Q.In fibrous proteins, polypeptide chains are held together by _______. (Note: one or more of the following options may be correct.)

The key idea is that fibrous proteins, like collagen or keratin, rely on a few dominant, specific interactions to hold their polypeptide chains together — not just any weak intermolecular force.

1. Disulphide linkages ($-S-S-$) form covalent bonds between cysteine residues, providing permanent, strong cross-links in proteins like keratin.
2. Hydrogen bonds occur between the $N-H$ and $C=O$ groups of the polypeptide backbone, stabilising the repeating secondary structures (e.g., the coiled-coil in keratin, the triple helix in collagen) that are characteristic of fibrous proteins. …

The question asks what holds polypeptide chains together in fibrous proteins. Fibrous proteins — like collagen, keratin, and myosin — are structural proteins with long, rod-like shapes. Their strength and stability come from two dominant interactions.

1. Hydrogen bonds are the backbone of secondary structure. In α-keratin (a fibrous protein), the α-helices are stabilized by hydrogen bonds between the carbonyl oxygen of one residue and the amide hydrogen of another, four residues away. In collagen, hydrogen bonds form between the three polypeptide strands of the triple helix. Without hydrogen bonds, the chains would simply drift apart. So (iv) is correct.

2. Disulphide linkages are covalent bonds between the sulfur atoms of two cysteine residues. These are especially important in keratin — the protein in hair, nails, and skin. The more disulphide cross-links, the harder the material (think horn vs. soft fur). So (ii) is correct.

3. Van der Waals forces (i) are weak, non-specific attractions that occur between all atoms when they are close together. They exist in every protein, but they are far too weak and non-directional to be the defining force holding fibrous-protein chains together, so this is not the answer the question is looking for. …

Method: Conceptual Elimination by Bond Type & Protein Structure

Step 1 – Recall the defining feature of fibrous proteins

Fibrous proteins (e.g., collagen, keratin, elastin) are structural — they form long, parallel polypeptide chains that are tightly packed to provide strength and elasticity.

Step 2 – List the bonds that stabilize any protein structure

• Primary structure: peptide bonds (covalent)
• Secondary & tertiary structure: hydrogen bonds, disulphide bridges, ionic (electrostatic) interactions, van der Waals forces
• Quaternary structure: same set of non-covalent bonds + sometimes disulphide links

Step 3 – Identify which bonds are especially important in fibrous proteins

Fibrous proteins rely heavily on inter-chain stability. The key stabilizers are:

• Hydrogen bonds — between N–H and C=O groups of adjacent chains (e.g., in collagen triple helix)
• Disulphide linkages — covalent cross-links between cysteine residues (e.g., in keratin)
• Electrostatic forces — ionic attractions between charged side chains (e.g., in silk fibroin) …

Here are the common mistakes students make on this question about Biochemical Bonds in Fibrous Proteins, along with how to avoid each.

Mistake 1: Selecting only one option (e.g., only hydrogen bonds)

Why it happens:

Students often memorize that fibrous proteins (like collagen, keratin, or silk fibroin) are stabilized by hydrogen bonds between polypeptide chains. They stop there, forgetting that fibrous proteins are multiply stabilized.

How to avoid:

Always recall the structural hierarchy of fibrous proteins. They are not just held by one type of bond. In a typical fibrous protein:

• Hydrogen bonds (between N–H and C=O groups) give strength and elasticity.
• Disulphide linkages (covalent –S–S– bonds between cysteine residues) provide permanent cross-linking (e.g., in keratin).
• Electrostatic forces (ionic bonds between charged side chains) also contribute.
• van der Waals forces (hydrophobic interactions) are present but are weak and not the primary stabilizers.

Key takeaway: The question says “one or more options may be correct.” Always check all options for relevance.

Mistake 2: Including van der Waals forces as a primary stabilizer

Why it happens:

Students think “all non-covalent interactions matter equally.” In fibrous proteins, van der Waals forces are present but are not the main forces holding polypeptide chains together — they are too weak and non-directional.

How to avoid:

Remember the relative strength of interactions in protein structure:

• Strongest: Covalent bonds (disulphide linkages)
• Moderate: Hydrogen bonds, electrostatic forces
• Weakest: van der Waals forces

For fibrous proteins, the primary stabilizers are hydrogen bonds, disulphide bonds, and electrostatic attractions. van der Waals forces are secondary and not exam-worthy as a “holding” force here.

Mistake 3: Confusing fibrous proteins with globular proteins

Why it happens:

Students mix up the bonding patterns. In globular proteins, hydrophobic interactions (van der Waals) are crucial for folding. In fibrous proteins, the chains are extended and parallel — so inter-chain hydrogen bonds dominate.

How to avoid:

Use a simple mnemonic:

• Fibrous → F for Filamentous, Forces = Hydrogen, Disulphide, Electrostatic.
• Globular → G for Globular, Gets Van der Waals (hydrophobic core).

Mistake 4: Forgetting that disulphide linkages are covalent and permanent

Why it happens: …