Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
Which one of the following is INCORRECT about protein structures?
Options:
(A) A protein fold is stabilized by favorable non-covalent interactions
(B) All parts of a fold can be classified as helices, strands or turns
(C) Two non-covalent atoms cannot be closer than the sum of their van der Waals radii
(D) The peptide bond is nearly planar
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.19)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Biochemistry (Protein Secondary Structure & Ramachandran Principles)
**2. Detailed Step-by-Step Analysis**:
- Let us evaluate each structural statement regarding protein architecture:
- **Statement A**: *Major force stabilizing secondary structures is hydrogen bonding.* This is completely true; secondary structures ($lpha$-helices, $eta$-sheets, $eta$-turns) are primarily stabilized by backbone hydrogen bonds between carbonyl oxygens ($C=O$) and amide hydrogens ($N-H$).
- **Statement B**: *Subunits of multimeric proteins are held together only by non-covalent interactions.* This is **INCORRECT**. While many multimeric proteins are held together exclusively by hydrophobic interactions, hydrogen bonds, and salt bridges, numerous multimeric proteins are stabilized by **interchain covalent disulfide bonds** (e.g., the heavy and light chains of immunoglobulins/antibodies, and the A and B chains of active insulin). Thus, the word "only" renders this statement false.
- **Statement C**: *Hydrogen bonds in an $lpha$-helix are parallel to the helix axis.* This is completely true; the intrachain hydrogen bonds between residue $i$ and $i+4$ align almost parallel to the central helical axis.
- **Statement D**: *Ramachandran plot displays allowed dihedral angles of amino acids in a polypeptide.* This is completely true; it plots backbone torsional angles $\phi$ (phi) and $\psi$ (psi).
- **Option-by-Option Evaluation**:
- **Option (A) [TRUE/NOT ANSWER]**: Accurately describes secondary structure stabilization.
- **Option (B) [INCORRECT STATEMENT / CORRECT OPTION]**: Multimeric proteins frequently utilize covalent interchain disulfide bonds (e.g., IgG, insulin), making the claim that they are held together *only* by non-covalent interactions false.
- **Option (C) [TRUE/NOT ANSWER]**: Accurately describes the orientation of $lpha$-helical hydrogen bonds.
- **Option (D) [TRUE/NOT ANSWER]**: Accurately defines the Ramachandran plot.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **B**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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