Q1. The portion of transmembrane proteins that cross the lipid bilayer are usually:
- α-helices
- β-sheet
- turns
- loops
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Answer: Option 1 (α-helices, ID: 68019147801).
Transmembrane proteins span the lipid bilayer, and their portions that cross it are predominantly α-helices due to their hydrophobic nature and structural stability in the nonpolar membrane core.
Option Analysis
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α-helices: These are the primary structure for most transmembrane segments, typically 20-25 hydrophobic amino acids long. The helix’s backbone hydrogen bonds satisfy polar groups internally, while side chains interact with lipid tails.
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β-sheet: Less common overall; seen in β-barrels (e.g., porins in bacterial outer membranes), where antiparallel strands form a hydrophilic pore barrel.
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Turns: Flexible connectors between secondary structures, usually hydrophilic and located in aqueous regions outside the bilayer.
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Loops: Extracellular or intracellular extensions, solvent-exposed and often polar, not suited for the hydrophobic bilayer core.
The transmembrane proteins lipid bilayer interaction is a core concept in biochemistry and molecular biology, especially for GATE Life Sciences aspirants. Understanding why the portion of transmembrane proteins that cross the lipid bilayer favors α-helices helps master membrane protein structure.
Why α-Helices Dominate Transmembrane Regions
α-Helices span the ~4 nm hydrophobic core with 20-25 residues, turning ~3.6 residues per turn. Hydrophobic side chains (e.g., leucine, valine) face lipids, while internal H-bonds stabilize the backbone—no unsatisfied polar groups. Examples: Bacteriorhodopsin (7 helices), G-protein coupled receptors.
Comparing Secondary Structures
Structure Role in Transmembrane Proteins Suitability for Lipid Bilayer Examples α-Helices Primary spanning segments High (hydrophobic, stable) Ion channels β-Sheet Forms β-barrels (rare) Moderate (in outer membranes) Porins Turns Connects helices Low (hydrophilic) Extracellular Loops Aqueous extensions Low (solvent-exposed) Cytoplasmic Biological Significance
Single-pass proteins use one helix; multi-pass have bundles (e.g., 12 in GLUT transporters). Mutations disrupting helices cause diseases like cystic fibrosis. For GATE Life Sciences PYQ, recall: α-helices are “usually” correct, as β-barrels are exceptions.
This transmembrane proteins lipid bilayer knowledge boosts exam scores and research in microbiology, genetics.
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