Lecture 4 - Pores, Channels And Transporters Flashcards
What is active transport?
Transport of ions or molecules against the concentration gradient using energy directly or indirectly from hydrolysis of ATP
What is primary active transport?
Active transport which directly uses the energy from the hydrolysis of ATP to transport molecules against their concentration gradient
What is secondary active transport?
Type of Co-transporter
When an electrochemical gradient is set up by an ATPase is then used to transport another ion or solute against its gradient
Use a pre-existing electrochemical gradient to co-transport a molecule against its concentration gradient
What are the 4 primary active transporters i need to know?
Na+/K+ pump (sodium pump)
Calcium ATPases (PMCA and SERCA)
K+/H+ ATPase (Proton pump)
F1/F0 ATPase (ATP synthase, doesn’t hydrolyse ATP)
Where is the SERCA transporter protein located?
Sarcoplasmic reticulum of skeletal muscle cells
What type of co-transport is occurring with a Na+/K+ ATPase or sodium pump?
Antiport
Where a sodium pump is concerned, what are the relative concentrations of Na+ and K+ inside and outside of the cell?
K+ high inside cell low outside
Na+ high outside cell, low inside
With the Na+/K+ ATPase (sodium pump), how many sodium ions and how many potassium ions are being transported?
3 Na+
2 K+
Which direction is Na+ being actively transported and which direction is K+ being actively transported with the sodium pump?
2 K+ pumped into cell
3 Na+ pumped out of cell
Why is a sodium pump an example of a primary active transporter?
Uses energy directly from the hydrolysis of ATP to actively transport both K+ and Na+ against their gradients
Which amino acid on the sodium pump gets phosphorylated from the Pi from hydrolysed ATP?
Phosphorylates Aspartate
Makes phosphoenzyme intermediate
How does the sodium pump work?
ATP hydrolysed (Aspartate phosphorylated) causes conformational change
3 Na+ taken into their high affinity sites
Conformational change opens the protein so Na+ can be released out of the cell
Conformational change allows 2 K+ to bind to pump
Conformational change opens pump to inside the cell phosphate released causing K+ site to have low affinity for K+ so released into cell
What is the structure of the sodium pump?
2 subunits
Alpha and beta
What is the function of the alpha subunit of the sodium pump?
Where all the work is done:
Where phosphate binds and where ions pumped
Has Ouabain receptor
What is the function of the Beta subunit of the sodium pump?
A glycoprotein which puts sodium pump on plasma membrane of cell
What is Ouabain?
Cardiac glycoside
Binds to sodium pump preventing it from functioning
What is the function of the Na+/K+ ATPase (Na+ pump)
Forms Na+ and K+ gradients for electrical excitability
Drives secondary active transport (set up electrochemical gradient)
What is the main contributor to maintaining membrane resting potential?
Leaky K+ channels
Which ion is toxic to the heart?
K+
What is the normally K+ serum levels in the blood?
4mM
The concentrations of which ions are normally always greater outside the cell compared to inside?
Ca2+
Na+
Cl-
Why is calcium toxic to cells?
Activates enzymes that shouldn’t be activated
Calcium phosphate crystallises in the cell killing the cell
Calcium influx in mitochondria causes apoptosis
Why is it useful the concentration of calcium being so much lower inside the cell compared to the outside?
Can utilise calcium for certain process
What is another name for a plasma membrane Ca2+ ATPase?
PMCA
What is the role of PMCA?
(Plasm membrane Ca2+ ATPase)?
Exclude Ca2+ ions from the cell
What type of transport does the PMCA primary active transporter use?
Uniport
What other transporter protein expels calcium from the cell?
NCX (Na+/Ca2+)
What is the difference between PMCA and NCX?
NCX = Secondary active transporter + antiporter
PMCA = Primary active transporter + Uniporter
How does the affinity and capacity of PMCA compare to NCX?
