All of the following would be considered a tertiary structure of a protein EXCEPT: A. transmembrane proteins B. globular proteins C. cooperative interaction of multiple subunits D. fibrous proteins E. all of the above
The correct answer and explanation is:
Correct Answer: C. cooperative interaction of multiple subunits
Explanation:
Proteins have several levels of structural organization:
- Primary structure – the sequence of amino acids in the polypeptide chain.
- Secondary structure – local folding patterns such as α-helices and β-sheets, stabilized by hydrogen bonds.
- Tertiary structure – the overall 3D shape of a single polypeptide chain, resulting from interactions among side chains (R-groups), including hydrogen bonding, ionic interactions, disulfide bridges, and hydrophobic packing.
- Quaternary structure – formed when multiple polypeptide chains (subunits) come together and function as one unit. This level involves cooperative interactions among these subunits.
Now, let’s examine each option:
- A. Transmembrane proteins: These are proteins that span a membrane and have a defined 3D structure within the membrane, typically part of their tertiary structure.
- B. Globular proteins: These are compact, water-soluble proteins with a complex tertiary structure, often involved in dynamic functions like enzymatic activity.
- C. Cooperative interaction of multiple subunits: This describes a quaternary structure, not tertiary. Hemoglobin is a classic example where multiple subunits (each with its own tertiary structure) work together, and their interaction leads to cooperativity in oxygen binding.
- D. Fibrous proteins: These have elongated shapes and structural roles, with a defined tertiary structure (e.g., collagen or keratin).
- E. All of the above: Incorrect because C is not a tertiary structure.
Conclusion:
The cooperative interaction of multiple subunits is characteristic of the quaternary structure, not the tertiary. Therefore, the correct answer is C, as it does not represent a tertiary structure.