Describe the four structural levels of proteins

Note: the 3 first protein structures (primary, secondary, tertiary) are referred to a single polypeptid, and not a protein that consists of more than one polypeptids. The quaternary structure is referred to a protein that is consisted by more than one polypeptids.

Primary structure:
The primary structure of the polypeptide is the sequence of its amino acids. If you know the sequence of the amino acids, then you know the primary structure.

Secondary structure:
Secondary structures involve structures that are the result of local interaction of the amino acids (mainly hydrogen bonds). You must also mention that the main types of secondary structures are alpha helix and beta sheet.

Tertiary structure
It is the three-dimensional structure of the protein. It is basically how the alpha helix and beta sheets (and other protein parts) folds and coils with each other to form a complex three-dimensional structure. More specifically, the tertiary structure tells us the location (coordinates) of all the atoms of the polypeptide (in three dimensions).
Tertiary structure is the result of interactions like hydrogen bonds, disulfide bonds, and non-covalent bonds as well.

Quaternary structure
It is the three dimentional stucture of a protein that is consisted by more than one polypeptids (subunits). Polypeptids interact with each other (hydrogen bonds, disulfide bonds, and non-covalent etc) to form a more complex three dimentional structure.
Similarly, the location (coordinates) of every atom of the protein is the quaternary structure.


Note that, if a protein is consisted by one polypeptid, then it does not have Quaternary structure (it only has primary, secondary and tertiary structures). Only proteins that are consisted by more than one polypeptids have quaternary structures.