Membrane Transport - Study Notes (BIOL 3302)

Learning Objectives

• Describe the function and mechanism of action of the various membrane transporters

The Lipid Bilayer and Permeability

• The biomembrane is essentially impermeable to most biomolecules, necessitating transport proteins for regulated exchange
• Permeability by category (from slide):
  • Gases and small nonpolar molecules cross readily
  • Hydrophobic and small polar molecules cross slowly
  • Large polar molecules and charged species are largely impermeable without transport proteins
• Examples across classes (illustrative):
  • Permeable: CO$<em>2$, H$</em>2$O (to some extent), H$^+$ (minor without channels)
  • Generally impermeable without transporters: Na$^+$, Ca$^{2+}$, Cl$^-$, Glucose, Amino acids
• Why design membranes to be essentially impermeable? To regulate precise cell–cell, cell–ECM, and intracellular exchange while maintaining compartmentalization
• Partition and permeability concepts introduced later justify why lipophilicity matters for membrane crossing
• Partition coefficient (lipophilicity measure): $K = \frac{[x]<em>{ ext{lipid}}}{[x]</em>{ ext{water}}}$
  • Higher $K$ (more lipophilic) generally yields higher permeability and faster diffusion across the bilayer
• Contextual links to diseases and physiology (e.g., regulated transport is essential for function)

Major Types of Transport Proteins (3 Main Classes)

• Exterior

• Cytosol

• ATPase (energy-consuming)

• Channel (gated or non-gated)

• Transporter/Carrier (uniporters, symporters, antiporters)

• Figure reference: Figure 11-2, Lodish et al.

• 3 main types with energy considerations:

  • ATPases: use energy from ATP hydrolysis to move substrates (primary active transport)
  • Channels: provide aqueous pores; may be gated; can be non-gated (always open) or gated
  • Transporters (carriers): bind substrate and change conformation to move substrates; can be uniporters, symporters, or antiporters

• Energy relationships:

  • ATPases: ATP hydrolysis provides energy to move substrates against gradients
  • Channels/Uniporters/Some symporters: operate down gradients (facilitated or passive transport)
  • Secondary transporters (co-transporters): use energy from downhill movement of one substrate to drive uphill transport of another

Facilitated Transport vs Active Transport

• Facilitated (passive) transport:
  • Moves down chemical or electrochemical gradient
  • No direct energy input required
• Active transport:
  • Moves against a gradient (chemical or electrochemical)
  • Requires energy input (direct ATP hydrolysis or from gradient energy for secondary transport)

Gradients and Energetics

• Molecular movement favors flow down gradients (entropy increases, disorder favored)
• Gradients involved:
  • Chemical gradient (concentration difference)
  • Electrochemical gradient (combined with membrane potential)
• Membrane potential: charge imbalance across the membrane influences transport
• Entropy perspective:
  • Entropy (S) tends to increase (S > 0) for spontaneous processes; lower entropy (S < 0) is less favorable

Channel Transport (Facilitated, Passive)

• Channels can be gated or non-gated
• Non-gated channels: always open (e.g., aquaporins for water)
• Gate regulated by:
  • Chemical signals (ligand-gated)
  • Electrical signals (voltage-gated)
  • Mechanical stimuli (mechanically-gated)
  • Temperature, light (others listed in slides)
• Ion and small molecule transport through channels is down its gradient and does not require energy input
• Gated channels opening mechanisms:
  • Ligand-gated: receptor binding induces conformational change to open pore
  • Voltage-gated: membrane potential triggers opening
  • Mechanically-gated: physical pressure or stretch opens pore
• Example: Ligand-mediated conformational change opens channel pore
  • Diagrammatic note: LGIC (Ligand-Gated Ion Channel) illustration
• Water transport through channels: Aquaporins
  • Structure: about 6 helices span the membrane per subunit; tetrameric pore forming complex
  • Non-gated channels; water moves down its gradient
  • Proton exclusion: Asn residue helps prevent proton flux (H$^+$) via aquaporins, preserving the proton gradient
  • Flux: very rapid—described as about a billion water molecules per second per aquaporin pore in some summaries

