Question

Which of the following are CORRECT about protein structure?
P. Secondary structure is formed by a repeating pattern of interactions among the polypeptide backbone atoms
Q. Tertiary structure is the three-dimensional arrangement of the polypeptide backbone atoms only
R. Quaternary structure refers to an assembly of multiple polypeptide subunits

• A. P and Q only
• B. P and R only
• C. Q and R only
• D. P, Q and R

Hint:

Remember: Secondary = backbone H-bonds; Tertiary = backbone + side-chain interactions in one chain; Quaternary = association of multiple polypeptide chains.

Answer 1

The Correct Option is B

Step 1: Check statement P (Secondary structure).
Secondary structure arises mainly from regular hydrogen bonding between polypeptide backbone atoms (C=O and N–H groups).
These repeating interactions form $\alpha$-helices and $\beta$-sheets.
So, P correctly describes secondary structure and is true.
Step 2: Check statement Q (Tertiary structure).
Tertiary structure is the overall three-dimensional folding of a \emph{single} polypeptide chain.
It depends on interactions involving both the backbone \emph{and} the side chains (R-groups): hydrophobic interactions, ionic bonds, hydrogen bonds, disulfide bonds, etc.
Since Q says “backbone atoms only”, it ignores side-chain interactions.
Therefore, Q is false.
Step 3: Check statement R (Quaternary structure).
Quaternary structure is defined as the spatial arrangement of multiple polypeptide chains (subunits) in a protein complex.
Examples include hemoglobin (four subunits) and many enzyme complexes.
Thus, R correctly states that quaternary structure refers to an assembly of multiple polypeptide subunits.
So, R is true.
Step 4: Combine the correct statements.
Correct statements: P (true), Q (false), R (true).
Hence, the correct option is (B) P and R only.