Membranes and Membrane Proteins

1. Importance of Membranes

• ~30% of the human genome encodes membrane proteins.

• Over 50% of all known drugs target membrane proteins.

• Lipid membranes make up only 6–12% of cytosolic volume, with 2–5% from the plasma membrane.

• Of ~1195 known protein folds, only 58 (~5%) are membrane protein folds.

2. Example of Membrane Protein Drug Target

GLP-1 Receptor

• GPCR family (G-Protein Coupled Receptors).

• Regulates glucose homeostasis.

• Process:

  1. Agonist binds extracellularly.

  2. Triggers intracellular signalling cascade.

  3. Results in increased insulin secretion.

• Targeted by drug Semaglutide.

3. Membrane Structure and Function

Structure

• Composed of lipids (15–75%) and proteins.

• Selectively permeable: allows specific molecule passage.

• Architecture varies by organism:

  • Bacteria: Gram-positive & Gram-negative.

  • Eukaryotes: Plasma membrane, organelles (e.g. mitochondria).

Functions

• Compartmentalisation

• Scaffold for biochemical reactions

• Selective permeability

• Solute transport

• Signal reception

• Intracellular communication

• Energy transduction

4. Bacterial Membranes

Gram-negative

• Two membranes.

• Thin peptidoglycan layer in the periplasmic space.

• Outer membrane is more permeable.

Gram-positive

• Thick peptidoglycan wall.

• Single membrane.

5. Eukaryotic Membranes

• Double lipid bilayers in organelles such as:

  • Mitochondria

  • Nucleus

6. Common Membrane Features

• Sheet-like, lipid & protein structure.

• Hydrophilic (head) and hydrophobic (tail) regions.

• Non-covalent assembly.

• Asymmetric, fluid, and electrically polarised.

7. Membrane Lipids

• 4 major biomolecule groups:

  1. Lipids

  2. Nucleic acids

  3. Proteins

  4. Polysaccharides

Types

1. Phosphoglycerides:

   • Glycerol backbone

   • 2 fatty acids (C1 & C2), phosphate at C3.

   • Variable alcohol group → affects charge and interactions.

2. Cardiolipin (CL):

   • Inner mitochondrial membrane (20% of lipids).

   • 2 phospholipids + glycerol.

   • Involved in cristae formation and apoptosis (cytochrome c release).

3. Sphingolipids:

   • Backbone = sphingosine (C18 amino alcohol).

   • Includes:

     • Sphingomyelin (phosphorylcholine)

     • Ceramide (with fatty acid)

     • Cerebrosides (with sugar)

     • Gangliosides (with sialic acid)

Cholesterol

• 30–40% of plasma membrane lipids.

• Rigid structure → decreases fluidity.

Lipid Diversity

• Lipid composition varies by:

  • Chemical structure → specific functions.

  • Ratio (composition) → affects behaviour.

8. Membrane Permeability & Asymmetry

• Impermeable to ions & most polar molecules.

• Small molecule permeability depends on hydrophobicity.

• Phosphatidylserine (PS):

  • Normally on inner leaflet.

  • If on outer → signals apoptosis.

9. Membrane Dynamics

• Lipids are not static; they hop between regions.

• Hop diffusion:

  • Rapid local diffusion.

  • Occasional hopping across barriers (e.g. spectrin-actin fences).

Membrane Skeleton

• Defines cell shape, esp. in erythrocytes.

• Spectrin (α-helical coil) + Ankyrin + integral proteins → structure and fencing.

10. Membrane Proteins

Types

• Integral: Require detergents for removal.

• Peripheral: Easily removed.

• Amphitropic: Associate reversibly (regulated by ligands or phosphorylation).

Fluid Mosaic Model

• Singer & Nicolson (1972): Proteins float in a lipid sea.

• Updated model includes:

  • Asymmetry

  • Lipid rafts

  • Immobile regions

11. Glycophorin (Erythrocyte Protein)

• N-terminal domain is trypsin-accessible (extracellular).

• Oligosaccharides only on outside.

• Important in malaria (plasmodium) invasion.

• Basis for M and N blood groups (Ser1Leu, Gly5Glu).

• Exists as a dimer, with helix-helix interactions (mediated by Gly, Ala, Ser).

Composition

• 60% carbohydrate: prevents cell clumping.

• 40% protein

• 20aa transmembrane helix: hydrophobic residues.

Hydropathy & Energetics

• Hydropathy scale used to predict membrane-spanning segments.

• Insertion energetics:

  • Hydrophobic gain: -36 kcal/mol

  • Dehydration cost: +26 kcal/mol

  • Net ΔG: -10 kcal/mol

12. β-Barrel Membrane Proteins

Structure Principles

1. Even number of β-strands.

2. N/C-termini in periplasm.

3. Tilt: ~45°.

4. Antiparallel strands.

5. Short turns (T1, T2) periplasmic side.

6. Long loops (L1, L2) extracellular.

7. Aromatic girdles (Tyr, Trp) at interfaces.

Example: Porin

• Outer membrane of Gram-negative bacteria, mitochondria, chloroplasts.

• Channels for small hydrophilic molecules (<600 Da).

• High-resolution structures exist (crystallisation is challenging).

13. Membrane Insertion and Composition

• Membrane core = non-polar (~3 nm), with polar headgroups (~1.5 nm interface).

• Amino acid location by region:

  • Core: Non-polar residues.

  • Interface: Tyr, Trp.

  • Aqueous phase: Charged residues.

14. Lipid Rafts

• Domains rich in sphingolipids + cholesterol.

• Thicker, more ordered, and less fluid.

• Enriched in GPI-anchored proteins.

• Size: ~500 nm, dynamic (lifespan in milliseconds).

• Aid in protein co-localisation.

15. Working with Integral Membrane Proteins

• Challenges:

  • Low expression levels.

  • Poor solubility in detergents.

  • Difficult purification.

  • Crystallisation issues.

  • Biochemical studies are more complex due to native lipid/protein interference