Chapter 13- Membrane channels and pumps Flashcards
3 classes of transporters that make membranes permeable to specific molecules
ATP-driven pumps, carriers, and channels. The nature of the membrane transporters dictates the biochemical processes that occur inside the cell.
Which type of molecules does not require the assistance of proteins to cross the cell membrane?
Lipophilic (or hydrophobic) molecules can pass directly through phospholipid bilayers down their concentration gradients in the process of simple diffusion. They do not require the assistance of proteins.
Which molecules require the presence of a protein channel to move across a membrane?
Polar or fully charged molecules require the presence of a channel to move across a membrane down their concentration gradient. Such movement is called facilitated diffusion or passive transport
Active transport
Moving a molecule across a membrane against its concentration gradient, requires energy
Membrane potential
The free energy of a solute that has an unequal distribution of a charged molecule. It takes into account the concentration differences and charge difference
Na/K pump
Most animal cells contain a high concentration of K+ and a
low concentration of Na+ relative to the external environment. To generate and maintain this membrane potential requires the
action of an active transport system, the Na/K pump. The pump is a member of the P-type ATPase family.
P-type ATPases
ATPases that form a phosphorylated aspartate intermediate.
What does the Na/K pump use as a source of energy?
ATP hydrolysis
What 2 conformations must pumps exist in?
One with the ion binding site facing into the cell and one with the binding site facing out of the cell. ATP hydrolysis powers the interconversion of the two conformations- as the conformation changes, the molecule is able to pass through the membrane
SERCA
Sarcoplasmic reticulum Ca2+ ATPase- a P-type ATPase, pumps calcium from muscle cytoplasm into the sarcoplasmic reticulum. It is a single polypeptide chain with a transmembrane domain consisting of 10 α helices.
3 domains of the cytoplasmic portion of the SERCA enzyme
- N domain- binds ATP
- P domain- accepts the phosphate during the reaction cycle;
- A domain- links the N and P domains
Steps of the catalytic/transport cycle of P-type ATPases (6)
- Binding of cytoplasmic calcium
- ATP binding by the N domain
- Transfer of a phosphoryl group to an aspartate in the P domain
- ADP is released and a structural change occurs so that the calcium binding site now faces the sarcoplasmic reticulum lumen and the calcium leaves the enzyme.
- The phosphoryl group in the P domain is hydrolyzed.
- The enzyme changes conformation so that the calcium binding site again faces the cytoplasm
Cardiotonic steroids
Found in foxglove- inhibit Na/K ATPase and are used to treat congestive heart failure. The decrease in the Na+ gradient
results in slower removal of Ca2+ from the cell, and The increase in cellular Ca2+ enhances the contractile ability of the heart.
Which other molecules are part of the same class of inhibitors as cardiotonic steroids?
Digitoxigenin and ouabain
The human genome contains how many genes encoding P-type
ATPases?
70
Which molecules are transported by p-type ATPases?
Some transport ions such as H+, Ca2+, or Na+, while others transport metals such as Cu2+ or even phospholipids with charged head groups
What is the reaction mechanism of p-type ATPases?
All members of this protein family have the same fundamental reaction mechanism: taking advantage of the free energy release associated with ATP hydrolysis to drive membrane transport by means of conformational changes induced by addition and removal of a phosphoryl group at a key aspartate site.
Multidrug-resistance protein
ATP-dependent pump that forces out small molecules from the cell, is a member of a family of transporters called ABC transporters
ABC transporters
Characterized by a common domain called the ATP-binding cassette (ABC). ABC transporters consist of two ABC domains and two membrane-spanning domains
Steps of the reaction cycle of the multidrug resistance pump (5)
- The channel is open to the cytoplasm.
- Substrates bind, causing conformational changes in the ABC domain.
- ATP binds to the ABC domains, causing more structural changes that orient the substrate so that it is facing the outside of the cell.
- The outward facing conformation of the transporter has reduced affinity for the substrate, allowing its release.
- ATP hydrolysis occurs, resetting the transporter to the initial state.
Secondary transporters/cotransporters
Use the energy of one gradient to power the formation of another
Symporters
Power the transport of a molecule against its concentration gradient by coupling the movement of that of another molecule down its concentration gradient, with both molecules moving in the same direction
Antiporters
Also use one concentration gradient to power the formation of another, but the molecules move in opposite directions
Uniporters
Transport a molecule in either direction, depending on the concentration difference across the membrane
Lactose permease
From E. coli- it is a symporter that uses an H+ gradient to power the entry of lactose into the cell
Transport cycle of lactose permease (6 steps)
- The cycle begins with the lactose-binding pocket facing the outside of the cell.
