Membrane Proteins Lecture Notes

Overview

• Membrane Proteins: Integral to or interact with biological membranes.
• Comprise about one-third of human proteins.
• Functions: Facilitate diffusion, active transport, cell connection, signal transduction, cell identification.
• Variation in types and amounts across different membranes.
• Membranes can consist of up to 75% protein by mass.

Types of Membrane Proteins

Integral or Intrinsic Proteins

• Permanent part of the membrane.
• Subtypes:
  • Integral Monotopic Proteins: Attached to only one membrane leaflet.
  • Transmembrane Proteins: Span the lipid bilayer (bi-topic or poly-topic).
  • Lipid-Anchored Proteins: Covalently attached to lipids in the bilayer (e.g., GPI anchors).

Peripheral or Extrinsic Proteins

• Transiently associated with membranes or integral proteins.
• Associations include hydrophobic, electrostatic, or other non-covalent interactions.

Membrane Structure and Behavior

• Phospholipid Bilayer: Composed of two leaflets.
  • Polar Heads: Hydrophilic (water-loving).
  • Nonpolar Tails: Hydrophobic (water-hating).
• Hydrophobic Effect: Drives the formation of lipid bilayers.
• Amphipathic Nature: Transmembrane proteins have both hydrophilic and hydrophobic regions.

Transmembrane Proteins

• Characteristics:
  • Hard to isolate; require detergents to disrupt lipid bilayer.
  • Two basic forms: alpha-helical and beta-barrel proteins.
  • Beta-barrel proteins: Found in outer membranes of gram-negative bacteria, mitochondria, and chloroplasts.
• Structure Prediction: Hydropathy plot (hydrophobicity index vs. amino acid number).
• Topology Classification:
  • Based on N and C termini positions, start/stop transfer sequences.
  • Example types: Type 1 (N-terminus extracellular), Type 2 (N-terminus cytosolic).

Protein Functions

• Gateways: Allow specific substances to cross the membrane.
• Transport Mechanisms:
  • Facilitated Transport: Passive, spontaneous process.
  • Active Transport: Requires energy, often against gradients.

Glycosylation

• Most transmembrane proteins in animal cells are glycosylated.
• Cell Coat: Carbohydrate-covered cell surface.

Conclusion

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