Q.42 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

Protein structures organize from primary amino acid sequences to complex 3D forms essential for function in biological systems like enzymes and signaling proteins.

Correct Answer

The correct choice is (B) P and R only. Statement P accurately describes secondary structure via backbone hydrogen bonds, while R correctly defines quaternary structure; Q is incorrect as tertiary involves side chains too.

Secondary Structure (P)

Secondary structure arises from repeating hydrogen bonds between carbonyl oxygen and amide hydrogen in the polypeptide backbone, forming alpha helices or beta sheets.
This local folding excludes side chains (R groups), relying solely on backbone interactions for stability.
Common in proteins like hemoglobin, these patterns enable compact folding.

Tertiary Structure (Q)

Tertiary structure represents the full 3D fold of a single polypeptide, driven by interactions among backbone and side chain R groups via hydrophobic effects, disulfide bonds, ionic bonds, and hydrogen bonds.
Q wrongly limits it to backbone atoms only, ignoring crucial R group roles that dictate overall shape and function.
Examples include myoglobin’s globular form stabilized by these diverse forces.

Quaternary Structure (R)

Quaternary structure assembles multiple polypeptide subunits into a functional complex, as in hemoglobin’s tetramer.
Held by non-covalent interactions between subunits, it applies only to multi-chain proteins.
This level enhances stability and regulation absent in single-chain proteins.

Why Other Options Fail

• (A) P and Q only: Fails due to Q’s inaccuracy on tertiary structure.

• (C) Q and R only: Incorrect as Q is wrong and omits valid P.

• (D) P, Q and R: Invalid because of Q’s error.