- A proton binds, followed by lactose binding.
- The permease everts.
- Lactose leaves the permease and enters the cell.
- A proton leaves the permease, entering the cell.
- The permease everts to complete the cycle
Ion channels
Allow the rapid movement of ions across membranes down their concentration gradients
Action potentials
The result of K+ channels and Na+ channels opening and closing in response to changes in membrane potential. The interior of a neuron has a higher concentration of K+ and a lower concentration of Na+ than the external environment; the resulting typical membrane potential is −60 mV. A nerve impulse or action potential is generated when the membrane potential is transiently depolarized beyond a critical threshold value. The membrane subsequently repolarizes
Patch-clamp technique
These measurements reveal the activities of single channels. A pipette is placed against a small portion of a cell membrane, and suction is applied to form a tight seal called a gigaseal. The gigaseal allows measurement of channel activity when a voltage is applied across the cell membrane. The patch of membrane sealed to the pipette can also be excised
Na+ channel structure
The Na+ channel was purified on the basis of its affinity for the neurotoxin tetrodotoxin. It consists of four repeated regions of similar sequence, and each region contains five hydrophobic segments and one hydrophilic segment
Ca 2+ channel structure
The Ca2+ channel is homologous to the Na+ channel
K+ channel structure
The K+ channel consists of four subunits, each homologous to one of the repeated units in the Na+ channel. A mutated version of the K+ channel, termed shaker, facilitated its purification.
How does the K+ channel transport ions?
The potassium channel selectively and rapidly transports K+ across the cell membrane. Larger ions are not transported because they are too big to enter the channel. Smaller ions are excluded because they cannot interact with the selectivity filter. Such ions are small enough that the energy of desolvation cannot be compensated for by interactions with the selectivity filter.
How does the structure of the K+ channel explain its speed of transport?
Charge repulsion among the four ion-binding sites in the potassium channel accounts for the rapid transport of K+ ions down their concentration gradient
Which ion channels are voltage gated?
The Na+ and K+ channels are voltage-gated channels; they change conformation with changes in membrane potential
K+ voltage gated channel
Segments S1 through S4 of the K+ channel function in voltage gating, with S4 being the voltage sensor. Segments S1 through S4 contain “paddle” domains. Changes in membrane potential alter the conformation of the paddles to open or close the channel
How can the K+ channel be deactivated?
It can be inactivated by physically blocking the channel. A segment of the channel, termed the ball, is tethered to the channel by a polypeptide segment called the chain- mutants lacking the ball and chain do not inactivate.
Protease
A protease can be used to cleave off the chain in a K+ channel, providing early evidence that the chain is flexible (protease-accessible)
How does depolarization inactivate the K+ channel?
Depolarization opens the channel and creates a binding site for the ball, which binds to and inactivates the channel
Acetylcholine
A neurotransmitter that is released into the synaptic cleft. Acetylcholine then binds to a channel, called the acetylcholine receptor, opening the channel to K+ and Na+ and triggering an action potential. The acetylcholine receptor, a ligand-activated channel, was isolated from the torpedo (electric ray)
Acetylcholine receptor
Ligand gated channel. The acetylcholine receptor is made up of four types of subunits with the stoichiometry α2βγδ arranged to form a
pentameric ring. Binding of acetylcholine causes conformational changes that rotate the membrane-spanning helices so that the pore opens to the passage of K+ and Na+ ions.
Nernst equation
Describes the membrane potential/equilibrium potential
If an ion X+ is unequally distributed across a membrane, it will
Tend to move down its concentration gradient. This movement will be inhibited by the accumulation of positive charges
What is the resting potential of a neuron?
-60 mV. This value is close to the equilibrium potential for K+ because a small number of K+ channels are open
Generation of an action potential
Activation of the acetylcholine receptor allows K+ to flow out of
the cell and Na+ to flow in. The change in membrane potential first activates the Na+ channels, then opens the voltage-gated K+ channels while the “ball” segment of the Na+ channel inactivates that channel. At the same time, the acetylcholine receptor is inactivated. With only the K+ channels open, the membrane potential drops toward the K+ equilibrium potential. The K+ channels are then closed by the “ball” segment, and the membrane potential returns to its initial state
Long QT syndrome
The recovery of the action potential is delayed- causes loss of
consciousness, heart arrhythmia, and sudden death. “QT” refers to a particular feature of the cardiac electrical activity pattern as measured by electrocardiography.
