# DBT JRF 2020 — Question 141

Exam: DBT JRF 2020 · Section: B · Topic: Biochemistry · Difficulty: Easy

## Question

The primary structure of a protein is stabilized by

## Options

- **A.** hydrogen bonds
- **B.** ionic bond
- **C.** covalent bond
- **D.** the angle formed between plane ABD and plane ACD

## Answer

Option C

## Explanation

Peptide bond in primary protein structure: The backbone of a polypeptide consists of $N$ -C $\alpha$ - $C$ (O) repeat units connected by PEPTIDE BONDS (amide bonds). The peptide bond is a covalent bond ( $\text{C-N}$ bond with partial double-bond character due to resonance, bond energy $\approx 350 \text{ kJ/mol}$ ). The peptide bond is: planar (due to resonance); usually in trans configuration; resistant to hydrolysis under normal physiological conditions (very stable, half-life ~1000 years at neutral pH in the absence of enzymes). Denaturation (heat, urea, SDS, extremes of pH) disrupts H-bonds (secondary structure), hydrophobic interactions, and electrostatic bonds (tertiary/quaternary structure) but does NOT break peptide bonds (covalent). Therefore the primary structure is stabilised by covalent (peptide) bonds. Hydrogen bonds stabilise 2° structure; ionic bonds contribute to 3° structure.

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