The ordered folding of polypeptide chain give rise to the 3-D conformation known as secondary structure of the protein such as helices, sheet and loops. Arrangement of the secondary structure gives rise to the tertiary structure. α-helix and β-sheet are connected via unstructured loops to arrange themselves in the protein structure and it allows the secondary structure to change their direction. Tertiary structure defines the function of a protein, enzymatic activity or a nature of structural protein. Different polypeptide chains are arranged to give quaternary structure (Figure 33.2).

Primary Struture: The amino acid sequence of the protein is known as primary structure (Figure 33.2). The order of amino acid determine the folding of the protein to achieve net minimum free energy. This is achived in multiple steps collectively known as folding.

Secondary Structure: The amino acid interact with each and as a result peptide chain folds into the secondary structures. These secondary structures are the building blocks for the tertiary structures. These secondary structures are as follows:

α-Helix: it is a helical structure termed as α-helix by Linus pauling. In this structure, the polypeptide backbone is wound around the central axis with R group of the amino acid protrude outward from the helix backbone (Figure 32.2). In most of the protein, α-helix is right handed.  

β-sheet: This is more extended conformation of polypeptide chain where R groups protrude from zigzag strture in opposite directions, giving a alternating structure (Figure 32.2). 

Turns: These secondary structure has no definite structure and these are present in protein structure to change the direction of running polypeptide (Figure 32.2). These are also found to the places to connect the successive α-helix and β-sheet. The number of amino acids and their preference in turn is not consistent. The two protein can adopt similar 3-D conformation by changing the length and keeping amino acids in the tuen region of the structure.

Tertiary structure: Secondary structure folds to give rise higher order organization, commonly known as tertiary structure. 

Quaternary Struture: If multiple polypeptides are involved in the constitution of protein, the tertiary structure of these different polypeptide chains come together to form quarternary structure.