Protein in Chemistry: Definition, Classifications, Functions and Examples

Imagine having a big steak or a salad with chickpeas; one savors the flavor, but have you ever thought about what proteins do for you? Far more than an important nutrient, they are truly at the core of life—participating in nearly every biological process from muscle growth to immune response, hormone production to enzyme production, crucial to our healthy well-being.

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This Story also Contains

1. What Are Proteins?
2. On the basis of molecular structure, proteins are classified as:
3. Type of Proteins and Their Importance
4. Relevance and Applications of Proteins
5. Some Solved Examples
6. Summary

We encounter proteins in our lives either as food, supplements, or even as medicines. Proteins are taken with great concern by athletes for the recovery and growth of muscles. Plant-based proteins are sought after by health-conscious people to add to their diets. Other than nutrition, proteins form a frontier of ongoing cutting-edge research in medicine and biotechnology that is changing the way new treatments and sustainable practices are being developed.

What Are Proteins?

Proteins are large, complex molecules consisting of chains of amino acids. Amino acids are organic compounds that contain carbon, hydrogen, oxygen, nitrogen, and on rare occasions, sulfur. They have vital roles in structure, function, and regulation in the body's tissues and organs. What makes each protein different in shape and function is its unique combination of amino acids. There are 20 different amino acids that are bonded in a sequence to form thousands of kinds of proteins, each tailored to perform certain jobs. There are two large classes of proteins: first, the structural ones that give support or shape to cells and tissues, and secondly, functional ones facilitating biochemical reactions. It is such knowledge of proteins, lying at the very heart of any knowledge in biology, for instance, since they play a part in nearly every cellular process and are thus so vital for life.

Proteins are considered biopolymers having a large number of amino acids linked to each other through peptide linkages containing three-dimensional (3D) structures. Protein structure can be studied at four levels, i.e, primary, secondary and tertiary, and quaternary structures.

1. Primary structure:

Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked with each other in a specific sequence and it is this sequence of amino acids that is said to be the primary structure of that protein. Any change in this primary structure i.e., the sequence of amino acids creates a different protein.

2. Secondary structure:

The conformation in which the polypeptide chains assume a shape as a result of H-bonding is called the 2^o structure on protein. Due to the fractional double bond nature of the C-N bond is peptide linkage, the amide part, i.e., -CO-NH- is planar and inflexible, i.e, free rotation about this bond is not possible. Secondary structure is further divided into α-helix and β-pleated sheet.

3. Tertiary structure:

The 3^o structure of a protein pertains to its entire 3D structure, i.e, the way in which the whole protein molecule folds up in the 3D space to form a particular shape.

4. Quaternary structure:

Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure.

The diagrammatic representation of all these structures are shown below:

On the basis of molecular structure, proteins are classified as:

1. Fibrous proteins:

They are linear or thread-like molecules, lying side by side to form fibres. The polypeptide chains are held by H-bonding and some by disulphide bonds, and hence have high intermolecular forces of attraction. Thus, they are insoluble in H₂O and are stable to moderate changes of pH and temperature. They are chief structural materials of animal tissues, e.g., keratin in skin, hair, wool, nails; collagen in tendons; fibroin in silk, etc.

2. Globular proteins:

The folding of polypeptide chains gives a spheroidal shape in which hydrophobic parts are inward and hydrophilic parts are outwards, thus water molecules interact strongly with the polar groups and hence they are water-soluble. They are very sensitive to small changes of pH and temperature. They include all enzymes, many hormones such as insulin from the pancreas, thyroglobulin from thyroid gland, antibodies, haemoglobin, etc.

Type of Proteins and Their Importance

Proteins can be categorized by property into structure and function. Major types include:

1. Enzymatic Proteins:

They are proteins that behave like catalysts in biochemical reactions. One example is amylase, which breaks starches down to sugars.

2. Structural Proteins:

They give cells and tissues support and shape. Examples would include collagen found in connective tissues and keratin in hair and nails.

