Chapter 6 Flashcards
Aquaporins are involved in
a. thirst mechanism
b. concentration of urine by kidneys
c. digestion
d. regulation of body temp
e. secretion and absorption of spinal fluid
f. secretion of tears, saliva, sweat, and bile
g. reproduction
Structure of aquaporins
a. Homotetramers
b. each subunit forms a pore
What are the three features that confer the water-specificity of aquaporin?
a. size restriction via a constriction region
b. electrostatic repulsion
c. water dipole orientation
In kidneys, what does the aquaporin-1 type protein channels do?
Help concentrate 180 liters of blood filtrate per day into a urine volume of 1.5 liters per day by reabsorbing
Where does the aquaporin-1 have a constitutive high water permeability?
In the epithelial cells of the proximal convoluted tubules and descending thin limbs of the loop of Henle
Vasopressin stimulates the expression of
Aquaporin-2
What does the stimulation of the expression of aquaporin-2 result in?
Increased urine concentration
Where is the aquaporin-2 expressed?
In the collecting ducts
What are the steps of action potential?
- opening of voltage-gated Na+ channels
- rapid flow of Na+ ions into the cell
- membrane depolarization
- depolarization stops within milliseconds, Na+ channels rapidly inactivate
- early repolarization begins, voltage-gated Ca2+ channels open
- transient outward K+ currents balance the Ca2+ channels
- more K+ ions rapidly exit the cell
- repolarization
- Na+/K+ - ATPase drives membrane potential toward repolarization to reestablish the resting negative membrane potential
What happens when depolarization stops?
Na+ channels rapidly inactivate
What happens when early repolarization begin?
voltage-gated Ca2+ channels open
What results after the K+ ions rapidly exit the cell?
repolarization
Ca2+ signaling regulates what?
muscle contraction and heart rhythm
Excitation-concentration coupling
Process in which membrane depolarization results in production of force by muscles (cardiac and skeletal)
What cell has much lower [Ca2+] compared to the extracellular or ER/SR concentrations?
Resting cells
Resting cells have much lower [Ca2+] compared to where?
the extracellular or ER/SR concentrations
When is the signal initiated at the plasma membrane?
when it is depolarized from an incoming action potential
What are the 4 steps of excitation-contraction coupling?
- Depolarization activates Ca2+ channels
- Ca2+ influx stimulates Ca2+ release from SR into cytosol
- Increased cytosolic [Ca2+] stimulates myofilament force development
- Relaxation occurs when cytosolic [Ca2+] decreases
What senses the change due to depolarization?
Voltage-dependent Ca2+ channels
Voltage-dependent Ca2+ channels respond to the change by
allowing a small influx of Ca2+ ions to enter the cell
What does the influx of Ca2+ ion stimulate?
Release of lots of Ca2+ from the SR through RyRs
RyRs
Ryanodine Receptors
What is RyRs?
Intracellular Ca2+ gated Ca2+ release channels
What does RyRs do?
bind the plant alkaloid ryanodine with high specificity, blocking the channel
Increase in cytosolic [Ca2+] activates
[Ca2+] sensitive protein troponin C
What does the [Ca2+] sensitive protein troponin C stimulate?
contraction of the myofilaments
What does the extrusion of Ca2+ from the cytosol cause?
causes the muscle to relax
How does the extrusion occur?
a. reuptake of Ca2+ ions into the SR by the SR Ca2+ - ATPase pump
b. removal of Ca2+ ions from the cytosol by the Na+/Ca2+ - exchanger in the plasma membrane
What allows the reuptake of Ca2+ ions into the SR?
SR Ca2+ - ATPase pump
What allows the removal of Ca2+ ions from the cytosol
Na+/Ca2+ - exchanger
What is an another type of intracellular Ca2+ release channel?
