11: Membrane transport of small molecules and the electrical properties of membranes Flashcards
What two main factors determine the rate of diffusion across a membrane?
Size
Relative hydrophobicity
Small, nonpolar (hydrophobic) = fast
What are the two main classes of membrane transport proteins? Differences?
Transporters:
Conformational change when bound to a specific solute. Alternate exposure of solute-binding site at each side of the membrane.
Passive or active.
Channels:
More weak interaction.
Form continuous pores that extend across the membrane.
Faster transport, passive down concentration (electrochemical) gradient.
What are the three conformations of a transporter protein?
Open
Closed
Inactive
(outward-open, occluded, inward-open)
What are the three main ways of active transport?
Coupled transporters:
One molecule uphill, one downhill
ATP-driven pumps:
Uphill transport coupled with ATP hydrolysis
Light- or redox-driven pumps:
Uphill transport coupled with energy from light or a redox reaction
Briefly describe the structure of a transporter
10 or more transmembrane α helices.
Solute- and ion binding sites in the middle of the membrane.
Pseudosymmetric: built from inverted repeats, the structure of the two halves are similar but inverted
=> alternate access to binding sites
Which type of antiport is generally used to maintain pH in cells? What is the normal intracellular pH?
Na+ antiport/exchangers
Intracellular pH~7.2
What are the basic differences between the three classes of ATP-driven pumps (transport ATPases)?
P-type:
Phosphorylate themselves
Ion pumps, maintain gradients of ex. Na+, K+, H+, Ca2+…
ABC transporters: Largest family ATP-binding cassette, 2 domains Pump small molecules Might function as transporters or gated channels
V-type:
Turbin-like, constructed from multiple subunits.
Transfer H+ into organelles.
Structurally similar to F-type (ATP synthase, work in reverse to drive ATP synthesis)
What is the role of the P-type Ca2+ pump and how does it work?
Pump Ca2+ actively out of a cell to maintain a steep concentration gradient.
Ca2+ pump is in the membrane of the sarcoplasmic reticulum. Action potential depolarizes a muscle cell membrane => influx of Ca2+ through Ca2+ release channels.
Ca2+ pump moves Ca2+ from the cytosol back into the SR.
Have 2 centrally binding sites for Ca2+. At non-phosphorylated state: only accessible from the cytosolic side of the SR memb.
Binding => phosphorylation and conformation change. Binding of new ATP opens to SR lumen side => Ca2+ out.
Replaced by H+ and H2O
What is the sarcoplasmic reticulum?
Network of specialized smooth ER that is important in transmitting the electrical impulse as well as in the storage of calcium ions.
Forms a network of tubular sacs in the muscle cell cytoplasm.
What type of transport ATPase is the Na+-K+ pump?
P-type:
Phosphorylate themselves
Ion pump
What is the general structure of an ABC transporter?
2 hydrophobic domains, each containing 6 transmembrane α helices.
Can be one or more polypeptides.
Have 2 cytosolic ATPase domains.
How does an ABC-transporter function?
Specific for a molecule or molecule class.
Variety of substrates: aa, oligo/polysaccharides, peptides, proteins, inorganic ions…
Some function as transporters, others as gated channels.
Eukaryotic: mostly export from the cytosol into the
extracellular space or ER (via TAP).
Prokaryotic: transport molecules in both directions
across membranes.
Important clinically
What is special about the P-type ATPases?
Phosphorylate and dephosphorylate themselves during each pumping cycle.
E.g. the Ca2+ pump and the Na+-K+ pump
Which ions do channels primarily transport?
Na+
K+
Ca2+
Cl-
What is special about aquaporins relative to ion channels?
Permeable to water, but not to ions.
What structural components of an aquaporin are important for its function?
Continuously open.
Narrow pore - one water molecule at a time, too narrow for hydrated ions.
Carbonyl oxygens on one side.
Hydrophobic aa on the other side.
2 asparagines in the middle bind to the O atom of the central H2O => bipolarity. Cannot participate in an H+ relay => pore impermeable to H+
What is a selectivity filter and where is it found?
The narrowest part of an ion channel.
Limits the rate of passage for ions.
Allows contact between the ions and the channel wall so that only ions of appropriate size and charge can pass.
What are the different types of ligands that bind ligand-gated ion channels?
Extracellular mediator e.g., neurotransmitter
Intracellular mediator e.g., ion
Nucleotide
What main types of stimuli can cause an ion channel to open?
- voltage across a membrane
- mechanical stress
- binding of a ligand
What characterize ion channels / how do we distinguish them from each other?
Ion it conducts
Mechanism by which it is gated
Abundance and localization in the cell/specific cells
What is the role of K+ leak channels?
Open in the plasma membranes of all cells, even in unstimulated/resting.
Makes the plasma membrane much more permeable to K+ than to other ions => maintains the membrane potential.
What structural components of the voltage-gated K+ channel are important for its function?
4 identical transmembrane subunits that together form a pore through the membrane.
Each subunit contributes 2 transmembrane α helices, tilted outward in the membrane and together form a cone. Wide end faces outside of the cell (K+ exit)
Polypeptide connecting transmembrane helices form a short α helix (pore helix) and a crucial loop extending into the wide section of the cone, forming the selectivity filter. Contains carbonyl O.
Pore helices guide K+ into the selectivity filter
-C=O O’s are too far away from each other for Na+ to pass
Gating: movement of the helices in the membrane to obstruct/open path for ions.
