briefly introduce the structue of a polypeptide?

Peptide bond has partial double bond character, and the atoms in the peptide bond all lie in the amide plane. The only possible rotation of the main chain is about the alpha carbon-carbon bond and the alpha carbon-nitrogen bond. Therefore, the amide planes define the polypeptide backbone. There are 4 levels of structure found in polypeptides and proteins. The primary structure of a polypeptide of protein determines its secondary, tertiary, and quaternary structures.

The primary structure of polypeptides is the sequence of amino acids in the polypeptide chain. The primary structure may be thought of as a complete description of all of the covalent bonding in a polypeptide chain or protein.

Secondary structure :The two main secondary structures are the alpha helix and the anti-parallel beta-pleated sheet. There are other periodic conformations, but the α-helix and β-pleated sheet are the most stable. A single polypeptide or protein may contain multiple secondary structures.

Tertiary structure is maintained by disulfide bonds. Disulfide bonds are formed between the side chains of cysteine by oxidation of two thiol groups (SH) to form a disulfide bond (S-S).

Quaternary structure is used to describe proteins composed of multiple subunits.The arrangement of the monomers in the three-dimensional protein is the quaternary structure. For instance, there is hemoglobin protein. It is a package of its monomeric subunits. In hemoglobin, there are two α-chains, each with 141 amino acids, and two β-chains, each with 146 amino acids. Because there are two different subunits, hemoglobin exhibits heteroquaternary structure. If all of the monomers in a protein are identical, there is homoquaternary structure.