What are the four levels of protein structure?

The four levels of protein structure are: primary, secondary, tertiary, and quaternary structure. 

1. The primary structure of a protein refers to the specific sequence of amino acids joined together to create a polypeptide chain. During the process of protein synthesis, each amino acid is connected to the next one by peptide bonds, forming the linear sequence of the protein.
2. The secondary structure of a protein consists of segments within the amino acid chain that are held together by hydrogen bonds formed along the backbone of the polypeptide. These hydrogen bonds contribute to the formation of specific shapes like alpha-helices and beta-pleated sheets within the protein's structure. The hydrogen bonds in both structures occur between a carbonyl group in one amino acid and an amino hydrogen group of another amino acid along the protein chain. 
3. The tertiary structure of a protein is formed by the interactions among the side chains (also known as R-groups) of amino acids within the polypeptide chain. These interactions contribute to the three-dimensional shape and overall folding of the protein. Interactions among R-groups that shape the tertiary structure of a protein involve various forces such as dipole-dipole interactions, hydrogen bonding, ionic bonding, and London dispersion forces.
4. The quaternary structure of a protein refers to the overall shape created by the interactions between two or more polypeptide chains' side chains. This arrangement of multiple protein subunits contributes to the functional and structural complexity of the protein. The same interactions responsible for forming the tertiary structure of proteins are also involved in holding together the subunits within the quaternary structure.

When a protein encounters alterations in pH, temperature, or exposure to certain chemicals, the interactions responsible for maintaining its three-dimensional structure can be disturbed. As a result, the protein may unfold and lose its specific shape, returning to an unstructured sequence of amino acids while retaining its original primary sequence. This process is known as denaturation. Denatured proteins typically lose their functions and are unable to perform their roles in the body.