Protein stability is dictated by the type of interactions that help the folding of the entire protein molecule. The primary structure of proteins is the most stable since the protein sequence is held together by covalent bonds between individual amino acids. Some of the tertiary structures that are held together by disulfide bonds are the most stable other than the primary sequence of the protein. Among the non-covalently bound structures, protein secondary structure is the most stable due to the predominant hydrogen bonding interactions.
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