Chapter 13: Membrane Channels and Pumps

Two classes of transmembrane proteins--pumps and channels-- can be used to circumvent the impermeability of the lipid bilayer.

Some of the structural and functional features of pro teins are described in this chapter.

The authors first differentiate between active transport and passive transport and then discuss how to quantitate the free energy stored in concentration gradients.
There are three types of active transport systems: the P-type ATPases, the ABC pumps, and the secondary transporters.
There are many structural and mechanistic features in the Na+-K+ and sarcoplasmic reticulum Ca2+-ATPases.
The family of ABC pumps that have recently been identified include the multidrug resistance protein and the cystic fibrosis transmembrane conductance regulator.
The mechanism of secondary transporters is examined in the discussion of active transport.

In addition to active transport, ion channels can be used.

The key properties of all channels are discussed by the authors.
Researchers can measure the activity of a single ion channel using the patch-clamp technique.
Two types of ion channels important in the propagation of nerve impulses are examined in detail.
The chapter ends with a discussion of gap junctions, which act as cell-to-cell channels and allow all polar molecules with less than 1 kDa to pass through.

You should be able to complete the objectives when you master this chapter.

There are two factors that determine whether a molecule crosses a membrane.

The rates of transport through channels are different from the rates of active transport.

List the sequence of steps in the action potential.

The structure, ion selectivity, and inactivation mechanism of the potassium channel are compared.

The structure of the channel should be compared to its rate of transport.

There are examples ofmol ecules that can pass through gap junctions.

The side of the mem brane where the various processes involved in the transport of Na+ and K+ will take place is determined by the orientation of the Na+-K+ pump.

The existence of four different states of the Ca2+-ATPase is the basis for the proposed model.

The exchanger is used for calcium and sodium transport.

If you want to describe the characteristics in the right column, you have to write them down.

List the events in the transmission of nerve impulses.

The action potential should be placed in their correct sequence.

The corre sponding properties are listed in the right column.

Active transport must be used since the transport is unfavorable in the direction indicated by the sign of DG.

There is a net efflux of one positively charged ion if three Na+ ion are transported out for every two K+ ion.

The answer is yes.

While prokaryotic ABCs are often multisubunit, eukaryotic ABCs usually have the same domain on the same polypeptide.

The answer is incorrect.

The exchanger transports the cations in the opposite direction.

A column with covalently attached cobratoxin can be used in the purification of the acetylcholine receptor from a mixture of macromolecules in the postsynaptic membrane that has been solubilized.

Correct answer (c) is incorrect.

The diameter of the channel is not uniform.

The concentrations of Na+ and K+ are 150 and 140 mM, respectively, in dog skeletal muscle.

The temperature is 25oC and the potential is -60 mV.

A molecule is transported from one side to the other.

The Na+-K+ATPase is one of the pumps in the cells.

One such pump is the H+-K+ ATPase, in which a hydrogen ion is removed from the cytoplasm in exchange for a potassium ion.
Explain why the Na+-K+ ATPase can contribute to the potential of the cell but the H+-K+ATPase can't.

In experiments to investigate the mechanism of transport of two substances, X and Y, across cell membranes, cells were incubated in media containing various concentrations of X and Y, and the initial rate of transport of each of the substances into the cell was determined.

It is helpful to refer to the discussion of the enzymes in the text.

If you want to see if the Na+-K+ ATPase reaction involves a stable enzyme-phosphate intermediate, you can design an experiment using the labeled ATP with 32P in the g position.

The rising phase of the action potential and the falling phase are caused by certain events.

If the ion flow occurs against concentration gradients or both, specify it.

There is a narrow region of the sodium channel.

Val is replaced by a mu tant protein.

9 x 298 x ln 5 is a negative number.

The total is 1.99 x 298 x ln.

The entry of a positively charged ion is aided by the membrane potential.

The free energy change will be less favorable if the calculation is repeated for a negatively charged molecule.

The sum of the two will be the total free-energy change.

9 x 298 x ln + 23 x 062

When Na+ is taken out of the cell, work must be done against both a concentration and electrical gradient.

The calculation for the K+ ion is done now.

9 x 298 x ln + 23 x 062

The concentration is being transported against an electrical gradient.

The sign for the electrical term in the equation is negative.