3. Transport Proteins:

These proteins transport substances in the body. One classical example of a transport protein is hemoglobin, responsible for oxygen carriage in the blood.

4. Antibodies:

It's the group of proteins that includes an important part of the immune system: proteins recognizing and inactivating foreign invaders like bacteria and viruses.

5. Hormonal Proteins:

These proteins act as messengers in integrating many physiological processes. The best example would probably be insulin, which is involved in the regulation of blood sugar.

Each of these proteins is essential to human health, and all are necessary for the body to function optimally; therefore, the human must consume a daily supply that represents a good variety of foods to provide an adequate intake of meats, dairy products, legumes, and nuts in the diet.

Relevance and Applications of Proteins

It cannot be limited to the area of basic nutrition; they are involved in many areas, from medicine and biotechnology to environmental sciences. Proteins have a very important role in medicine, in the development of therapies and treatments. For instance, recombinant DNA technology in the treatment of diabetes is required for the generation of synthetic insulin, which is a protein whose normal function is to regulate the levels of blood sugar.

Proteins are used in biotechnology in producing enzymes for industrial processes, production of biofuels, and biodegradable plastics. With the art of engineering proteins to obtain a given function, industries are revolutionized, setting new routes toward sustainability.

Moreover, knowledge of proteins is highly significant in research and academic circles. Researchers in this field investigate different protein structures and functions to understand diseases better. This has resulted in some very important discoveries in drugs, advances in the area of personalized medicine, etc. This can be well explained by the fact that research into cancerous diseases is focused on protein interactions and mutations, which give way to tumor growth, hence opening up selective ways of treatment.

It is, therefore, important to study proteins within nutrition science as well. Herein, understanding the amino acid profiles of foods underlies dietary recommendations and helps solve key health problems worldwide, one of which is malnutrition.

The importance of proteins thus is not only the basis of life but extends to huge implications in many other respects. In other words, thanks to their versatility and key roles, they become a focus that reaches from health care to environmental sustainability.

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Some Solved Examples

Example 1

Question: Insulin production and its action in the human body are responsible for the level of diabetes. This compound belongs to which of the following categories?
1) A co-enzyme
2) A hormone
3) An enzyme
4) An antibiotic

Solution: Insulin is a globular protein and is a biochemically active hormone secreted by the pancreas. Hence, the answer is option (2).

Example 2
Question: Which bond holds the alpha-helix in protein together?
1) Covalent bond between C and N atoms
2) Covalent bond between N and O atoms
3) Intermolecular hydrogen bonds
4) None of these

Solution: Polypeptide chains form all possible hydrogen bonds to create the alpha-helix shape. Hence, the answer is option (3).

Example 3
Question: In ________, all peptide chains are stretched out to nearly maximum extension and then laid side by side which are held together by intermolecular hydrogen bonds.
1) alpha helix structure
2) gamma plate structure
3) beta pleated sheet structure
4) None

Solution: Beta-Pleated sheets consist of two or more alpha-helix structures aligned and held together by intermolecular hydrogen bonds, forming a sheet-like structure. Hence, the answer is option (3).

Example 4

Question: Which of the following forces stabilize the tertiary structure of proteins, i.e., further folding of the secondary structure?
1) Disulphide linkages
2) Hydrogen bond
3) Electrostatic forces of attraction
4) All of these

Solution: Tertiary structure is stabilized by various interactions including ionic bonds, disulphide bonds, hydrogen bonds, and van der Waals forces. Hence, the answer is option (4).

Summary

They are large, complex molecules integral to life, made up of amino acids, which play nearly every known function in the human body. They can be divided into many different categories, enzymatic, structural, transport, antibody, and hormonal proteins being a few. Moreover, their importance extends beyond the basics of nutrition because they are very vital in medicine, biotechnology, and research in view of therapeutic progress, new industrial applications, and dietary changes.