Aquaporins: Non-Gated Water Channels

• Key features:
  • 6 α-helices per subunit, forming a pore; four subunits create the functional tetramer
  • Pore diameter is just sufficient for a water molecule
  • Water moves passively down its osmotic gradient; no conformational changes during transport
  • Asn side chains participate in hydrogen bonding to allow water passage while blocking proton transport
• Relevance to physiology:
  • Water reabsorption in kidney and other tissues; dysregulation linked to nephrogenic diabetes insipidus and related conditions
• Example context from literature: Rapid water transport through aquaporin channels supports rapid fluid balance adjustments

Uniporters, Symporters, Antiporters (Transporters)

• Uniporter: one substrate moves down its gradient via transporter conformational change
  • Example: GLUT family transport glucose across membranes
• Symporter (cotransporter): transport two or more substrates in the same direction
  • Example: Na$^+$/glucose cotransporter (SGLT) moves Na$^+$ and glucose into the cell together; substrate movements are coupled
• Antiporter (exchanger): transport substrates in opposite directions
• Transporters vs channels:
  • Channels: faster flow through a pore; gating can regulate opening
  • Transporters: alternate conformation to move substrate; typically slower than channels; transporter number influences maximal rate (V$_{max}$)
• Example context: Amino acid import relies on Na$^+$-driven secondary transporters, coordinated with Na$^+$/K$^+$ ATPase activity
• Co-transport theory demonstrated via pumps, channels, and transporters working together (Lodish Figure 11-3):
  1) Na$^+$/K$^+$-ATPase maintains Na$^+$ gradient
  2) K$^+$ flow down gradient across membrane generates membrane potential
  3) Na$^+$ moving down electrochemical gradient powers uphill amino acid transport

Transport Kinetics: Vmax and Km

• V$_{max}$: maximum rate of substrate uptake; depends on the number of transporters on the membrane
• Km: substrate concentration at which transport rate is half of V$_{max}$
• Michaelis-Menten relationship (general form):
  $v = \frac{V<em>{ ext{max}} [S]}{K</em>m + [S]}$
• Higher Km indicates lower affinity of the transporter for the substrate
• Examples of affinity differences:
  • Glucose transporters: GLUT1 vs GLUT2
  • Km values: $K<em>m( ext{GLUT1}) ext{ ≈ } 1.5 ext{ mM}, \, K</em>m( ext{GLUT2}) ext{ ≈ } 20 ext{ mM}$
• Practical note: higher affinity (lower Km) means transporter becomes saturated and operates efficiently at lower substrate concentrations

GLUTs: Glucose Transporters

• GLUT1: insulin-independent; expressed in blood/endothelial cells
• GLUT4: insulin-responsive; translocates to plasma membrane in muscle and adipose tissue in response to insulin; deficiency contributes to diabetes symptoms

Na$^+$/Glucose Cotransport (Symporter)

• Mechanism:
  • Co-transport of Na$^+$ and glucose from extracellular space into cytosol
  • Transporter cycles through outward-facing, occluded, and inward-facing conformations to move glucose against its gradient using the Na$^+$ gradient energy
• Energy source:
  • Energy stored in the Na$^+$ electrochemical gradient established by the Na$^+$/K$^+$-ATPase

Na$^+$/K$^+$-ATPase and Gradient Formation

• Na$^+$/K$^+$-ATPase creates and maintains Na$^+$ and K$^+$ gradients across the plasma membrane
• Consequences:
  • Gradient drives secondary active transport (e.g., amino acid import)
  • Membrane potential contributes to electrochemical gradients used by various transporters

