What are Proteins?

We generally tend to think of proteins only from a dietary lens, as a component of what we eat. However, they are among the most important and abundant organic macromolecules in the human body, with diverse structures and functions. Every cell contains thousands and thousands of proteins, each with specific functions. Some help in the formation of cellular membrane or walls, some help the cell to move, others act as messages or signals and flow seamlessly from one cell to another, carrying information.  

Are there Different Types of Proteins? 

There are different types of proteins having different but specific functions. For instance, enzymes are proteins that act as catalysts. They help speed up biochemical reactions. Enzymes can recognize substrates and, after binding to a specific substrate, act as a catalyst to kick start reactions. Thus, every enzyme has a specific function and participates in a specific reaction to break down or build its products.   

The saliva secreted by the salivary glands present in the mouth contains an enzyme called salivary amylase. It helps to break down amylose and amylopectin (the two polysaccharides which form starch) into smaller sugar molecules such as dextrin and maltose. This sugar makes starchy foods often taste sweeter, as the salivary amylase works hard to break down the starch into sugar.   

Hormones are also a type of protein. These are released by the endocrine glands and act as chemical messengers that control physiological processes such as growth, metabolism, development, reproduction, and many other functions. Some hormones are steroid-based, and the ones based on proteins are called peptide hormones.  

You’ve probably heard of insulin, a peptide hormone that is critical in regulating blood glucose levels. When the glucose concentration in blood increases, especially after meals, a signal is sent to specific cells of the pancreas. They release insulin, which then binds to liver cells and others, signaling these organs to uptake glucose molecules through glucose transporters. Insulin thereby helps in restoring blood sugar to its normal levels. When an individual's body is unable to regulate insulin, the condition is referred to as diabetes.  

What does a Protein look like?

Proteins are present in different shapes and sizes. Some proteins are globular in shape and known as globular proteins. Some proteins are long, thin, and fibrous and thus referred to as fibrous proteins. Hemoglobin, the protein that carries oxygen in your blood, is a globular protein. Keratin, which is present in the hair and the outer layer of the skin, is an example of fibrous protein. The protein shape, to some extent, dictates the function of the protein owing to various chemical bonds that are required to hold this shape. Any variation in pH or temperature can negatively affect the protein’s structure or shape, causing it to become nonfunctional or function differently. This process is called denaturation, and the protein is said to be a denatured protein.   

What does Denaturation mean? 

Think of an egg white. When uncooked, it is clear, but as soon as it starts to cook, it turns opaque. The change in opacity is because egg white has a type of protein called albumin, which has a specific shape due to its chemical bonds. The heat disrupts these bonds and brings the hydrophobic amino acids to the top, usually present inside the protein. Now, these hydrophobic amino acids stick to one another and form a new type of bond or aggregates, which changes the structure of the egg white. From clear, egg white turns white and opaque due to protein denaturation.   

Composition of Proteins  

Proteins are made up of monomers called amino acids. Polypeptide proteins are made up of linear chains of amino acid residues. There are about 20 types of amino acids that make up proteins. Their basic structure consists of a central carbon atom - the alpha carbon (α) - which bonds with amino groups (NH₂), carboxyl groups (COOH), and hydrogen. Amino acids are generally considered neutral; at the isoelectric point, they occur as a zwitterion. Both the negative charge and positive charge on the amino acid balance each other.  

Based on whether the side chain of an amino acid is acidic, basic, polar or nonpolar the properties of the proteins and how it will behave in different environments is determined. For example, valine and leucine - both amino acids - are nonpolar and hydrophobic. Similarly, serine and glutamine are polar and hydrophilic. Lysine and arginine have positively charged side chains and are considered basic amino acids.   

In a protein, amino acids are bonded together by covalent bonds called peptide bonds. The formation of these bonds results in a dehydration synthesis reaction where the carboxyl group of one amino acid reacts with the amino group of another amino acid, releasing a water molecule.

Why are Proteins so Important? 

As mentioned earlier, proteins are the building blocks required by organisms to survive and sustain themselves. They not only help the body to grow but also involved in repair, cell structure maintenance, and providing support. Muscles are majorly made up of proteins, and as we already know, are crucial for the body to function. Besides, proteins are required for development and communication within the body, for example, enzymes and hormones. In short, life without proteins seems impossible.  

As proteins are constantly required for the upkeep of the body,  consumption of a protein-rich diet is important. Thus, consuming milk, eggs, and lean meat in limited quantities is great for the body.   

Plant-based foods also contain proteins; however, they are incomplete proteins. They lack certain essential amino acids that the body requires. Thus, plant-based proteins may not be able to completely support the growth and development of the body or repair and maintenance of the tissues. Plant-based proteins include nuts, legumes, beans, cereals, grains, lentils, peas, etc. Thus, plant-based protein should ideally be combined with supplements to provide complete nutrition to the body.  

How much protein you need in your diet depends on your age, body size, metabolism, quality of the protein you consume, and the condition of your body. For instance, a high-school athlete will need more protein than an average person and therefore needs a high-protein diet.  

Did you Know?  

• Lack of protein can slow down body activities. The protein deficiency makes you tired, lose weight, lose muscle mass, and lack energy.  
• Inadequate intake of proteins during childhood can cause stunted growth.  
• Long-term protein deficiency can damage the liver and even cause death.  
• After water, protein is the 2nd most abundant component of the human body.  
• The heart, liver, lungs, and most other organs in the body are made up of proteins. 
• Proteins are essential for your immune system. Antibodies produced by the body to fight off infections and neutralize antigens are made up of proteins. 
• Your hair, skin, and nails are made up of proteins. 
• Muscles are made up of proteins and are denser compared to fat. So, you can lose fat and gain muscle - which is the healthier way to go - and still not lose an ounce of weight!  

Common Misconceptions 

1. Proteins are not just globular or fibrous; they can be classified at four levels: primary, secondary, tertiary, and quaternary.  
2. Amino acids don’t make proteins - they are the building blocks that form the primary structure of the proteins. The functional structure of the protein is determined by different types of bonds between different polypeptides resulting in secondary, tertiary, and quaternary structures.  
3. Both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) sequences are responsible for protein coding. Thus, mutations in the sequence of RNA or DNA sequence can affect the protein structure and function. 

 Context and Application 

This topic is important for both Bachelors of Science and Master of Science entrance Exam as well as medical entrance exams: 

• Bachelors of Science in Biochemistry 
• Master of Science in Biochemistry 
• Bachelors and Masters of Science in Life sciences. 

Related Topics 

• Protein synthesis 
• Denaturation of Proteins 
• DNA (Deoxyribonucleic acid) 
• RNA (Ribonucleic acid) 
• Genetics