Introduction
Protein is the most important bio molecule. this so called macromolecules is composed of linear array of 20 different amino acids. this makes proteins more complex than DNA which is composed of four different nucleotide. proteins are also called polypeptide because they are assembled of peptides and peptide links (An amide linkage).

Structures
Traditionally biochemists defined four structure level for proteins. This classifications greatly help studying these structures.
Primary Structures
This structure is defined as linear array of amino acid in a protein:
Ala - Gly - Ser - Pro - Phe - ...
Secondary Structures
These structures are commonly repeated structures that can be observed in most proteins:

• Alpha Helix
• Beta Pleated sheets
• Random Coil
• Turns

Tertiary Structures

These structures are based on three dimensional positioning of secondary structures.
Motifs are common combination of secondary structures that protein poses.
Several example of motifes:

• Beta - Alpha - Beta
• Alpha - Alpha
• Greek Key
• Jelly roll

Domains are usually distinct part of protein that separately fold and often form globular portions.
Quartenery Structures
Often an active protein is multimer, that is, it is composed of several subunits(Like Hemoglubin and Rubisco). the orientation and translations of these subunits constitute the Quartenery structure.