PMCA = High affinity BUT Low Capacity (Ca2+ binds well but cant move a lot)
NCX = Low affinity BUT High capacity (Ca2+ doesn’t bind well but can move lots at once)
What is the name of the calcium transporter found on the membrane of the sarcoplasmic reticulum?
SERCA
(SarcoEndoplasmic reticulum Ca2+ ATPase)
What type of Ca2+ transporter exists in the mitochondria?
Ca2+ uniporter
What is the pathology of Duchenne muscular dystrophy?
Too much Ca2+ enters into mitochondria
Water drawn into mitochondria
Mitochondria burst
Replaced with adipocytes
How can Ca2+ entering mitochondria cause cell death?
Ca2+ can activate caspases causing apoptosis
What is unique about the F1/F0 ATPase (ATP synthase)?
Synthesises ATP rather than hydrolysing it
Briefly describe how the F1/F0 ATPase (ATP synthase) transporter works:
Proton gradient from inter membrane space to matrix exists
H+ binds to F0 which rotates allowing H+ to pass back into the matrix
F1 rotates rotates and Phosphorylates ADP and Pi to ATP
How many H+ must pass through the F1/F0 ATPase complex to produce 1 ATP molecule?
3 H+ = 1 ATP
What type of transporter is Sodium Glucose Cotransporter 1/2 (SGLT-1 or SGLT-2)?
Secondary active transporter
Symporter
How many Na+ does SGLT-1 move in for every glucose?
2 Na+
How many Na+ does SGLT-2 move in for every glucose?
1 Na+
What is the importance of Organic Anion Transporter (OAT) and Organic Cation Transporter (OCT)?
How many drugs enter into cells
What is the role of the Na+/Ca2+ exchanger (NCX)?
Expels intracellular Ca2+ from cell during recovery (while cell is repolarising)
What type of transporter is NCX?
Antiporter
Secondary active transporter
How many molecules of Na+ are moved into the cell and how many Ca2+ ions are moved out of the cell per cycle by NCX?
3 Na+ in
1 Ca2+ out
How does charge within the cell usually change as NCX is active?
Becomes more positive by +1 each exchange (3+ in cell , 2+ out of cell net gain of +1)
What is reperfusion?
Restoration of blood flow to a region normally following an arterial occlusion allowing for wastes to begin to be removed from the tissues
What happens to NCX when the membrane is polarised?
Ca2+ pumped into cell
Na+ pumped out
Function of NCX flips
How does NCX function in ischaemia?
Blood supply cut of leads to lack of O2 so depletion of ATP
Sodium pump cant act
Na+ accumulates in cell and cell becomes very positive so is depolarises
Depolarised membrane causes NCX function to flip so 3 Na+ expelled from cell as 1 Ca2+ pumped into cell
Ca2+ builds up in cell, cell dies
What are the 2 primary active transport methods of controlling resting Ca2+ conc in the cell?
PMCA
SERCA
What is the secondary active transport method of controlling resting calcium concentration?
NCX
What is the method of controlling resting calcium concentration via facilitated diffusion?
Mitochondrial Ca2+ uniports
Buffer potentially damaging calcium concentrations
What does the Na+/H+ exchanger transport? (NHE)
1 Na+ into cell
1 H+ out of cell
No net change in charge
What type of transport is used by NHE (Sodium proton exchanger)?
Secondary active transport
Antiport
What is the role of NHE (Na/H+ exchanger)?
Regulates pH
Regulates cell volume
How does NHE regulate cell volume?
As sodium brought into cell, water is brought with it
What is Amiloride?
Drug that targets the NHE
What is the function of amiloride?
Inhibits NHE
Prevents Na+ and water being drawn back into kidney cells
Is a diuretic (does take potassium with it)
What is the importance of the sodium pump (Na+/K+ ATPase) carrier protein in regulating cellular pH?
Establishes Na+ gradient (electrochemical gradient) so secondary active transport can take place at other transporters to regulate pH
Which transporters lead to acid extrusion from a cell?
NHE
Sodium Bicarbonate Chloride transporter
Which transporters cause alkali influx into cells?