IP3R
IP3R
Inositol 1,4,5-triphosphate receptor
What are the two distinct gene families of glucose transporter proteins that function in the plasma membrane?
a. GLUTs
b. Na+/glucose cotransporters
What are GLUTs?
uniporters which mediate facilitated transport of glucose down its concentration gradient
What does Na+/glucose cotransporter do?
couple the energy of the transmembrane Na+ gradient to the transport of glucose
GLUTs are part of what family?
Major facilitator superfamily (MFS)
What are MFS?
largest superfamily of proteins involved in the membrane transport
In whom are GLUTs found?
In all living organisms
What does GLUTS mediate?
the transport of solutes into or out of cells, depending on the solute concentrations
Why is GLUT-1 important?
to facilitate glucose into the brain
How does GLUT-1 facilitate glucose to brain?
By transporting glucose from the blood into endothelial cells, and then transporting glucose from the endothelial cell into the ECM, and then from the ECM into an astrocyte
What kind of transporter is GLUT-4
insulin-responsive transporter
What does GLUT-4 mediate?
mediates glucose uptake by muscle and adipose tissues
Where are GLUT-4 proteins located?
in the intracellular vesicles that fuse with the plasma membrane
What does the GLUT-4 proteins do?
Delivers the GLUT-4 transporters to the plasma membrane
What disease occurs if there is not enough GLUT-4 in the plasma membrane?
Type II diabetes
What two conformations to GLUT proteins alternate between?
a. glucose binding site faces the extracellular space
b. glucose binding site faces the cytosol
How does the binding of the glucose affect the orientation?
reorientation of the glucose-binding sites tot he opposite side of the membrane and in release of glucose
What do the symporters and antiporters move?
one solute against its transmembrane concentration gradient
What energy do the symporters and antiporters use?
uses the gradient energy of the second solute moving down its transmembrane concentration gradient
Symporters and antiporters are part of what family?
Major facilitator superfamily (MFS)
What is LacY?
bacterial lactose permease
a monomeric oligosaccahride/H+ symporter
What energy does LacY use to drive the accumulation of nutrients?
H+ gradient
LacY use what gradient to generate an H+ gradient?
Lactose gradient
How is the H+ gradient generated?
By a combination of ETS and by the F1F0 ATPase
Combination of ETS and F1F0 ATPase can couple?
ATP hydrolysis to the export of protons from the cytosol
Release of the lactose and protons into the cytosol induces a transition back to what conformation?
Outward-facing conformation
What are the 4 examples of a primary active transport protein that maintains the Na+ gradient?
a. voltage-dependent Na+ channels
b. epithelial Na+ channels
c. Na+/substrate transporters
d. Na+ dependent transporters involved in pH regulation
What are the 3 cotransporters of Na+/substrate transporters?
a. Na+/glucose symporter
b. Na+/iodide cotransporter
c. Na+/prooline cotransporter
How does Na+/glucose cotransporter work? (steps)
- Na+ binds
- conformational change
- sugar binds
- conformational change exposes Na+ and sugar to the intracellular side of the membrane
- released into the cytosol
- conformational, change and resetting to starting position
What is Na+/Ca2+ exchanger in excitable cells?
primary Ca2+ extrusion system to the ECM side of the plasma membrane
What does Na+/Ca2+ exchanger do?
transport 3 Na+ ions in exchange for 1 Ca2+ ion, generating a net electrogenic current of 1+ per cycle
What does Na+/K+/Cl- cotransporter mediate?
mediates electroneutral transport with a stoichiometry of 1:1:2
Under physiological conditions, where does Na+,K+,Cl- contransported to?