To get the total energy expenditure, we have to account for the transport of Na+ and K+.

The energy furnished by the hydrolysis of a single ATP is sufficient.

A molecule is transported from one side to the other.

The molecule will move down its concentration gradient if the concentration on side 1 is higher than the concentration on side 2.

The rate of a chemical process is always the same as the rate constant.

The rate of transport will be greater in (b) than in (a).

The difference between the two systems is their ex change of ion.

The inside of the cell is more negative for each pump cycle due to the fact that there are three Na+ ion and two K+ ion taken up.
The H+-K+ ATPase extrudes one H+ ion for each K+ taken up, so its operation is neutral.

Resistance to one drug can make cells less sensi tive to other drugs.

There is a correlation between the expression and activity of a 170 kDa protein called Multidrug ResistanceProtein (MDR) and the development of multidrug resistance.

The drugs are pumped out of cells before they can exert their effects.

If a carrier is involved in the transport of substance X, the curve for X shows saturation.

The curve for Y shows no saturation, which is consistent with the idea that substance Y diffuses without a carrier.
Such behavior is shown by substances that can enter cells without a carrier.

In the first half-reaction, the first product of the reaction,ADP, is created when the two main components of the reaction, ATP and unmodified, interact in the presence of Na+ and Mg2+.

In the second half-reaction, the phosphorylated enzyme is hydrolyzed by water in the presence of K+.

An overall reaction would be suggested by the following experiment.

Incubate a fragmented preparation with K+ ion to hydrolyze anyphosphate that might be bound to the enzyme.

After washing the preparation to remove K+, transfer it to a medium containing g-labeled ATP, Na+, and Mg2+.

After a suitable period of time, wash the preparation.

You can detect the presence of labeled phosphate with scintillation counting.
The presence of radioactivity in the fraction would suggest that a stable intermediate had been formed.
There is a covalent aspartylphosphate derivative at the active site.

Nerve cells have a higher concentration of K+ inside than out side and a higher concentration of Na+ outside than inside.

The inside of the cell is negative with respect to the outside.

Only a small number of Na+ and K+ ion enter and leave the cell for each action potential that is generated in an axon.

In a poisoned nerve cell, many tens of thousands of impulses may be conducted before ionic equilibrium is achieved.
In the long run, the active transport of Na+ and K+ is necessary, but not in the short run.

The Na+ conductance would have been decreased if a Mutant pro tein was found in the narrow region of the sodium channel.

The charge between Na+ ion and carboxylate anions is important in drawing Na+ into the channel.
The un charged side chain of Val won't attract Na+ ion.

There are no charged groups in the side chain of Val that would show a sensitivity to pH.

The positively charged guanido group is likely to interact with the negatively charged group in the sodium channel.

The action potential is created by the flow of sodium from the outside of the cell to the inside.

The magnitude of the action potential would be reduced due to the reduced amount of sodium conductance in the Mutant.

Form reconstituted vesicles with each oriented in the Membrane so that its face is towards the inside of the vesicle.

The hydrogen ion will be created by bacteriorhodopsin when the vesicles areilluminated.
The entry of Lactose will be driven by the hydrogen ion gradient.

The electrochemical gradient for Na+ influx is larger than that for K+efflux.

Na+ moves from the positive to the negative side of the membrane, whereas K+ moves from the negative to the positive side, which favors Na+ influx.

There is a cations binding substance at the entrance of the potassium channel.

The energy required to dehydrate Na+ and smaller cations is too large and is not compensated by favorable polar interactions that occur in the case of K+.

The ball and chain model of channel inactivation is supported by two pieces of experimental evidence.

The Na+ or K+ channel can be trimmed with trypsin if it is treated on the cytoplasmic side.
There are two things that show that the N-terminal splice variant of the potassium channel has changed.
A deletion of 42 amino acids at the N-terminus causes the channel to open but not inactivate.
Adding a syntheticpeptide corresponding to the deleted amino acids restores inactivation to the channel.

It is 105 x (5 x 10-2)4 for four ligands.

The fractions of open chan nels are 10-5, 2 x 10-4, 3.98 x 10-3, 7.41 x 10-2, and 0.615.

The three molecules all contain highly reactive phosphoryl groups that readily react with the active-site serine of acetylcholinesterase to form a stable derivative.