ATPases: The Four Major Classes

• P-Class ATPases
  • E.g., Ca$^{2+}$-ATPase (SERCA in ER, PMCA in plasma membrane)
  • Mechanism: E1/E2 cycle with phosphorylation of a conserved aspartate residue; moves Ca$^{2+}$ against its concentration gradient
• V-Class H$^+$ Pumps
  • Proton pumps in vacuolar/endomembrane systems; use proton motive force to acidify compartments; contribute to electrochemical gradient
• F-Class H$^+$ Pumps
  • Found in mitochondria and bacterial membranes; synthesize ATP using the proton motive force (ATP synthase)
• ABC Transporters (ATP-Binding Cassette)
  • Use solute-binding proteins or substrate-recognition domains to bind solutes; channel opens following ATP binding/hydrolysis; transports a wide range of substrates including lipids, drugs, and peptides
• Key figure reference: Figure 11-2, Lodish et al.

P-Class Ca$^{2+}$ ATPase: Mechanism

• Function: Move Ca$^{2+}$ against gradient from cytosol into SR lumen or external medium
• Core components:
  • Ca$^{2+}$-binding sites on cytosolic side
  • Phosphorylated aspartate on the ATPase (phosphorylation by ATP)
  • Conformational changes between E1 (Ca$^{2+}$-binding, high affinity) and E2 (Ca$^{2+}$ release, low affinity) states
• Process outline:
  • Ca$^{2+}$ binding in E1 state; ATP binds and is hydrolyzed; aspartate residue is phosphorylated
  • Phosphorylation drives conformational change to E2; Ca$^{2+}$ released to lumen/cytosol
  • Dephosphorylation resets pump to E1

V- and F-Class H$^+$ Pumps; ATP Synthase

• V-Class: hydrolyze ATP to pump H$^+$ across membranes, acidifying organelles like lysosomes
• F-Class: use proton motive force to synthesize ATP from ADP + Pi (ATP synthase) but can also run in reverse under certain conditions to pump protons
• Concept image: ATP synthase uses the energy from the H$^+$ electrochemical gradient to generate ATP; conversely, ATP hydrolysis can power proton pumping

ABC Transporters: Mechanism and Roles

• General mechanism (illustrated in slide 49):
  1) Solute-binding protein binds substrate (in prokaryotes; or substrate directly binds in some cases)
  2) Binding protein/substrate interacts with the transmembrane channel
  3) Channel undergoes conformational change to open
  4) ATP hydrolysis provides energy for opening channel and movement of solute
• Key points:
  • They often handle bulky, diverse, or hydrophobic substrates
  • Play roles in nutrient uptake and drug resistance

Disease Contexts Linked to Transport Defects

• Diabetes mellitus (Type 1 and Type 2): impaired glucose uptake/transport into cells via GLUTs and/or insulin signaling
• Cystic Fibrosis (CF): defective CFTR Cl$^-$ channel; categories of CFTR mutations
• Osteopetrosis: defective bone resorption due to impaired H$^+$ transport in resorption lacuna (osteoclast dysfunction) via H$^+$-transporting pumps and related pathways
• Additional context provided in slides includes specific CFTR mutation classes and their effects (see table)

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Mutations

• Table: Mutations grouped into classes by effect on CFTR function
• Class I: No functional CFTR (protein production blocked or truncated)
  • Example mutation: W1282X (premature stop codon)
  • Effect: protein fails to reach the cell membrane
• Class II: Protein misfolding and trafficking defects; example AF508 (ΔF508)
• Class III: Defective regulation of CFTR channel gating; example G551D
• Class IV: Decreased chloride conductance; example R117H
• Class V: Reduced CFTR production due to splicing or promoter defects; example 3120+1G>A
• General notes:
  • Class I–III: more severe loss of function
  • Class IV–V: milder functional reductions
• Visuals link CFTR channel states: open vs closed with mucus/airways consequences

