Explain the types of protein structures based on the provided image.

(N/A) The image illustrates the $\alpha$-helix structure,which is a type of secondary protein structure. Proteins have four levels of structural organization:
$1$. Primary Structure: The specific sequence of amino acids in a polypeptide chain.
$2$. Secondary Structure: Refers to the shape in which a long polypeptide chain can exist. It includes $\alpha$-helix and $\beta$-pleated sheet structures,stabilized by hydrogen bonding between the $C=O$ and $N-H$ groups of the peptide backbone.
$3$. Tertiary Structure: The overall three-dimensional folding of the entire polypeptide chain,stabilized by various interactions like hydrogen bonds,ionic bonds,disulfide linkages,and van der Waals forces.
$4$. Quaternary Structure: The spatial arrangement of two or more polypeptide chains (subunits) in a multi-subunit protein.