Into cells
Why is the Na+/K+/Cl- cotransporter important?
a. to maintain intracellular Cl- concentration
b. for the reabsorption of NaCl from the kidney to filtrate
What does Na+/Mg2+ exchanger do?
transports 2 Na+ ions in for each Mg2+ extruded, thus transport is electroneutral under physiologic conditions
Why is the Na+/Mg2+ exchanger important?
to get rid of excess Mg2+ that constantly permeates into the cytosol at a low rate
What energy does Na+/H+ exchanger and the Na+/HCO3- contransporter use?
energy of the transmembrane Na+ gradient to regulate pH
Lungs and kidneys help maintain the acid-base balance of the plasma by?
excreting CO2 out of the lungs and H+ into the urine
What secretes H+ into the lumenal filtrate (urine)?
apical membrane Na+/H+ exchanger (NHE)
The secretion of H+ into the lumenal filtrate is coupled to?
transport of an equal number of bicarbonate ions into the blood
What transports bicarbonate ions into the blood?
Na+/HCO3- cotransporter in the basolateral membrane of epithelial cells of the proximal tubule
What functions as the intracellular Ca2+ storage compartment?
ER
When does the resting Ca2+ concentrations reestablished?
after Ca2+ signaling has occurred
What are the main types of Ca2+ transport protein to extrude Ca2+ from the cytosol?
Ca2+ ATPase in the ER and in the plasma membrane
What in muscle cells gets most of the Ca2+ out?
SERCA (sarcoendoplasmic reticulum Ca2+ ATPase)
What sequences does the SERCA pump reaction cycle consist of?
sequence of phosphorylation and dephosphorylation events that power the uphill transport of 2 Ca2+ ions into the SR per hydrolyzed ATP in exchange for 2 H+ ions
PMCA (plasma membrane Ca2+ ATPase) pump function
transport 1 Ca2+ per ATP hydrolyzed
Similarity of SERCA and PMCA
a. P-type pumps
b. ATP dependent
c. use ATP to autophosphorylate a conserved aspartic acid residue
5 Functions of membrane
- maintain homeostasis
- control solute concentrations across membrane
- adapt to altered metabolic situation
- process info
- transport nutrient in and waste products out
What creates the electrochemical gradients?
the ion concentration difference inside and outside the cell
Which ions have higher extracellular concentration (in the outside)?
Na+, Ca2+, and Cl-
Which ions have higher intracellular concentration (inside)?
K+
The charge inside the cell is _________ relative to the outside
negative
How is the speed of the solute different in channel proteins and carriers?
Solute pass through channel proteins at high rate while they pass through carrier proteins much slower
What three traits do channel proteins have?
a. high solute selectivity
b. a rapid rate of solute permeation
c. a gating mechanism
What are 5 examples of channels?
a. ion channels
b. aquaporins
c. gap junctions
d. NPCs
e. ER protein translocators
What does the selectivity filter allow?
Allow channel proteins to discriminate among different solutes
How are channel proteins regulated?
By gating
What are 4 types of gating?
a. ligand-gated
b. voltage-gated
c. stretch-activated
d. temperature-activated
Electrochemical dictates the
direction of the movement
What forms pores
channel proteins
How do carrier proteins work?
solutes bind on one side of the membrane, undergo an allosteric change, and release them on the other side of the membrane
Carrier proteins transduce energy from
a. electrochemical gradients
b. ATP
c. other energy sources
What are the two main types of a carrier protein?
transporters and pump
Transporters
couple energy from electrochemical membrane gradients to facilitate movement of substrate across the membrane
What are the three types of transporters?
a. uniporters
b. symporters (cotransporters)
c. antiporters (exchangers)
Pumps
uses energy directly to drive energetically less favorable substrate accumulation or efflux
What does primary active transport (pumps) do?
drive transport of solutes against their electrochemical gradients
What energy does primary active transport use?
ATP
Primary active transport works to
maintain gradient of solutes across membranes
What 2 examples of primary active transport?
a. Ca2+ ATPase
b. Na+/K+ ATPase
What does secondary active transport (transporters) do?
drives trans-membrane solute transport
What energy does secondary active transport use?
use the free energy stored in the electrochemical gradients
do not use ATP directly
Ions in solution surrounded by
water molecules are attracted by their dipolar partial negative and partial positive charges
What is formed around each ion
hydration shell
Formation of hydration shell is energetically _________.
favorable because it requires lots of energy to move into a lipid bilayer
Size of the hydration shell depends upon
the charge density and size of the ion
Ion channels enable the ___________ ______________ of ions as they travel through
partial dehydration
As ions goes through the ion channel, it forms a weak bond with what and why?
amino acid residues, which help make the transport process energetically favorable and selective
When does a membrane potential exist?
when there is an electrochemical gradient
A membrane potential requires
a. the ion concentration differences across the membrane resulting in a charge separation
b. a membrane which is selectively permeable for at least one of the ionic species
What three ions are major contributors to the membrane potential?
