Discuss the structure and shape of proteins.

(N/A) Proteins are complex polymers of $\alpha$-amino acids. Their structure is described at four levels:
$1$. Primary structure: The specific sequence of amino acids in a polypeptide chain.
$2$. Secondary structure: The conformation of the polypeptide chain,which includes $\alpha$-helix and $\beta$-pleated sheet structures,stabilized by hydrogen bonding.
$3$. Tertiary structure: The overall three-dimensional folding of the polypeptide chain,resulting from interactions like hydrophobic interactions,ionic bonds,hydrogen bonds,and disulfide linkages.
$4$. Quaternary structure: The spatial arrangement of two or more polypeptide chains (subunits) in a multi-subunit protein.
Based on shape,proteins are classified as:
$(i)$ Fibrous proteins: Long,thread-like molecules held together by hydrogen and disulfide bonds (e.g.,keratin,myosin).
(ii) Globular proteins: Spherical shapes resulting from the folding of polypeptide chains (e.g.,insulin,albumin).