What are the structures of protein?

Proteins are polymers. They are large molecules made up of smaller amino acid molecules. Each amino acid molecule is made up of a carbon atom, an amino group, a carboxyl group, and a side chain (also known as an R group). The amino acid sequence of a polypeptide chain determines the final 3D structure of the protein. The function of a protein is directly related to its structure. 

There are four levels of protein structure - primary, secondary, tertiary, and quaternary. 

1. Primary Protein Structure - consists of a simple polypeptide chain in which the amino acids are bound together as a linear chain via covalent bonds. This is the most basic type of protein structure. Even a tiny mutation in the amino acid sequence of the polypeptide chain can affect protein folding, impacting the overall structure and function of the protein. 
2. Secondary Protein Structure - is formed by the repetitive folding of the polypeptide chain. When the polypeptide chain folds up, hydrogen bonds form between the atoms of the chain. These hydrogen bonds help to maintain the secondary structure. There are two main types of secondary protein structures: the α-helix and the β-pleated sheet. The α-helix appears as a coil formed by hydrogen bonds between the carbonyl group and the amino group of different amino acids. The β-pleated sheet is formed by the hydrogen molecule of an amino acid on one sheet and hydrogen bonds between the carboxyl groups of one amino acid on another sheet. 
3. Tertiary Protein Structure - is a unique 3D structure formed by further folding of secondary proteins. The formation of the tertiary protein structure occurs via several types of bonds including covalent bonds, electrostatic or ionic bonds, hydrophobic bonds, or hydrogen bonds. Tertiary proteins are globular in shape and contain a binding site for the protein action. A protein is fully functional only when it has this 3D shape. 
4. Quaternary Protein Structure - is formed when multiple polypeptide chains link together to create a functioning unit. This structure is formed via bonds between the R groups of different amino acids within the polypeptide chains. This bonding contributes to the shape of the protein. At this point, the protein is fully functional and capable of performing its specific role or roles within the body. Hemoglobin is one of the most notable examples of a quaternary protein structure. It consists of four polypeptide chains, and is specially adapted to bind oxygen in the blood.