K+, Na+, and Cl-
What two ions contribute little to the resting membrane potential?
Ca2+ and Mg2+
K+ leak channels allow
K+ ions out of the cell, down their electrochemical gradient
K+ leak channels help
to make the inside of the cell more negative
Na+/K+ ATPase function
pumps 2 K+ ions into the cell for every 3 Na+ it pumps out
Na+/K+ ATPase helps to
a. make inside more negative
b. increase the K+ gradient so the leak channels can keep working
How does depolarization occurs?
when Na+ or Ca2+ channels open, the ions flow into the cell
How does repolarization occur?
when K+ channels open, K+ moves out of the cell
Depolarization makes the membrane potential more
positive
Repolarization makes the membrane potential more
negative
K+ channels form a
narrow water-filled pore
Structure of K+ channels
a. tetramers
b. each identical subunit contributes to a central pore
What are the two main parts of the K+ channel?
central cavity and selectivity filter
Central cavity help
stabilize K+ ions before they go through
Selectivity filter has to ________ the ions in order for them to pass through
dehydrate
How is the dehydration of ion enabled by the selectivity filter?
enabled due to the partial negative charge of the oxygen atoms in the selectivity filter which acts as surrogate water molecules
What are three K+ channel subfamilies?
- gates sensitive to metabolic state of the cell
- gates sensitive to ligand binding
- gates sensitive to voltage
2 examples of K+ channels
a. Ca2+ activated K+ channel
b. voltage-gated K+ channel
Voltage-gated channels sense
changes of the membrane electric field via several positively charged amino acid residues in their voltage-sensing module of their transmembrane domains
Voltage-dependent Na+ channels require what gradient
electrochemical Na+ gradient
The electrochemical Na+ gradient is generated by
Na+/K+ ATPase
IMPs formed with
single pore-forming subunit
IMPs transport
Na+ ions down their electrochemical gradient
After voltage-dependent activation what occurs very quickly?
voltage-dependent inactivation of Na+ channels
The selectivity filter of Na+ channels bind to
tetrodotoxin from puffer fish
Binding of tetrodotoxin to the selectivity filter causes
paralysis by inactivating voltage-gated Na+ channels involved in the initiation and propagation of action potentials in nerve cells
Na+ channels are targets for
local anesthetics and drugs used to treat cardiac arrhythmias
Drugs that used to treat cardiac arrhythmias inhibit
membrane depolarization
ENaC
Epithelial Na+ channels
ENaCs are regulated by
hormones
ENaCs first found in
epithelial cells
ENaCs mediate
bulk flow of Na+ ions, influence water transport across cell layers
ENaCs function depends on
Na+ gradient established by the Na+/K+ ATPase
Where are ENaCs located?
in the apical membrane of epithelial cells in the distal tubule and collecting ducts of each kidney nephron
ENaCs allow
Na+ ions from the filtrate to enter the cells down their gradient
Na+/K+ ATPase help by
removing Na+ from the epithelial cell and transporting it back into the blood capillary
Reabsorption of Na+ is regulated by
aldosterone from the adrenal glands and vasopressin from the pituitary gland
Aldosterone bind to receptors on
kidney cells
Mutation in ENaC, increasing Na+ reabsorption results in
high blood plasma volume, hypertension, and low plasma K+
A diuretic drug that blocks the Na+ reabsorption by ENaC
amiloride
Amiloride blocks Na+ reabsorption by ENaC in?
lumenal membrane of the kidney distal tubule and collecting duct
Amiloride results in
decreased Na+ reabsorption, lower Na+ concentration in blood, and lower or normalized blood pressure
Where are Ca2+ concentrations high?
