BIOS 301 - Membrane Transport

Use of SNARE proteins in membrane fusion

• Local increase in calcium ion signals release of neurotransmitter

• t-SNARE (target membrane), v-SNARE (vesicle membrane), and SNAP-25 (target membrane) intertwine to form a coiled bundle of 4 alpha-helices

• Two membranes are drawn together and fused

• ATP is not required for SNARE coiling, but is required for SNARE disassembly

Passive transport (general)

Movement of a substrate down its concentration gradient

Active transport (general)

Movement of a substrate against its concentration gradient or electric potential

Simple passive diffusion

Cell membranes are permeable to small, nonpolar molecules that diffuse across the membrane to achieve concentration/charge equilibrium

Delta G equation for membrane transport

• Cin, Cout = concentration of substrate inside vs. outside cell

• Z = charge of ion

• F = Faraday’s constant

• Delta psi = membrane potential

• R = ideal gas constant

• T = temp in Kelvin

• If Cout > Cin, 1st term makes delta G more negative (favorable)

• If delta psi is negative and Z is positive, 2nd term makes delta G more negative (favorable)

Flux equation for passive membrane transport

• J = flux of particles passing through the membrane

• Cin-Cout = concentration gradient (steepness)

• l = thickness of membrane (distance)

• D = diffusion coefficient across membrane (composition of medium)

• K = partition coefficient (molecule identity)

  • Very small for most polar molecules

Permeability coefficient

• Measures the rate at which a substance passes through the membrane during passive diffusion

• Higher value means greater permeability

• Is an experimentally measured lumped parameter of l, K, D

Facilitated diffusion

• Involves passive diffusion of polar/charged molecules, facilitated by transporters

• Transporter is lined with hydrophilic amino acid side chains, allows substrate to be solvated in channel

• Transporter binds substrate through many weak, noncovalent interactions (facilitates dehydration)

  • Solutes are surrounded by shell of water molecules before they pass through membrane

Three classes of transport systems

• Uniport, symport, antiport

• Can be either active or passive transport

Uniport (definition, example)

• A single solute moves in one direction across the membrane

• Ex: glucose transporter in erythrocytes (GLUT1)

  • Moves glucose (down its concentration gradient) from blood plasma into erythrocytes

Glucose transporter (structure/function)

• Has 12 transmembrane domains

• Transmembrane domains consist of amphipathic alpha helices

  • Polar residues interact with glucose

  • Nonpolar residues interact with membrane lipids

Aquaporin selectivity

• Determined by amino acid properties within the protein channel

  • Size exclusion - His narrows pore to sterically block larger molecules

  • Electrostatic repulsion - Arg provides electrostatic barrier that repels cations (e.g. protons, H3O+)

  • Water dipole reorientation - NPA motifs flip water’s dipole to make it energetically impossible for proton travel through channel

    • Ensures only neutral water passes

K+ ion channel selectivity

• To enter the channel, solvated K+ must shed its H2O shell

• Backbone carbonyl oxygens of channel residues orient to perfectly mimic geometry of H2O molecules surrounding solvated K+

  • Means minimal energy penalty for K+ to enter channel

• Na+ is smaller than K+ - carbonyl oxygens cannot replace its hydration shell

  • Energetically costly

Antiport (definition, example)

• Moves two different substrates in opposite directions

• Ex: chloride bicarbonate exchanger

  • CO2 produced by respiring tissues is converted to blood soluble HCO3- by carbonic anhydrase

  • As HCO3- enters erythrocytes, Cl- exits

    • Keeps cell equilibrated (no net charge transfer)

  • HCO3- is converted back to CO2 by carbonic anhydrase and released into lung space for excretion

Primary active transport

Solute transport is coupled directly to an exergonic reaction (e.g. ATP hydrolysis)

Secondary active transport

Uphill transport of solute 2 is coupled to downhill flow of solute 1 (originally pumped uphill by primary active transport)

Na+/K+ ATPase

• Primary active antiporter

  • Facilitates movement of Na+ and K+ against their electrochemical gradients

• Contains three domains:

  • N (nucleotide binding domain) binds ATP/Mg2+ and phosphorylates Asp in P domain

  • P (phosphorylation domain) contains key Asp residue

  • A (actuator domain) removes phosphate from Asp with each pump cycle

• Mechanism

  • Transporter binds 3 Na+ from inside of cell

  • Phosphorylation alters enzyme shape/affinity - releases Na+ and binds 2 K+ from outside of cell

  • Dephosphorylation alters enzyme shape/affinity - releases K+

Symport (definition, example)

• Moves two different substances in the same direction

• Ex: Na+/glucose symporter (secondary active transport)

  • Na+ concentration is high extracellularly (gradient set by Na+/K+ ATPase) → draws Na+ inward

  • Provides energy needed to transport glucose from gut to epithelial cell (against concentration gradient)

  • Glucose uniporter moves glucose from epithelial cells → blood (via passive diffusion)

ABC transporter

• ABC = ATP Binding Cassette

• Hydrolyze 2 ATP to transport a specific substrate (primary active transporters)