Proteins are biomolecules present in all living cells and play a very important role in its functioning and also its survival. Proteins are present in many forms in our body like hair, cartilage, muscles, tendon, skin and also ligaments. Proteins are also called as polypeptides. Proteins are made up of linear chains of amino acid molecules, which are linked together by a special bond known as polypeptide bonds.

Amino Acids:

Basic units of proteins are known as amino acids. Naturally 20 amino acids are present. These 20 amino acids are arranged in many different manners to form different types of proteins.

All amino acids have same basic structure but differ in their side-chain groups. The chemical nature and also physical nature of amino acids depend such as size, charge, structure and also reactivity depends on these side chains.

Peptide Bond:

Amino acids are linked with each other by a special chemical association called as peptide bond. These peptide bonds are formed as a result of dehydration reaction between the amino group of one amino acid with the carboxyl group of the adjacent amino acid.

Formation of a Peptide Bond:

Peptide bond is basically an amide bond which is formed as a result of the linkage between the C-terminal of an amino acid with that of N-terminal of another amino acid. This reaction required the hydrolysis of high energy phosphate bond or in other words this is an endergonic reaction.

Structure of Proteins:

Proteins are heteropolymers that is they are formed by the polymerization of the amino acids. The structure of proteins is depicted at four hierarchical levels in basic biology. They are

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

1. Primary Structure:
The linear arrangement of amino acid or in other words sequence of amino acids is called as the primary structure of protein. Primary structure of the protein gives us the positional information of the amino acids present in them. This helps us to understand the first amino acid present in the chain, second one and so on.

In another words, proteins are imagines as a line, where the left end of this line is represented by the first amino acid and the right end of this line is represented by the last amino acid molecule. The first amino acid present in the protein is termed as the N-terminal amino acid and the last amino acid of the protein is called as the C-terminal amino acid.

2. Secondary Structure:

A protein does not exist as a single linear structure. The primary amino acid structure folds in the form of a structure known as helix, which resembles a revolving stair case's cource. In biological conditions only some parts of the protein are usually arranged in the form of a helix, which is popularly known as localized organization.

Other region or remaining regions of the same protein thread may be folded into other forms of secondary structure such as alpha helix and a beta sheets. Alpha helix is rod like structures, where as beta sheets are a planar structure formed by the combination of more than two beta strands. This arrangement of amino acids or proteins is known as secondary structure.

Tertiary Structure:
Third level of structural hierarchy is known as tertiary structure. The long chain protein molecule can also be folded into complex hollow woollen ball this gives rise to a particular structure; this also gives rise to a three dimensional structure or view to a protein molecule. Tertiary structure is very much necessary for the normal biological functioning of the protein molecules. Tertiary structure is the highest level of organization for monomeric proteins or in other words proteins which consists of a single polypeptide chain.

Quaternary Structure:
Some proteins are made up of more than single polypeptide chain are known as multimeric proteins. These proteins structure are such that the individually folded polypeptides or subunits are arranged one upon another to form a complex structure such as a cube or a plate. This kind of arrangements which consists of two or more subunits is termed as the quaternary structure of proteins. Quaternary structure of a protein is very much important to conduct normal biological functions of a protein.

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