In the extracellular fluid and in ER and SR
Ca2+ channels are gated by
extracellular ligands, voltage changes, or Ca2+ itself
Influx of Ca2+ increases the intracellular [Ca2+] to a level that triggers responses such as
a. muscle contraction
b. hormone or neurotransmitter release
c. activation of Ca2+ dependent signaling cascades
d. gene transcription
In in vitro assays, Cl- channels function as
nonselective anion channels
What are the three different gene families?
- CLC gene family
- Cystic fibrosis transmembrane conductance regulator
- Ligand-gated (GABA receptor and glycine receptor family)
Cl- is the most abundant anion
in vivo assay
Cystic fibrosis transmembrane conductance regulator is part of what family
ABC transporter family
ABC
ATP binding cassette where every member of family needs to bind to ATP
What are the 2 major components of the electrochemical gradient across the eukaryotic plasma membrane?
Na+ and K+ gradients
What does the negative resting membrane potential regulate?
osmotic pressure
What does the negative resting membrane potential allows
secondary Na+ dependent transport of molecules
Electrochemical gradient is generated and maintained by
Na+/K+ ATPase
Na+/K+ ATPase belongs to the family of
P-type ATPases
Belonging to the family of P-type ATPase means
it autophosphorylates an aspartic acid residue as an intermediate during ion transport
Terminal phosphate is transferred from
ATP to the active site in the enzyme
For each ATP hydrolyzed, how many Na+ are moved out and K+ are moved in/from where?
3 Na+ out and 2K+ from ECF into the cytosol
Through Na+/K+ ATPase, what is created across the plasma membrane?
an electrical potential difference and an osmotic ion gradient
Sodium potassium is _________ but not under _________ conditions
reversible; physiological
What targets the sodium potassium pump?
various toxins and drugs
plant steroids called cardiac glycosides
Cardiac glycosides function
inhibit ion transport by sodium potassium pump
Where does digitalis come from?
foxglove plants
Digitalis is used to
treat heart failure because a small amount will increase cytoplasmic [Na+] which results in higher cytoplasmic [Ca2+] which increase contractility of the heart
Target of digitalis
Sodium potassium pump
F1F0 ATP synthase couples
H+ movement to ATP synthesis or hydrolysis
F1F0 ATP synthase is a
molecular motor
F1F0 ATP synthase couples the energy of the electrochemical proton gradient across
the plasma membrane of prokaryotic cells or the mitochondrial inner membrane of eukaryotic cells to ATP synthesis
The transmembrane domain of F1F0 ATP synthase is the
F0 region
F0 region is involved in
translocation of protons down their electrochemical gradient
F0 region is the
transmembrane domain
The cytoplasmic or mitochondrial matrix globular domain is the
F1 region
F1 region contains the
catalytic sites responsible for ATP synthesis
F1 region is the
cytoplasmic or mitochondrial matrix globular domain
In F1F0 ATP synthase, per ATP synthesized, how many protons are transported?
4
In F1F0 ATP synthase, ATP synthesis occurs at a rate of
~100/sec
F1F0 ATP synthase can work in
reverse
In F1F0 ATP synthase, some bacterial cells can use
ATP hydrolysis to generate a H+ gradient which other membrane proteins can use to move solutes
H+ ATPase transport
protons out of the cytosol
V-ATPase
Vacuolar-type proton pumps
V-ATPases are
H+ ATPases
V-ATPases essential for
maintaining the pH of organelles such as lysosomes, endosomes, which need a more acidic environment than the cytosol
2 functional domains of H+ ATPases
V1 (cytosolic) and V0 domain
V1 function
binds and hydrolyses ATP, providing the energy for proton translocation across the membrane bound V0 domain
