What are the main interactions that stabilize each level of protein structure?

The primary structure is held together by covalent peptide bonds. They are formed during the process of protein biosynthesis, where the amino acids lose one water molecule per reaction to attach to another amino acid.

The secondary structure is determined by hydrogen bonds between the main-chain peptide groups.

The tertiary structure is held by multiple types of bonds and forces, including hydrophobic interactions, hydrogen bonding, disulfide bridge, ionic bonding, as well as van der Waals forces. Among these forces, the non-specific hydrophobic interaction is the main force driving the folding of protein, while hydrogen bonds and disulfide bonds are responsible for maintaining the stable structure.

The quaternary structure is also stabilized by the non-covalent interactions and disulfide bonds as in the tertiary structure, where more than one polypeptide is held together to form a single functional unit called multimer.