Which level of protein structure reflects interactions among multiple polypeptide chains?

Which level of protein structure reflects interactions among multiple polypeptide chains?

• A. Secondary structure
• B. Tertiary structure
• C. Quaternary structure
• D. Primary structure

Answer: (C)

Quaternary structure describes how multiple polypeptide chains assemble into a single functional protein complex. After each chain folds into its own three-dimensional shape (tertiary structure), the subunits come together and interact through noncovalent forces—hydrophobic contacts, hydrogen bonds, ionic interactions—and sometimes disulfide bridges to stabilize the whole assembly. This level is exemplified by hemoglobin, a tetramer composed of two alpha and two beta chains that work together to transport oxygen. Not all proteins have this level of organization; some exist as a single polypeptide, in which case only the primary, secondary, and tertiary structures are relevant.

Quaternary structure describes how multiple polypeptide chains assemble into a single functional protein complex. After each chain folds into its own three-dimensional shape (tertiary structure), the subunits come together and interact through noncovalent forces—hydrophobic contacts, hydrogen bonds, ionic interactions—and sometimes disulfide bridges to stabilize the whole assembly. This level is exemplified by hemoglobin, a tetramer composed of two alpha and two beta chains that work together to transport oxygen. Not all proteins have this level of organization; some exist as a single polypeptide, in which case only the primary, secondary, and tertiary structures are relevant.