Neurobiology Chapter 4: Channels and Transporters

What are the 3 postulated molecular properties of voltage gated channels?

-Have to be capable of allowing ions to move across the membrane at high rates -Have to be selective -Have to be able to sense voltage

What are the domains on an ATPase pump?

-NB (nucleotide binding domain) -P (phosphorylation domain) -AD (actuator domain - removes phosphate from P domain)

What is this diagram showing?

Some voltage gated potassium channels will only conduct current when calcium is available as an activator, and some will only conduct current with the appropriate pH.

Describe the kinetics of the voltage gated sodium channel in Graph B based on the current.

The channel usually opens quickly and then inactivates quickly. However, sometimes the channel will flicker, which is when it opens and inactivates in rapid succession.

What is macroscopic current?

The current flowing through a large number of channels.

What is microscopic current?

The current flowing through a single ion channel.

Why does the peak of an action potential never reach sodium's reversal potential?

The driving force becomes too small for sodium to continue exiting the cell.

What is the equilibrium potential of chloride?

-61 mV

On its cytoplasmic side, the mammalian voltage gated potassium channel has a regulatory _________ subunit and a _________ domain that links the subunit to the channel.

-Beta -T1

Name two intracellular ligands that open channels.

-Calcium -cAMP

A bacterial potassium channel is formed by _________ subunits that each cross the plasma membrane twice; between these two helical membrane-spanning structures is a ________ ________ that inserts into the plasma membrane.

-Four -Pore loop

What are some examples of ligand gated channels?

-Glutamate gated channels, allows passage of sodium into cell and potassium out -Calcium gated channels, allows passage of potassium out of the cell -cAMP activated channels, allows sodium into cell and potassium out

What are two examples of antiporters in the cell? Which way does each ion move and which is being transported actively?

-Na+/Ca++ exchanger. Sodium moves in and calcium moves out. Calcium is transported actively. -Na+/H+ exchanger. Sodium moves in and H+ moves out. H+ is transported actively.

What are two examples of co-transporters? Which way do the ions move and which is being transported actively?

-Na+/K+/Cl- transporter. All ions move into the cell. Potassium is transporter actively. -K+/Cl- transporter. Potassium and chloride exit the cell, and Cl- is transported actively. -Na+/neurotransmitter transporter. All move into the cell. NT's such as GABA and dopamine are transported actively.

Depolarization pushes voltage sensors (inward/outward), and hyperpolarization pushes voltage sensors (inward/outward).

-Outward -Inward

What two things determine the size of current through voltage gated channels?

-Probability of channels being open -The potential at which the membrane is clamped at

Voltage-gated ion channels that are selectively permeable to each of the major physiological ions have been identified. What are these ions?

-Sodium -Calcium -Potassium -Chloride

What does this diagram tell you about the sodium/potassium pump?

-Sodium and potassium fluxes cannot be separated -ATP is required

Give two examples of ATPase pumps. Which ions do they move and in which direction?

-Sodium potassium pump (moves sodium out and potassium in) -Calcium pump (moves calcium out)

Describe the alpha and beta subunits in the SR calcium pump.

-The alpha subunit has a large intracellular component -The beta subunit is predominantly cytoplasmic

How do the kinetics of symporters and antiporters compare to ions channels?

-They are slower -They can saturate

How do the kinetics of an ATPase pump compare to an ion channel?

-They are slower -They can saturate, meaning all the binding sites on the pump can be taken up and no more ion can pass through

What may be the reason for the doubled current in the top recording?

-Two potassium channels may be clamped -It may be a different kind of potassium channel

What are three different types of gated channels?

-Voltage gated -Chemically gated -Gated by mechanical stimuli

(Movement of calcium by the SR calcium pump) 1. _______ binds to the P domain. 2. The pump is _______ and ADP is released. 3. A conformational change causes ________ release, and two _______ come in as placeholders. 4. Two new ________ ions bind.

1. ATP 2. Phosphorylated 3. Calcium, protons 4. Calcium

(Translocation of sodium and potassium by the Na+/K+ pump) 1. Confirmation change allows _________ release and _________ binding. 2. The pump is _________ and ADP is released. 3. _________ is bound but occluded. 4. Confirmation change causes _________ release and _________ binding. 5. A __________ is removed from the pump. 6. __________ is bound but occluded.

