🏳️‍⚧️ Transmembrane Proteins

Phospholipid Organization

• Phospholipids naturally arrange themselves into stable structures when placed into water (micelles/liposomes)

Transmembrane/Integral Proteins

• e.g. Glycophorin A Dimer:

  • Transmembrane protein domain is hydrophobic peptide sequence (uncharged) that spans across PM

  • Sequence permanently attaches protein to PM

  • Structure typically in form of α helices

  • N and C terminals on opposite sides of the membrane

• α Helix:

  • Most common protein structure element crossing biological membranes

  • Nine amino acids with hydrophobic side chains:

    • Glycine (Gly)

    • Alanine (Ala)

    • Valine (Val)

    • Leucine (Leu)

    • Isoleucine (Ile)

    • Proline (Pro)

    • Phenylalanine (Phe)

    • Methionine (Met)

    • Tryptophan (Trp)

Tetraspanins (Transmembrane 4 Superfamily Proteins, TM4SFs)

• Family of membrane proteins found in all multicellular eukaryotes

• TM4SFs have four transmembrane α-helices and two extracellular domains:

  • One short (EC1)

  • One longer (EC2)

• Some tetraspanins can be glycosylated (attachment of carbohydrate molecule) on the long extracellular loop

• Play role in:

  • Cell adhesion

  • Motility

  • Proliferation

Movement of Substances Across Cell Membranes

• Lipid bilayers do not allow many compounds or molecules to pass freely

• Small uncharged molecules cross easily

  • O₂, CO₂, NO

• Large, polar, or charged compounds cannot cross easily

• Specific mechanisms required for controlled transport

• Examples needing controlled transport:

  • K⁺

  • Na⁺

  • Ca²⁺

  • Glucose

Mechanisms for Moving Molecules Across Membranes

1. Simple diffusion - Passive, nonmediated

   • High to low gradient

   • Small, uncharged molecules

2. Diffusion through a channel - Passive, nonmediated

   • Channel opens and closes

   • Small ions

3. Facilitated diffusion - Passive, transporter mediated

   • Transporter takes molecule, attaches something or causes conformational change, and transporter opens towards inside of cell to let new molecule through

   • Large molecules (e.g. glucose)

4. Active transport - Active, transporter mediated

   • Primary: Requires ATP to transport against gradient

   • Secondary: Uses an electrochemical gradient

• Some mechanisms require energy

• Active transport requires energy input

• Diffusion-based processes do not require energy

Passive Mechanisms

• Moves across cell membranes based on molecular concentrations

• Does not require energy

Simple Diffusion

Down a concentration gradient

• Flow is downhill

• Works only for very small uncharged molecules like O₂ and CO₂

• Water is also uncharged due to covalent bonds

• In covalent bonds, electrons are shared equally, so no net charge on atoms or molecule

Why water doesn’t easily pass through membranes:

• Even though water is uncharged, it is polar

• Lipid bilayer is hydrophobic, so polar molecules like water don't pass easily

• Water needs special channels called aquaporins to cross the membrane efficiently

Channels

Channels

• Protein channels used for passive transport

• Effective for small charged molecules like Na⁺, K⁺, Ca²⁺, Cl^-

• Ions move down concentration gradients → downhill flow

• Channels are selective → Allow only specific ions to pass through

• Ion channels are made up of integral membrane proteins

• Usually composed of multiple subunits that line an aqueous pore for ion movement

Ion Channels

• Channels are often are gated → Can be open or closed

• Gating allows response to different stimuli (e.g. neurotransmitters)

• Channels can be turned ON or OFF in response to signals

2 Types of Gated Ion Channels

1. Voltage-gated

2. Ligand-gated

Voltage-Gated Ion Channels

• e.g. Na⁺ and K⁺ channels

• Respond to changes in charge across the membrane

• Na⁺ concentration higher outside the cell, lower inside

• Example: Neuron action potentials

• Under non-depolarized conditions, neurons have low Na⁺ inside

• Opening Na⁺ channels under these conditions leads to action potential and depolarization of membrane

• Action Potential: Passage of electrical signal down nerve axon

Ligand-Gated Channels

• Channel responds to binding of specific molecule (ligand) on its surface

• Ligand binding produces conformational change in receptor/channel structure

• Only a ligand adapted to the binding site can produce an effect

• e.g. Acetylcholine receptors

  • Ligand-gated ion channels

  • Acetylcholine binds to the receptor

  • Channel opens, allowing ions (Na⁺, Ca²⁺) to enter

  • Triggers muscle contraction or nerve signal transmission

  • Channel closes after acetylcholine detaches

Toxins Targeting Ion Channels - Tetrodotoxin (TTX)

• Tetrodotoxin (TTX): Potent neurotoxin

• Found in pufferfish, blue-ringed octopuses, and moon snails

• Effect of TTX:

  • Blocks Na⁺ channels

  • No sodium influx

  • No action potentials

  • No muscle contraction

• Result:

