Protein Structure

You might not know, but protein is the most abundant substance in your body after water. Every single cell in your body consists of a protein. Proteins have a unique 3-D structure, enabling it to perform a variety of functions. Protein structures refer to a condensation of amino acids which forms peptide bonds. There are four types of structure in proteins. They are the primary structure of protein, the secondary structure of protein, tertiary, and quaternary. The primary structure is nothing but the sequence of amino acids in the protein. Secondary structure refers to dihedral angles of peptide bonds, and tertiary structure refers to the folding of proteins chains. In this article, we have explained the essential protein structure in an easy-to-digest format.   

Protein Structure: Definition

Proteins are nothing but biological polymers. They are polymers of amino acids joined together by amino acids. You must know that amino acids are the building blocks of proteins. It means that proteins have a chain-like structure, where amino acids are the primary ingredient. 


These amino acids get linked together with peptide bonds. When such a few bonds get linked together, it becomes a polypeptide chain. When one or more of these polypeptide chains gets twisted or folded, it forms a protein.  


The size of the protein varies significantly. It is dependent on the numbers of polypeptide molecules it holds. Insulin is one of the smallest protein molecules out there. And Titin is the largest protein molecule, having 34,350 amino acids.  


Classification of protein: Fibrous and globular are two types of proteins, decided by their molecular shape. When polypeptide chains run parallel, bonded by hydrogen and disulfide, you get a fibre-like structure. And when chains coil around, they give out a spherical shape.  


Also, four types of structure make up a protein molecule. You can learn about them as below. The image below can help you with understanding protein structures. 


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Primary Protein Structure 

The primary structure of a protein refers to a unique formation and sequence in which amino acids get combined. They all get linked together to produce a protein molecule. The primary structure is responsible for giving particular properties to protein. 


In total, there are 20 amino acids in the human body. They all have two groups – carboxyl and amino group. However, each one has a variable group, called an ‘R’ group. That R group is accountable for lending a unique structure to a protein. 


All the protein gets determined by the sequencing of the amino acids. This formation and sequence of amino acids in proteins is exceptionally unique. If you change even a single amino acid in the chain, then you can end up with a non-functioning protein.   


Secondary Protein Structure 

This secondary protein structure gives a unique shape to the protein. It’s where the peptide backbone of a protein structure gets fold onto itself. The folding of the polypeptide chains occurs because of the interaction between the carboxyl group and amine groups of the peptide chains. 


Secondary protein structure gives out two types of shapes; they are α-helix and β-pleated sheets.

  • α-helix – The backbone of protein follows a helical structure. Across different layers of the helix, the hydrogen forms bonds with oxygen, rendering a helical structure.

  • β-pleated sheet – In this shape, polypeptide chains get stacked next to each other. The external hydrogen molecules of these chains form intramolecular bonds, giving it a sheet-like structure.  

Tertiary Protein Structure 

Tertiary structure is responsible for the formation and 3-D shape of the protein. As amino acids form bonds during secondary structure, they give out shapes such as helices and sheets. Further, the structure can coil or fold randomly, and that’s what you call the tertiary structure of proteins. When the structure gets disturbed, the protein becomes denatured. Such a protein gets chemically affected, and the structure becomes distorted.    

Quaternary Protein Structure 

The spatial arrangement of two or more peptide chains leads you to quaternary protein structure. You should know that proteins don’t necessarily need to have a quaternary structure. Also note that primary, secondary, and tertiary structures of proteins are available in all-natural proteins. But, that’s not the case for quaternary structure. Thus, when a particular protein has the first three structures, it can qualify to be a protein. 

FAQ (Frequently Asked Questions)

1. Explain the classification of proteins. 

Proteins can get classified into two different types, depending on their molecular shapes. One is a fibrous protein, and another one is globular protein. 

Fibrous protein: When the polypeptide chains are running parallel, and it gets held together by hydrogen and disulfide bonds, then you get a fibre-like structure. These proteins are not soluble in water. Keratin and myosin are fibrous proteins, to name a few.

Globular proteins: When chains of polypeptides coil around to render a spherical shape, the resulting structure is a globular one. These proteins are soluble in water. Albumins and insulin are examples of the globular proteins.


2. What is a protein structure? State its stages. 

Primary structure of a protein is nothing but a sequence of amino acids in a polypeptide chain. During the process of protein biosynthesis, peptide bonds get created, which holds the primary structure in place. There are four levels of protein structures. Those four stages are principal, secondary, tertiary, and quaternary. By knowing the purpose and role of each level of protein structure, you can understand how the protein functions. The primary level gives particular properties to a protein referring to a sequence of amino acids in it. Secondary level gives a unique shape, and the tertiary level refers to 3-D shape and folding of protein chains.