Proteins are polymers of amino acid residues. They are versatile and responsible for different cellular functions, including DNA replication, molecular transport, catalysis, and structural support. Proteins have a hierarchical structure comprising at least three levels of organization: primary, secondary, and tertiary structure. Some large proteins have a quaternary structure where individual protein subunits are linked together.

The primary structure of a protein is its amino acid sequence. Amino acids are linked through peptide bonds to form a polypeptide chain with two ends: the amino terminus (N-terminus) and the carboxyl terminus (C-terminus). The sequence of amino acids determines the final folded form of the protein. Twenty different amino acids are arranged in different sequences to create a variety of polypeptides.

The secondary structure refers to locally folded regions of amino acid chains that are stabilized by hydrogen bonds. There are two types of secondary structures: alpha-helices and beta-pleated sheets. These secondary structural elements are connected by simple loops.

As the secondary structures assemble further, the final folded form of the protein is generated. Such a three-dimensional or tertiary structure of a protein is stabilized by the interactions between amino acid side chains. Hydrogen bonds, electrostatic forces, disulfide linkages, and Vander Waals forces stabilize the folded form, which is often the native or functional state of the protein.

When two or more folded polypeptide chains called protein subunits form a complex, a quaternary protein structure is created.