Which ion channel is important for generating and propagating action potentials in nerve cells?
Voltage-gated cation channels.
Na+ channels in nerve and skeletal muscle cells.
What happens during the depolarization reaction in a membrane?
Key words: depolarization, Ca2+, Na+, neurotransmitter
Depolarization of the membrane at the axon terminal => voltage-gated Ca2+ channels in presynaptic memb. open, Ca2+ influx.
Release of neurotransmitters from synaptic vesicles into synapse. Bind to transmitter/ligand-gated receptors at neighbor cell => opening of cation channels.
When sufficient depolarization: Opening of voltage-gated Na+-channels => Na+ influx (diffusion)
=> more depolarization
=> more Na+ channels open (self-amplification, positive feedback)
After reaching +50 mV:
- Inactivation of Na+ channels
- Opening of voltage-gated K+ channels, K+ efflux
=> restoring of natural memb. potential (-70 mV, neuron, to -40 mV)
What structural components of the voltage-gated Na+ channel are important for its function?
Chapter 11 - p. 622 Central channel (4 ppt) Every domain has: Voltage sensor Selectivity filter
Between 3rd and 4th domains: inactivation gate
What is the “refractory period”?
Time necessary for a sufficient number of Na+ channels to recover from inactivation to support a new action potential.
What type of channel are channelrhodopsins and what is the associated photopigment?
Photosensitive ion (cation) channels.
Has a covalently bound retinal group that absorbs light and undergoes an isomerization reaction, which triggers a conformational change in the protein, opening an ion channel in the plasma membrane.
How can light-responsive ion channels be used in science?
Use genetic engineering techniques to express it in target cells e.g., specific neurons in the brain.
Implant an optic cable near the relevant brain region.
Flash of light activates/inactivates the neurons to fire action potentials depending on the type of channel.
Frequency of light flashes determines the frequency of action potentials.
Which cell types form the myelin sheath of neurons?
Nonneuronal supporting cells, glial cells:
- Schwann cells in the peripheral nerves
- Oligodendrocytes in the CNS
What are the advantages of the nodes of Ranvier in the myelin sheath?
Nodes of Ranvier:
Regularly spaces without myelin where almost all the Na+ channels of the axon are located.
Action potential “jumps” from node to node => faster, more effective.
Energy conservation as the excitations is confined to small regions of the membrane.
How are the secreted neurotransmitters removed?
Example: acetylcholine
Destroyed by enzymes in the synaptic cleft.
Acetylcholine is hydrolyzed and degraded by acetylcholinesterase.
Taken up by the presynaptic nerve terminal or by surrounding glial cells.
Reuptake mediated by Na+ dependent NT symporters.
Rapid removal => temporal and spatial precision of signaling at a synapse.
What is the main function of a transmitter-gated ion channel?
Rapidly conversion of extracellular chemical signals into electrical signals at chemical synapses.
Open transiently in response to the binding of NT molecules => brief permeability change of membrane.
Size of action potential depends on the number of NTs released at the synapse and how long it persists there.
Relatively insensitive to memb. potential, cannot produce a self-amplifying excitation.
What are the main differences between excitatory and inhibitory chemical synapses?
Both are generally immediate, simple, and brief.
Excitatory:
- Excitatory NTs (acetylcholine, glutamate) open cation channels (often Na+, or Ca2+)
- Depolarization of the postsynaptic membrane, action potential
Inhibitory:
- Inhibitory NTs (GABA, glycine) open Cl- or K+ channels
- Suppresses firing, hyperpolarization
- Harder to fire action potential
Sum of excitatory and inhibitory signals
decide strength of the signal
Name some features of the acetylcholine receeptor
5 transmembrane ppt, where 2 are identical and bind acetylcholine.
Ring of hydrophobic aa blocks ion flow in the closed state.
Clusters of negatively charged aa at both ends help to attract cations and repel anions.
Na+ and K+ (some Ca2+) enter by diffusion => little selectivity
Opened transiently after binding of 2 acetylcholine molecules and conformational change.
Acetylcholinesterase hydrolyzes acetylcholine and returns the receptor to its original state.
If acetylcholine is present for a prolonged time (>20 milliseconds): desensitivization.
Give a brief description of the different types of ion channels at the axon hillock at a neuron
Voltage-gated Na+
1 selective for Ca2+
3 selective for K+: delayed, rapidly activating, and Ca2+-activated K+ channels
Delayed K+ channels:
Repolarize the membrane after each action potential by K+ efflux. Close after.
Slower kinetics, only open when the Na+ channels are inactivated.
Rapidly activating K+ channels:
Ensure a firing rate that is proportional to the strength of the depolarization stimulus.
Ca2+-activated K+ channels:
A neuron that is stimulated continuously for a prolonged period (more Ca2+ in) becomes gradually less responsive to the constant stimulus; adaptation. K+ leakage out of the cell => harder to depolarize
What is long-term potentiation (LTP) and when does it happen?
Occurs in a specific subset of neurons in the hippocampus and is responsible for molecular processes forming the prerequisite for learning.
Occurs when a presynaptic cell fires simultaneously with the post-synaptic membrane already being depolarized.
Glutamate - main excitatory NT in the mammalian CNS.
AMPA and NMDA receptors mediate the process => Ca2+ influx, depolarization.
AMPA receptors mediate normal signaling via membrane depolarization, NMDA regulate
the abundance of AMPA receptors dependent on glutamate presence and membrane depolarization
