A new drug is developed which selectively cleaves covalent bonds between two sulfur atoms of non-adjacent amino acids in a polypeptide chain. Which level of protein structure in affect molecules would be most directly affected by the drug?
A Quaternary
B Tertiary
C Primary
D Secondary
E All of the above would be equally affected
The correct answer and explanation is:
The correct answer is B Tertiary.
Proteins have several levels of structure: primary, secondary, tertiary, and quaternary. The primary structure refers to the linear sequence of amino acids in a polypeptide chain. The secondary structure involves local folding patterns, such as alpha helices and beta sheets, stabilized by hydrogen bonds. The tertiary structure refers to the overall 3D shape of the protein, which is stabilized by interactions between the side chains (R groups) of the amino acids, including disulfide bonds. Finally, the quaternary structure involves the interaction of multiple polypeptide chains to form a functional protein complex.
In this scenario, the drug targets disulfide bonds, which are covalent bonds formed between the sulfur atoms of cysteine residues that are often non-adjacent in the primary sequence. Disulfide bonds play a crucial role in stabilizing the tertiary structure of a protein. These bonds help to lock the protein into its specific three-dimensional conformation. By cleaving these disulfide bonds, the drug would disrupt the protein’s tertiary structure, potentially causing it to unfold or misfold.
The primary structure (the sequence of amino acids) would not be directly affected by the drug, as the amino acid sequence remains unchanged. Secondary structure elements like alpha helices and beta sheets may also be affected indirectly because they depend on the overall folding of the protein, but they are not directly stabilized by disulfide bonds. The quaternary structure, if it exists, could also be indirectly affected, but the primary target of the drug is the tertiary structure. Therefore, the tertiary level of protein structure would be most directly affected by the drug.