Respiratory paralysis can be caused by synaptic transmission being impossible without active acetylcholinesterase.

The log (240) x 1.36 is the free-energy change during the binding of the first acetylcholine.

The closed/open ratio will be the same effect as the binding of the ligand.

The channel is not perfect.
The two chains are not in the same place.
The presence of desensitized states in addition to the open and closed ones indicates that a more complex model is required.

The voltage needed to open half the channels is -22 mV.

The table below shows the information that was given.

The slope is 173 4.

Substituting, we get 4.5 x 23 kcal V-1 mol-1 x 0.05 V.

The channels do not provide any energy or allow any passive transport of ion.

In the opposite direction, potassium ion flow.
Active pumps need energy to establish the ion gradients.

There is a single positive charge in the guanidino group of tetrodotoxin.

The tetrodotoxin molecule is too large to pass through a sodium channel, so the guanidino group is likely to bind to it.

The channel is blocked.

Ion-channel blocking molecules can cause paralysis by disrupting electrical activity in the nervous system.

The snail toxins can be poisonous at low concentrations.
Such toxins could be useful for identifying and labeling new types of ion channels and for investigations of the channels' mechanisms of action.

This is a very important concept.

Let's take a look at the channels.

Na+ concentration can't be "sensed" by the cell when the sodium channels are closed, and there is no pathway by which Na+ ion could diffuse to adjust their concentrations toward equilibrium values.

A small number of the voltage-gated sodium channels open when the action potential is only 20 mV.
The opening of only a few channels causes a small initial increase in the sodium permeability, which causes a further increase in the membrane potential and more channels to open.
More and more channels are opened until the signal from the sodium ion peaks within 1 ms. All of the sodium channels will close after 1.5 ms from the initial triggering event.
During the short time that it is open, each channel only allows the passage of a small amount of Na+).

An initial triggering event can be amplified as many channels become involved, and the nerve cell can recover quickly and transmit a new impulse every few milliseconds, if the ability to generate a signal using only a small number of ion is taken into account.

It needs repolarization to return to a closed state.

Since BTX keeps the sodium channels open after depolarization, it blocks the transition from open to inactivated state.

chloride ion will flow into the cell

The action potential of the depolariza tion causes the chloride flux to be inhibitory.

There is a high concentration of lactose in the inner volume of the vesicles.

The binding of Lactose to the inner face of the permease would be followed by the binding of a protons.
Both sides would never give up.
Lactose and the proton will leave the permease because of the low concentration on the outside.
The uphill flux of protons will be driven by the downhill flux of Lactose.

For proper nerve activity, the change in permeability caused by the opening of channels must be short-lived.

It is important to remove the source of stimulation in order to close the channels.
Once initiated, the nerve impulse moves on and the postsynaptic membrane must return to its resting state in order to be ready to receive and propagation another signal.

Acetylcholinesterase has a similar mechanism to the serine proteases.

As with chymotrypsin and trypsin, the active site of acetylcholinesterase has serine.
The mechanism will involve the use of acyl and tetrahedral intermediates in which the substrates are bonded to the active-site Ser.

The Ser OH group gives a carbonyl group that acts as a base and accepts H+).

The acetyl group will be left to join the Ser in the acyl-enzyme intermediate.
In the second half of the reaction, water will act as a nucleophile to attack the acyl enzyme.
The free enzyme will be regenerated when the second product leaves.
The process can be repeated.

After the application of the toxin, the currents from these channels are completely stopped for about 60 s.

At the end of the first set of record ings in part A, the ASIC1a channels are starting to recover.

The estimate is based on the logarithmic scale.

The channels with the bV266M are open for longer.

There are possible explanations for the slower closing rate.
A tighter binding of acetylcholine could keep the channels open longer.

The normal rate of release of chyln could be delayed by the mutation, which could slow the transition from the open state to the closed state.

The rate of indole transport is determined by the concentration of indole.

This finding shows that indole can diffuse without a specific mechanism for transporting it.
By constrast, the rate of transport reaches a saturation point with no further rate increase.
There is a correlation between the finding for glucose and the fact that it would bind to a specificProtein and then be transported across the Membrane.
The transport rate would go up when all of the sites are occupied by the same molecule.
The inhibition of glucose transport by ouabain suggests that it requires energy and that it may be linked to the transport of Na+ or K+.