1. Potassium release and sodium binding 2. Phosphorylated 3. Sodium 4. Sodium release and potassium binding 5. Phosphate 6. Potassium

How do the sodium and potassium channels differ in this diagram?

After prolonged depolarization, sodium channels inactivate before closing, whereas potassium channels stay open. Sodium channels also open faster than potassium channels.

(Patch clamp) What is outside-out recording?

Allows you to measure current in a single ion channel. The extracellular portion of the channel is exposed to solution.

(Patch clamp) What is whole cell recording?

Allows you to measure current in a whole cell.

(Patch clamp) What is inside-out recording?

Allows you to measure current through a single ion channel. The intracellular portion of the membrane is exposed to solution.

What are the two subunits of the sodium/potassium pump?

Alpha and beta. The alpha subunit spans the membrane 10 times and contains all functional sites. The beta subunit has only one membrane spanning region.

What is ouabain in a sodium potassium pump?

An inhibitor

Explain the change in sodium current at these different potentials.

At +20 mV, the driving force is large because the membrane potential is far away from the reversal potential. The driving force decreases until reversal potential is reached at -58 mV. At -70 mV, potassium exits the cell because it is past it's reversal potential.

Explain the change in potassium current at these different potentials.

At -20 mV, the driving force is large because the membrane potential is far away from the reversal potential. The driving force decreases until reversal potential is reached at +58 mV. At +70 mV, sodium exits the cell because it is past it's reversal potential.

Which voltage gated channel has two pores? Sodium, calcium, potassium or chloride?

Chloride

What are active transporters?

Create gradients that are used by channels to create electrical signals.

Describe the paddle model of voltage gated potassium channels.

Depolarization causes voltage sensors to move towards the extracellular surface of the cell, and causes the channel to open. Hyperpolarization cases voltage sensors to move towards the intracellular surface of the cell, which causes the channel to close.

What does this diagram show?

Different types of voltage gated potassium channels behave differently in response to voltage clamping.

(Secondary active transport) What are antiporters?

Exchange intracellular and extracellular ions

(True/false) Secondary active transport requires ATP.

False. It uses electrochemical gradients as energy.

How many pore loops make up the pore in a bacterial potassium channel?

Four

How many subunits are there in a mammalian potassium channel?

Four

Name an extracellular ligand that opens an ion channel.

Glutamate

Which graph is microscopic current and which is macroscopic current?

Graph C is microscopic and Graph D is macroscopic.

Which graph is microscopic current and which is macroscopic current?

Graph C is the microscopic current and Graph D is the macroscopic current.

How do thermosensitive channels open?

Heat displaces membrane lipids which causes a structural rearrangement and subsequent channel opening.

When voltage sensors move in response to depolarization or hyperpolarization, they exert a force on the _______ _______ connecting the sensors to the channel pore, pulling the channel pore open (depolarzied) or pushing it closed (hyperpolarized).

Helical linkers

The gates of sodium and potassium channels are closed when the membrane is (depolarized/hyperpolarized).

Hyperpolarized

What is the difference between the beta subunits in sodium channels and calcium channels?

In sodium channels the beta subunit is in the membrane. In calcium channels the subunit is intracellular.

Rectification is not an inherent property of a channel protein itself, but reflects strong voltage dependence of channel block by what?

Intracellular divalent cations such as Mg and Ca

In outside-out recording, when a positively charged ion moves into the channel and towards the pipette, is it an inward or an outward current?

Inward

What is an inward rectifier potassium channel?

It conducts much larger inward currents at membrane voltages negative to the K+ equilibrium potential compared to outward currents at voltages positive to it.

Ligand gated channels are usually (more/less) selective than voltage gated channels.

Less

How do mechanosensitive channels open?

Mechanosensitive channels have three subunits and large bodies called blades on the extracellular side. Mechanical force disrupts the blades, leading to a rearrangement of the structure of the rest of the channel, and causes it to open.

The selectivity filter of a bacterial potassium channel is (wide/narrow).

Narrow

A bacterial potassium channel has central _________ charged pores formed by pore loops and adjacent helices.

Negatively

The structure of a potassium channel pore is well suited for conducting potassium; the narrowest part is near the outside mouth of the channel and is so constricted that only a ___________ potassium ion can fit through the bottleneck.

