GF

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.

Peptidoglycan: Polymer of amino acids + sugars. Unique to bacteria.

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