.

Related Topics to Study

Summary

As we have seen, all the biological functions of proteins and their importance to our body functions are very high. Proteins carry out various tasks in our bodies. From growth, cell repair to immunity, many more functions is carried out by proteins. They are important for children, teens, and pregnant women.

FAQs

The frequently asked doubts about Proteins are answered here:

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