Nonhydrated

Where in a cell are TRP channels located?

On the cell membrane. They are peripheral receptors.

How many proteins make up a voltage gated sodium channel? What about a voltage gated potassium channel?

One for sodium and four for potassium.

At rest, is there more Cl- inside or outside of the cell?

Outside

In outside-out recording, when a positively charged ion moves out of the cell and away from the pipette, is it an inward or an outward current?

Outward

Which technique was used to obtain the data in this graph?

Patch clamping. Voltage was clamped at -20 mV and current was measured.

In mammalian potassium channels, voltage sensors contain _________ charges that enable movement within the membrane in response to changes in membrane potential.

Positive

Why does potassium have a greater peak conductance compared to sodium?

Potassium channels remain open for a longer amount of time, so more current can flow through the channel. Sodium channels rapidly inactivate, so they can only conduct current for a short period of time.

Deep within a potassium channel is a water-filled cavity that connects to the interior of the cell. What occurs here?

Potassium is collected from the cytoplasm, and the negative charges from inner surface of the channel dehydrates the potassium. These "naked" ions can then move through four potassium binding sites (carbonyl oxygens) within the selectivity filter to eventually reach the extracellular space.

Are ATPases primary or secondary active transport?

Primary

Why does secondary active transport indirectly rely on primary active transport?

Primary active transport such as the Na+/K+ set up the electrochemical gradient which is used for energy in secondary active transport.

What is the difference between primary and secondary active transport?

Primary active transport uses ATP as energy to move ions. Secondary active transport uses an electrochemical gradient - generated by active transport - as an energy source to move molecules against their gradient.

Are ion transporters primary or secondary active transport?

Secondary (ion transporters are symporters and antiporters).

Which part of the potassium channel is responsible for the selective permeability of potassium?

Selectivity filter

Why is the probability of sodium channels opening greater than the probability of potassium channels opening when at a depolarized potential?

Sodium channels rapidly activate, whereas potassium channels take longer to open.

What model of voltage gated potassium channel is this?

The paddle model

Why can't sodium enter a potassium channel?

The pore walls of a potassium channel are too far apart to stabilize a dehydrated sodium ion.

Describe the kinetics of the voltage gated potassium channel in Graph B based on the current.

The potassium channels does not open right away, and once it's open, it remains open for several seconds (in contrast to sodium).

What causes the rapid flux of potassium ions through a potassium channel?

The presence of up to four potassium ions in the selectivity filter causes electrostatic repulsion between the ions. This pushes the ions through quickly.

What does Graph E show?

The probability that sodium channels will open at different membrane potentials. Probability increases as the neuron depolarizes.

What does Graph E show?

The probability that voltage gated potassium channels will open at a certain membrane potential. Probability increases as the membrane becomes more depolarized.

What model of voltage gated potassium channel is this?

The rotation model

Describe the rotation model of voltage gated potassium channels.

There are six transmembrane proteins, one of which is positively charged. When depolarized, this charged protein rotates and causes a confirmational change among the other subunits, causing the pore to open.

Explain this patch clamp recording an acetylcholine channel.

There's a grouping of channel openings into bursts and clusters and an increase in open probability within clusters at the Ach concentration was increased.

What happens to divalent cations when an inward rectifier potassium channel is hyperpolarzied?

They are removed from the channel pore so potassium can enter.

When activated, do voltage gated potassium channels always remain open or do they flicker?

They flicker between open and closed.

What are TRP channels and what is their function?

Transient receptor potential channels. The mediate a variety of sensations such as pain, temperature, different kinds of tastes, pressure, and vision.

The four __________ structures of a mammalian potassium channel have voltage sensors which move in response to membrane depolarization.

Transmembrane

(True/false) Chemically gated channels can be activated by either an extracellular or intracellular signal.

True

(True/false) The membrane spanning regions that contain voltage sensors in voltage gated channels may either be four subunits within one molecule or four molecules that come together to form a quaternary protein.

True

How many subunits are required for a chloride channel?

Two

What is the patch clamp method? How does it work?

Voltage clamping a single channel to observe current. A glass pipette with a small opening clamps a portion of the membrane using suction. Current is then measured using an amplifier attached to the pipette.

Why do voltage sensors move outward when depolarized?

Voltage sensors are positively charged, so they are repelled by a depolarizing stimulus.