  • Paralysis, including of diaphragm

  • Respiratory failure

  • Death

Toxins Targeting Ion Channels - Curare

• Mixture of organic compounds from plants (e.g., Strychnos species) in Central/South America

• Used as a paralyzing poison and hunting tool historically

• Curare as a competitive antagonist:

  • Binds to the nicotinic acetylcholine receptor (nAChR)

  • Occupies the same site as acetylcholine (ACh)

  • Has equal or greater affinity than ACh

  • Does not produce a response

  • Example of a non-depolarizing muscle relaxant

• Effects of Curare

  • Binds to nAChR (nicotinic acetylcholine receptor)

  • Blocks acetylcholine binding

  • No acetylcholine binding

  • No muscle contraction

  • Paralysis (muscle relaxation)

  • Used in medicine: Muscle relaxant during surgery

Carriers - Facilitated Diffusion

Facilitated diffusion:

• Compound binds specifically to an integral membrane protein called a facilitative transporter

• Change in transporter conformation allows compound to be released on the other side of the membrane

• Compound moves down a concentration gradient

Carriers: Glucose Transporter - Facilitated Diffusion

• Most animal cells import glucose from the blood into cells down a concentration gradient via facilitated diffusion

1. Transporter ready to accept glucose molecule

2. Glucose is accepted by transporter

3. Intracellular side of transporter opens

4. Glucose is released and cycle repeats

Another Type of Carrier: Symporter - Active Transport

• Symporters and antiporters use active transport

• Cells need to move substances from lower to higher concentrations in certain situations (e.g., glucose reabsorption in kidney cells after blood filtration)

• Cells can't rely on a concentration gradient alone because equilibrium would stop glucose reabsorption

• Solution: Use the chemical gradient of another molecule that doesn't reach equilibrium between extracellular and intracellular sides

The Na⁺ - Glucose Symporter - Active Transport

• Simultaneous binding of 2 Na⁺ and 1 glucose to the outward-facing transporter

• This binding causes a conformational change to the transporter (occluded conformation)

• The transporter adopts an inward-facing conformation

• The two Na⁺ molecules dissociate into the cytosol, pushing glucose into the cell

• The transporter returns to the outward-facing conformation to repeat the cycle

• Called active because glucose moves against its concentration gradient

• No ATP is used directly in this process

• Energy comes from Na⁺ gradient created by the Na⁺/K⁺ ATPase pump, which does use ATP

• The transporter is a symporter since both Na⁺ and glucose move in the same direction (into the cell)

A Third Type of Carrier: Antiporter - Active Transport

• Uses the concentration gradient of one molecule to move another molecule in the opposite direction

• Example: Sodium-proton exchanger (Na⁺/H⁺ exchanger) in the kidney nephron

• Transports Na⁺ into the cell and protons (H⁺) out of the cell

• Maintains pH and sodium levels in specific kidney cells

Active Transport

• Molecule binds to a protein in the membrane called an active transporter

• ATP is used to change the shape of the transporter protein, allowing molecules to be moved across the membrane

• This process moves molecules from low to high concentration (against the gradient)

• Requires energy (ATP) to function

Na⁺/K⁺ Pump - Active Transport

• Maintains the concentration of Na⁺ and K⁺ inside and outside the cell using ATP

• For each ATP hydrolyzed:

  • 3 Na⁺ ions are pumped out of the cell

  • 2 K⁺ ions are pumped into the cell

• Creates and maintains concentration gradients for Na⁺ and K⁺

• Helps regulate cell volume and maintain resting membrane potential

• Requires ATP to sustain Na⁺ chemical gradient

• Supports activity of Na⁺-glucose symporter by maintaining Na⁺ gradient

• Also helps return neuron to non-depolarized (quiet) state after action potential

Maintaining Cell Size Through Active Transport

• High intracellular Na⁺ concentration can affect cell volume

• Active transport mechanisms like Na⁺/K⁺ pump help regulate ion balance

• Red blood cell size changes with different solute concentrations (hypertonic, hypotonic, isotonic solutions)

• Aquaporin channels allow water to move in/out of cells, helping maintain osmotic balance

• Proper ion gradients prevent excessive swelling or shrinking of cells

Summary

• Integral membrane proteins have α helix regions made of hydrophobic amino acids that cross the membrane

• Molecules cross membranes using passive (no energy) or active (uses energy) transport

• Ion channels can open or close (gated)
   – Some open when charge changes (voltage-gated)
   – Some open when a specific molecule binds (ligand-gated)

• TTX blocks sodium ion channels → stops nerve signals → causes paralysis

• Curare blocks acetylcholine receptors → stops muscle contraction → causes paralysis

Transmembrane Protein Summary

• Transporters

  • ACh nicotinic receptor (ligand-gated channel)

  • Na⁺ channels (ion channel)

  • Glucose Transporter (facilitated diffusion)

  • Na⁺ -glucose symporter (secondary active transport)

• Enzymes

  • Acyl transferase

  • Phosphatase

  • Choline phosphotransferase

  • Flippase

  • Floppase