Sodium Bicarbonate Chloride transporter (NBC)
Sodium Bicarbonate transporter
Which transporter extrudes alkali from then cell no which cells are they commonly in?
Anion exchanger (AE)
Like band 3 in RBCs
Where in the kidney is blood glucose controlled?
Nephron
What would happen if selective reabsorption did not happen in the nephron?
Na+, HCO3-, H2O and glucose would all be lost from the body in urine
RAPID DEHYDRATION AND COMA
What type of kinetics do carrier proteins experience and why?
Michaelis Menton Kinetics
All sites get saturated and rate conformational changes occurring can’t change
In the gut (enterocyte) which glucose transporter is present on the apical surface?
SGLT-1
(Sodium glucose co-transporter 1)
In the gut (enterocyte) which glucose transporter is found on the base lateral surface?
GLUT-2
Where SGLT-1 is present, what other transporter will always be found on the basolateral surface?
GLUT-2
What type of active transport does the K+/H+ ATPase transporter do?
(Proton pump)
Primary active transport
Directly uses energy from hydrolysis of ATP
What is the role of the K+/H+ ATPase in the enterocyte?
Sets up proton gradient
So proton and sucrose can by symporter from gut lumen into enterocyte across Sucrose-proton cotransporter
What type of active transport is used by the sucrose-H+ Cotransport at the gut?
Secondary active transport
How many sections are there of the PCT of the nephron?
3
Which SGLT transporter is used in segment 1 of the PCT to transport glucose back in to the epithelial cell?
SGLT2
What SGLT transporter is used in segment 2 and 3 in the PCT to reabsorb glucose into the epithelial cells?
SGLT1
Which glucose exporter is used to put glucose back in the blood in segment 1 in the PCT?
GLUT 2
Which glucose exporter is used to put glucose back in the blood in segment 2 and 3 of the PCT?
GLUT1
Action of SGLT2 and SGLT1 inhibitors in Type 2 diabetes Mellitus?
SGLT2 inhibitors inhibit glucoses uptake in kidney
SGLT1 inhibitors inhibit intestinal absorption
What is phlorizin?
SGLT2 inhibitor
What is the main glucose transporter in the liver?
GLUT 2
What is the main glucose transporter in muscles and adipose?
GLUT 4
What hormone regulates expression of GLUT 4?
Insulin promotes
Where is GLUT1 found?
CNS (Astrocytes)
What are MCT proteins?
Symporters of H+ and lactate ions from Astrocyte to neurone
What pathways is glucose transported in the CNS?
Glucose into Astrocyte via GLUT1
GLucose converted to lactate
Lactate transported to neurone via MCT
Glucose via GLUT1 via endothelial cells into interstitial space
GLUT3 into neurone
What type of transport happens across GLUT transporters?
Facilitated diffusion
What drives the transport of glucose across SGLT transporters?
Na+/K+ ATPases
How does glucose uptake into skeletal muscles and adipose tissue increase with insulin?
Insulin binds to Tyrosine kinase receptor
Receptor phosphylates itself
Activates secondary messenger cascade
Vesicles containing GLUT 4 proteins fuse to plasma membrane of cells
Which glucose transporter is present at the blood brain barrier?
GLUT-1
Which glucose transporter is present in neurones?
GLUT-3
Why it essential that GLUT-1 and GLUT-3 have very low Km for glucose?
So brain and neurones receive as much glucose as possible to prevent death
Describe how glucose is transported from the gut lumen into the blood:
Na+/K+ ATPase pumps 3 Na+ out of cell into blood and 2K+ from blood into cell (basolateral surface)
SGLT1 Moves 2 Na+ with 1 glucose from apical surface into cell
GLUT2 moves glucose into the blood from the cell
Which has a lower Km, SGLT1 or SGLT2?
SGLT 1 since it works at lower concentrations (think of the kidney PCT)
Which glucose transporter reabsorb most of the glucose back into the PCT?
SGLT2
