# DBT JRF 2022 — Question 119

Exam: DBT JRF 2022 · Section: B · Topic: Biochemistry › Membrane Proteins & Hydrophobic Segments · Difficulty: Medium

## Question

A protein sequence has twenty consecutive hydrophobic amino acids followed by a few hydrophilic residues and this pattern repeats itself five to seven times. Which kind of a protein is described here?

## Options

- **A.** A secretory protein
- **B.** A transmembrane protein
- **C.** A cytoplasmic soluble enzyme
- **D.** A ribosomal protein

## Answer

Option B

## Explanation

Transmembrane domain prediction from sequence: Alpha-helical transmembrane (TM) domains in integral membrane proteins: • A stretch of ~18–25 consecutive hydrophobic amino acids in an $\alpha$ -helical conformation can span the lipid bilayer (~30 Å = 20 residues at 1.5 Å/residue rise). • Each TM helix is typically 20–25 amino acids of predominantly hydrophobic residues (Leu, Ile, Val, Ala, Phe, Met, Trp). • Multiple such hydrophobic stretches (repeats) indicate a MULTI-PASS transmembrane protein. Prediction tools: TMHMM, TMbase, Phobius, HMMTOP. If the sequence shows 5–7 hydrophobic stretches of ~20 aa each, the protein likely has 5–7 TM helices (e.g., a GPCR has 7 TM helices, ion channels have multiple TM repeats). Answer: ~20 consecutive hydrophobic residues = one TM helix; multiple such repeats = multi-pass transmembrane protein.

Source: https://mypyqbuddy.com/questions/a-protein-sequence-has-twenty-consecutive-hydrophobic-amino-6a104cd5920734408bc0c786