# DBT JRF 2022 — Question 127

Exam: DBT JRF 2022 · Section: B · Topic: Biochemistry › Protein Structure - Amino Acid Localization · Difficulty: Medium

## Question

Where is the greatest likelihood of the occurrence of a proline residue in a protein structure?

## Options

- **A.** In the active site pocket
- **B.** In the beta sheets
- **C.** In the turns and loops
- **D.** In the alpha helices

## Answer

Option C

## Explanation

Proline in protein structure: turns and loops: Proline is a unique amino acid with a pyrrolidine ring side chain that is covalently bonded to its own backbone nitrogen: • This ring structure constrains the backbone torsion angle $\phi \approx -60^\circ$ (fixed). • This rigidity makes proline INCOMPATIBLE with regular secondary structures: - $\alpha$ -helix: Pro disrupts the regular H-bonding pattern (no NH hydrogen to donate — the nitrogen is in the ring) → called a 'helix breaker' - $\beta$ -sheet: Pro's fixed $\phi$ angle does not fit the $\phi/\psi$ angles required for $\beta$ -sheets • Pro is most commonly found in: - TURNS (especially Type I and Type II $\beta$ -turns, position i+1) - LOOPS and coil regions where backbone flexibility requirements are less strict - At the beginning of $\alpha$ -helices where it can be accommodated Statistical analysis of protein structures confirms enrichment of Pro at turns and loops. Answer: Proline's fixed $\phi$ angle makes it a helix/sheet breaker; found most commonly in turns and loops.

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