Movement across membranes, channels and transporters

Transmembrane Proteins

Transmembrane domain (TMD):

• an alpha helical peptide sequence that is largely hydrophobic (uncharged) and spans the membrane; consists of amino acids with hydrophobic side chains (also uncharged)

• Permanently attaches the protein to the plasma membrane

  • Hydrophobic fatty acid tails interacts with hydrophobic TMD

• Can facilitate protein-protein interactions

• These alpha helices make up a dimer

Movement of Substances Across Cell Membranes

• Lipid bilayers do not allow many compounds/molecules to pass through them freely

• Small, uncharged molecules cross membranes relatively easily (H2O, oxygen, carbon dioxide, nitrogen monoxide)

• Large/polar/charged compounds cannot easily cross lipid bilayer

• Specific mechanisms are required for the controlled transport of many substances across membranes

• 4 basic mechanisms for moving molecules across membranes

  • simple diffusion

  • diffusion through a channel

  • facilitated diffusion

  • active transport (REQUIRES ENERGY)

Passive Mechanisms

• Passive movement of substances across cell membranes relies on molecular concentrations of the substance across the membrane

• moves from a high to low concentration (down concentration gradient)

• DOES NOT require energy

Passive Mechanisms: Simple Diffusion

• Down a concentration gradient—flow is downhill (no energy required)

• Simple diffusion works only for very small and uncharged molecules like water, oxygen, and carbon dioxide

• aquaporins are specific water channels; water moves through aquaporin channels in “single file” down the concentration gradient

Passive Mechanisms: Channels

• channels are formed by integral membrane proteins (typically multiple subunits) that line an aqueous pore

• This mode is particularly effective for small, charged molecules like sodium, potassium, chlorine

• Ions move down concentration gradients

• Channels are selective. allowing only particular types of ions to pass

Passive Mechanisms: Ion Channels

• ion channels are often gated (can be opened or closed)

• in other words, they can be turned on/off in response to different signals/stimuli

• 2 types of gated ion channels:

  • voltage gated channels

  • ligand-gated channels

Voltage-gated channels

• Some channels can respond to changes in charge across membrane

• e.x. action potentials in neurons

  • under non-depolarized conditions, neurons have low sodium inside

  • sodium wants to enter, causing a change in charge allowing the voltage gate to open causing the inside to be relatively more positive

Ligand-gated Channels

• The channel responds to binding of specific molecules on its surface — a ligand

• binding of a ligand produces conformational change in the structure of the receptor/channel

Toxins Targeting Ion Channels — Tetrodotoxin

• Tetrodotoxin (TTX) is a very potent neurotoxin

• the molecule was discovered in the pufferfish, but it is also found in several other aquatic animals like blue-ringed octopuses and moon snails

• TTX is a sodium channel blocker. It inhibits the firing of action potentials in neurons by binding to the voltage-gated sodium channels in nerve cell membranes and blocking the passage of sodium ions into the neuron. This prevents the nervous system from carrying messages to muscles, including the diaphragm. TTX intoxication consequently causes death via respiratory failure

Toxins Targeting Ion Channels — Curare

• Curare is a mixture of organic compounds found in different plants originating from Central and South America like members of the Strychnos species

• Was used as a paralyzing poison and hunting tool before modern techniques

• Curare is a competitive antagonist (inhibitor) of the nicotinic acetylcholine receptor (nAChR). It occupies the same position on the receptor as ACh with an equal or greater affinity, and elicits no response. It is a non-depolarizing muscle relaxant

Passive Mechanisms: Carriers

Facilitated diffusion:

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

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

• Compound moves down a concentration gradient; no energy requirement

Carriers: Glucose Transporter

Most animal cells import glucose from the blood into cells down a concentration gradient via a facilitative transporter

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 continues

   • The opening facing the intracellular space is an unstable conformational change, so it returns to be open extra cellularly to accept more glucose

Carrier: Symporter

• Under certain circumstances, cells need to move substances from a lower concentration to higher concentrations. Thus, against the concentration gradient

  • e.x. reabsorption of glucose in kidney cells after blood filtration to prevent its loss through urine

• In this case, cells can’t rely on concentration gradient of glucose as kidney cells would stop reabsorbing glucose when an extracellular and intracellular equilibrium is reached

• solution: rely on chemical gradient of a second molecule that wouldn’t reach extracellular and intracellular equilibrium

  • both molecules are transported in the SAME direction

Sodium Glucose Symporter

1. Simultaneous binding of 2 sodium and 1 glucose to the transporter with outward-facing binding sites

2. This causes a conformational change in the transporter (occluded conformation)

3. Eventually the transporter adopts an inward facing conformation that allows the dissociation of the 2 sodium molecules in the cytosol. As a result, the glucose molecule gets pushed in as well

4. Return to the outward-facing conformation to repeat the cycle

Carrier: Antiporter

• The concentration gradient of one molecule is used to transfer a second molecule in opposite directions

• An example is the sodium proton exchanger in the nephron of the kidney. This antiporter transports sodium into the cell and protons out of the cell. This carrier is specifically responsible to maintain pH and sodium levels in specific kidney cells

Movement of substances Across Cell membranes: Active Mechanisms

Active transport:

• Compound binds specifically to integral membrane protein called an active transporter

• Change in the conformation of the transporter caused by the hydrolysis of an ATP molecule allows molecules to be released on other side of the membrane

• Using this mechanism, compounds can move against a concentration gradient

• requires input of energy in the form of ATP molecules

Example of Active Transport: sodium potassium pump

the sodium/potassium ATPase maintains cellular sodium and potassium using ATP