CHEM 114A: Chapter 9 - Lecture 3

Cellular Membranes – Fundamental Roles

  • Define cell boundary: separate extracellular space from cytoplasm.
  • Regulate molecular traffic (selective permeability):
    • Hydrophobic solutes cross easily.
    • Polar/charged solutes require specialized transport.
  • Serve as platforms for membrane-associated proteins that mediate signaling, transport, catalysis.
  • Intracellular membranes (eukaryotes) generate micro-compartments:
    • Permit chemistry incompatible with bulk cytosol.
    • Products released once reactions conclude.
  • Mechanical properties:
    • Highly flexible (bending, budding, fusion).
    • Self-healing: breaks spontaneously reseal via entropic forces.

General Membrane Composition & Physical Parameters

  • Lipid phase is heterogeneous: many lipid classes with extensive sub-classes.
  • Asymmetric distribution: outer vs inner leaflet differ in lipid & protein composition.
  • Fluidity is tunable by lipid composition, especially cholesterol content.
  • Core hydrophobic thickness ≈ 35 A˚35\,\text{Å} ("~35 inches" stated in slide).
    • Including both polar head-group regions ⇒ total thickness ≈50–75 A˚\approx 50\text{–}75\,\text{Å}.
  • A lipid bilayer is never protein-free; protein mass fraction depends on function:
    • Myelin sheath ≈ 19%19\% protein (insulation).
    • Inner mitochondrial membrane ≈ 76%76\% protein (high respiratory activity).
    • Typical eukaryotic plasma membrane ≈ 50%50\% protein.
  • Structure–function correlation: membrane composition is tailored to cellular role.

Three Broad Classes of Membrane-Associated Proteins

  1. Integral (IMP) – permanently embedded; span or deeply penetrate bilayer.
  2. Lipid-Linked – covalently attached lipid anchor tethers protein to bilayer.
  3. Peripheral – associate non-covalently with polar head groups / integral proteins; easily dissociated by salt or pH.

Integral Membrane Proteins (IMP)

  • Stabilized mainly by hydrophobic interactions; extraction requires detergents (form protein–detergent micelles).
  • ~30%30\% of all known proteins.
  • Amphiphilic: polar extramembranous regions + hydrophobic trans-membrane (TM) segments.
  • Hydropathy plots (sliding window ≈15 aa) predict TM segments:
    • Positive hydropathy >0 ⇒ hydrophobic; negative ⇒ hydrophilic.
  • Example: Glycophorin A
    • Three domains:
    1. N-terminal, extracellular, highly glycosylated & polar.
    2. Single hydrophobic α\alpha-helix TM (rich in Leu, Ile, Val, Gly, Ala).
    3. C-terminal, cytosolic, polar/charged.
    • Asymmetrical orientation is fixed.
  • α\alpha-helix spanning rule: > 17.517.5 residues (~26 Å) required to cross bilayer.

Seven Topological Variants (not mutually exclusive)

  1. Single TM α\alpha-helix (bitopic).
  2. Multi-pass α\alpha-helical (polytopic).
  3. β\beta-barrel polytopic (mixed α/β\alpha/\beta possible).
  4. Amphipathic α\alpha-helix lying parallel to surface (hydrophobic face toward core, polar face toward heads).
  5. Shallow hydrophobic loop inserted into leaflet.
  6. Lipid-linked helix that enters only part-way.
  7. Peripheral proteins bound via ionic/electrostatic interactions.

β\beta-Barrel IMPs

  • Contain 8–228\text{–}22 antiparallel β\beta-strands; exterior hydrophobic, interior hydrophilic ⇒ form pores.
  • Often house internal α\alpha-helix “gate.”
  • Porin family (gram-negative bacteria, mitochondria):
    • ~16 strands; function as monomers or trimers.
    • Continuous polypeptide loops back; N & C termini adjacent forming “knot” closure.
    • Transport polar solutes through central channel.

Lipid-Linked (Covalently Anchored) Proteins

• Lipid moiety inserts into one leaflet; protein may reside entirely on one membrane face.
• A single protein can bear multiple lipid anchors.

Prenylated Proteins

  • Attach isoprenoid chains to C-terminal CaaX motif (Cys-aliphatic-aliphatic-Y):
    • If Y = Ala/Met/Ser ⇒ C15C_{15} farnesyl.
    • If Y = Leu ⇒ C20C_{20} geranylgeranyl.
  • Steps: thioether linkage to Cys → removal of “aaX” → C-terminal carboxyl methylation (reduces polarity, enhances membrane affinity).

Fatty-Acylated Proteins

  1. Myristoylation (C$_{14}$, amide to N-terminal Gly): permanent.
  2. Palmitoylation (C$_{16}$, thioester to internal Cys): reversible; enzymatic addition/removal regulates membrane association & signaling.

GPI-Anchored Proteins

  • Found in all eukaryotes; always exposed on extracellular surface.
  • Architecture: phosphatidylinositol (PI) + core tetrasaccharide (Man-Man-Man-GlcN) + phosphoethanolamine + amide linkage to C-terminal protein.
  • Fatty-acyl chains (R$1$, R$2$) in PI embed in outer leaflet; anchor is virtually permanent.

Peripheral Membrane Proteins

  • Bind via electrostatic & H-bond interactions to lipid head groups or other proteins.
  • Some possess shallow hydrophobic loops/“fingers” that dip into bilayer for added affinity.
  • Easily released by high salt or pH shift; no detergents required.

Functional & Practical Implications

  • Membrane flexibility & self-healing underpin vesicle trafficking, exo/endocytosis.
  • Selective permeability drives need for diverse transport proteins (channels, carriers, pumps).
  • Lipid composition tuning (e.g., cholesterol) is a cellular strategy to adapt to temperature, pressure, and functional demands.
  • Covalent lipidation acts as a reversible “zip-code” directing proteins to correct membrane locales, modulating signaling cascades.
  • Isolation/purification challenges: integral proteins mandate detergents; peripheral proteins can be stripped under mild conditions.