Which mutations cause LQTS?
The most common mutations identified in LQTS patients inactivate K+ channels or prevent the proper trafficking of the channels to the plasma membrane. This loss of K+ permeability slows repolarization of the membrane and delays heart contraction
EKG definition
Record of electrical event in the myocardium that can be correlated with mechanical events
P wave
Depolarization of atrial myocardium and signals onset of atrial contraction
QRS complex
Ventricular depolarization and signals onset of ventricular contraction. Repolarization of the atria occurs simultaneously
T wave
Repolarization of ventricles, precedes ventricular repolarization
PQ interval/PR interval
.16 seconds, atria contract and begin to relax, ventricles begin to contract
QT interval
.36 seconds. Ventricles contract and begin to relax
Gap junctions
Also called cell to cell channels, allow passage of materials between adjacent cells and are important for intercellular communication. All polar molecules with a mass of less than 1 kDa can pass through gap junctions
What are gap junctions composed of?
A cell-to-cell junction is composed of 12 molecules of connexin. Six hexagonally arranged connexins form a half-channel
called a connexon or hemichannel. Connexons from adjacent cells form a gap junction, and gap junctions traverse two membranes and allow cytoplasm-to-cytoplasm communication.
Properties of cardiac muscle
Cardiac muscle cells are elongated, branching cells containing 1-2 centrally located nuclei. Contains actin and myosin filaments and intercalated disks. Electrically, cardiac muscle of the atria and of the ventricles behaves as a single unit.
Intercalated disks
Specialized cell-cell contacts in cardiac muscle. Cell membranes interdigitate, desmosomes hold cells together, gap junctions allow action potentials to move from one cell to the next.
Aquaporins
Aquaporins allow rapid and specific movement of water
across membranes. Specific positively charged residues toward the center of the channel prevent the transport of protons through
aquaporins.
Why is the membrane potential important in the heart?
The heart uses coordinated changes in membrane potential to
create efficient muscular contractions that allow effective
pumping of blood throughout the body
How can the heart beat spontaneously, without any input from
the rest of the body?
Certain cardiac cells are known as pacemakers; the most
important region is the sinoatrial (SA) node. The cells of the
SA node spontaneously generate about 100 action potentials
per second. These cells have an unusual capacity for mixed Na+/K+
conductance when their membranes are hyperpolarized; this
membrane permeability is referred to as the funny current (If).
Path of electrical current in the heart (4 steps)
- Action potentials originate in the SA node (the pacemaker) and travel across the wall of the atrium from the SA node to the AV node.
- Action potentials pass through the AV node and along the AV bundle, which extends from the AV node through the fibrous skeleton into the interventricular septum.
- The AV bundle divides into right and left bundle branches, and APs descend to the apex of each ventricle along the bundle branches
- APs are carried by the Purkinje fibers from the bundle branches to the ventricular walls and papillary muscles
Valve positions when blood is flowing into the left ventricle
The bicuspid valve is open. The cusps of the valve are pushed by the blood into the ventricle. The aortic semilunar valve is closed. The cusps of the valve overlap as they are pushed by the blood in the aorta toward the ventricle.
Valve positions when blood is flowing out of the left ventricle
The bicuspid valve is closed. The cusps of the valves overlap as they are pushed by the blood toward the left atrium. The aortic semilunar valve is open. The cusps of the valve are pushed by the blood toward the aorta.
Funny current
This current is active only at rest, leading to a gradual depolarization of the membrane to a level that triggers the initiation of an action potential. Then it is off during the action potential itself and back on once the membrane returns to its resting potential. Unusual: these channels are opened by hyperpolarization and close when the membrane is depolarized.
Which channels are responsible for the funny current?
The channels responsible for the funny current are known as HCN (hyperpolarization-activated cyclic nucleotide-gated) channels, with the HCN4 isomer most highly expressed in the SA node.
Sick sinus syndrome
Mutations in the HCN4 gene have been identified in patients with sick sinus syndrome. Greater hyperpolarization may be required to achieve opening of the mutant HCN4 receptor, and fewer mutant HCN4 channels may be open at a given resting potential, leading to
slower depolarization. Symptoms can include abnormally low heart rate, fainting, and fatigue.
