What are the levels of protein structure?

There are four levels of protein structure responsible for a protein's unique conformation (shape, which determines its job and how it functions).

The primary structure refers to the unique sequence of amino acids.

The secondary structure results from hydrogen bonding within the polypeptide molecule. It refers to how the polypeptide coils or folds into two distinct shapes: an alpha helix or a beta-pleated sheet.

Tertiary structure is the intricate three-dimensional shape or conformation of a protein that is superimposed on its secondary structure. Tertiary structure determines the protein's specificity, or how few ligands the protein's binding site can bind to.

Quaternary structure refers to proteins that consist of more than one polypeptide chain. For example, hemoglobin consists of four polypeptide chains.