CFTR Channel Illustration and Consequences

• Mutated CFTR channels lead to thick, sticky mucus in lungs; fewer functional channels on membranes
• CFTR dysfunction disrupts Cl$^-$ transport and fluid balance across epithelia

Osteopetrosis: Mechanistic Insight

• Osteopetrosis involves failure of osteoclast-mediated bone resorption
• Key components shown include vesicles, CIC-7, CH, H$^+$-ATPase, and resorption lacuna
• The disease reflects impaired proton pumping and acidification of the resorption lacuna

Gases & Small Uncharged Polar Molecules Diffusion Across Bilayer

• Passive diffusion across the lipid bilayer: down its concentration gradient, no energy input
• Small uncharged molecules diffuse more readily than larger polar molecules or ions
• Practical implication: only certain species cross readily without transporters; others require specialized proteins

Partition Coefficient and Lipophilicity (Revisited)

• Partition coefficient: $K = \frac{[x]<em>{ ext{lipid}}}{[x]</em>{ ext{water}}}$
• Correlates with lipophilicity and membrane permeability: higher $K$ yields higher permeability and faster diffusion

Demonstrations of Partition Coefficient and Lipophilicity

• Example discussion: diethylurea vs urea shows higher $K$ for diethylurea due to increased lipophilicity (illustrative trend)

3 Main Types of Transport Proteins ( recap )

• ATPase: energy from ATP hydrolysis; moves substrates against gradient
• Channel: aqueous pore; gated or non-gated; facilitates rapid movement down gradients
• Transporter (carrier): binds substrate and undergoes conformational change to move substrate; includes uniporters, symporters, antiporters
• Reference figure: Figure 11-2, Lodish et al.

Channel Examples and Transport Kinetics

• Aquaporins (water channel) as a primary example of non-gated channels
• K$^+$ channel (voltage-gated example) shows selective passage and voltage-dependent opening
• Transport kinetics concept: transport rate can be assessed via V$_{max}$ and Km; channels may show rapid flow relative to carriers
• Assay concepts: transport kinetics can be measured with dye-based or substrate-uptake assays to determine V$_{max}$ and Km
• Important practical point: V$_{max}$ depends on transporter density; higher density increases the maximum rate; Km provides affinity insight

V$_{max}$ and Km: Detailed Definitions

• V$_{max}$: the rate of molecule uptake when all transporters are working at full capacity
• Km: the substrate concentration at which uptake rate is half of V$_{max}$
• Expressions:
  • $v = \frac{V<em>{ ext{max}} [S]}{K</em>m + [S]}$
• Interpretation:
  • Higher Km => lower affinity for substrate
  • For multiple substrates and transporters, Km is akin to the affinity constant for the transporter–substrate interaction

Summary of Practical Implications

• Membrane transport proteins mediate cell–cell, intracellular, and cell–ECM communication and transfer across an essentially impermeable biomembrane
• Transport efficiency depends on transporter type, gating, energy source (ATPase vs gradient), and transporter density
• Proper function is essential for physiologic processes (glucose uptake, ion homeostasis, water balance, bone remodeling, mucus clearance)

Quick References and Figures (from Slides)

• Figure 11-2 (Lodish et al.) shows the three main transporter types and their energy relationships
• Figure 11-3 (Lodish et al.) demonstrates how pumps, channels, and transporters cooperate in amino acid import
• Aquaporin structure and gating notes: non-gated, high-throughput water transport; selectivity and proton exclusion by specific residues
• CFTR mutation classes and implications for channel function and disease severity
• GLUT1 vs GLUT2 Km values illustrating affinity differences and tissue-specific transport roles
• ATPase classes: P-class (Ca$^{2+}$), V-class (H$^+$), F-class (H$^+$ synthase), ABC transporters

Final Takeaway

• Membrane transport proteins are essential to overcome the lipid bilayer’s impermeability, enabling regulated exchange across compartments and supporting key physiological processes and disease states.