What is the 4 levels structure of protein??????????????

3D structure of protein

The wide variety of 3-dimensional protein structures corresponds to the diversity of functions proteins fulfill.

Proteins fold in three dimensions. Protein structure is organized hierarchically from so-called primary structure to quaternary structure. Higher-level structures are motifs and domains.

Above all the wide variety of conformations is due to the huge amount of different sequences of amino acid residues.

Primary structure 

Each protein is built up from a set number of amino acids, joined and shaped in a particular way.

There are 20 different types of amino acids, so for a simple dipeptide there are 400 possible combinations;  8000 combinations for a tripeptide.
Clearly the number of possible combinations is almost infinite when larger numbers of amino acids are combined to form a polypeptide.

The primary level of protein structure is not just the number and  identity of the component amino acids in the protein, but the order or sequence in which the specific amino acids are combined  (by condensation, forming peptide bonds) in the polypeptide chain. 

Secondary structure

The polypeptide chain can fold back on itself in a number of ways. Each bond in the backbone formed by alternating amino acid central carbons and -CONH - peptide linkages can rotate completely, and so a number of shapes are possible.
It will become stabilised in a position where >NH groups (from the peptide bonds) become close enough to form hydrogen bonds with  >C=O groups on another peptide bond, in another chain, or further along in the same chain.
Often, these weak interactions between several successive amino acid residues result in a coiled (helical) structure.
The usual way in which the polypeptide chain is coiled is called the alpha helix, in which interactions take place between groups 3 - 4 amino acid residues apart. Some amino acids put a kink or bend in these regions of helical structure, allowing the chain to bend back on itself and form a more globular molecular structure.

Tertiary structure

The 3-dimensional structure of a protein's polypeptide chain or chains may be locked in place by other stronger bonds. These bonds are formed between components of  the -R groups of the amino acid residues. The types of bonds may include:

• ionic bonds or salt bridges (between acidic negatively charged and basic positively charged side groups) 
• hydrogen bonds (between -COOH and -NH₂ and -OH groups on side chains)
• hydrophobic forces between non polar side groups
• disulphide bridges - strong covalent bonds - between thiol groups on pairs of the amino acid cysteine 
• covalent links to other groups, such as the iron-containing haem unit in myoglobin and haemoglobin, or to other types of molecules like carbohydrates or lipids (forming glycoproteins and lipoproteins).

Quaternary structure

Not all proteins have a quaternary level of structure.

A protein with a quaternary structure consists of more than one practically identical sub-unit, not joined by strong bonds like those above.
A well known example is haemoglobin, which consists of 2 alpha and 2 beta chains, consisting of 141 and 146 amino acid residues respectively.
Many enzymes consist of  several sub-units, often as dimers (2 sub-units) and tetramers (4 sub-units) combined in this way.

Here is a video regarding the 3D structure of protein: