3.1 Transporters (Passive and Active Transport) Flashcards
why do we study channels, transporters and receptors
30% of total human genome encodes for membrane proteins and 90% of all pharmaceuticals target membrane proteins
passive transport
facilitated diffusion
- down concentration gradient
- use of a channel or a transporter
active transport
- uphill against electrochemical gradient
- coupled to source of metabolic energy (either chemical reaction of downhill transport of another molecule)
simple diffusion
passive transport:
diffusion of nonpolar compounds down concentration gradient
facilitated diffusion
passive
- for: large molecules (sugar, AA, vitamins)
- depends on: transporter, [substrate]
- transport is saturable and specific
- relatively slow
in facilitated diffusion, are any chemical bonds made/broken?
no
in facilitated diffusion, which has more affinity for the transporter: substrate inside or outside
neither, they have the same affinity to the binding site of the passive transporter
graph of GLUT1 activity (label axises and line)
slide 8
why would you want a low Km
ensures that the transporter works even at low substrate concentrations, especially important if it’s something that the cell really needs
what kind of transporter is the glucose transporter
passive transporter (facilitated diffusion)
steps of glucose transport (4)
1) glucose binds to T1
2) binding lowers Ea and triggers transition to T2
3) Glucose released from T2 to cytoplasm
4) Ea rises and transporter returns to T1 conformation
how many conformations does the glucose transporter exist in
2
- T1: glucose-binding site on outside
- T2: glucose-binding site on inside
what is Km
concentration of solute when the transport rate is half its maximum
what does Vmax measure
rate which the carrier can flip its two conformations
GLUT1
PM of RBC
- Km = 1.5 mM (blood [glucose]=5mM)
why does glucose not get transport back out even though it is a passive transport system
as soon as glucose transported into the cell, it is phosphorylated to glucose-6P which has no affinity to GLUT1
GLUT2
liver and B cells of pancreas
- Km = 20 mM
- transports glucose out of hepatocytes to replenish blood glucose (don’t want to it to use up a lot of glucose)
GLUT3
neuronal cells
- Km = 0.15 mM
- needs constant influx of glucose so very high affinity
GLUT4
skeletal muscle
- Km = 5 mM
draw the activity graphs for GLUT1 with GLUT2,3,4
slide 13?
which GLUT is regulated by insulin
GLUT4 (found in skeletal muscle, fat, heart)
how is GLUT4 regulated by insulin
- normally GLUT4 is sequestered in secretory vesicles in the cytosol
- after a rich meal, blood glucose levels exceeds 5 mM and triggers the release of insulin from pancreas
- insulin receptor triggers movement of vesicles to PM
- GLUT4 more abundant at the PM
- glucose uptake increases
- when insulin release slows down, GLUT4 reabsorbed into secretory vesicles again
what is type 1 diabetes and what happens?
diabetes mellitus
- insulin not released
- GLUT4 stays in vesicles
- glucose in the blood not transported into the cell so blood sugar remains high
active transport types
1) coupled: uphill transport of one solute coupled to downhill transport of another
2) ATP-driven: couples uphill transport to hydrolysis of ATP
coupled transport energy source
electrochemical gradient of another solute
symporters
coupled transporter
- transport in same direction (co-exchangers)
antiporters
coupled transporter
- coupled transport in opposite direction (exchangers)
what is the main driving force of active transport
Na+
concentration gradient of Na+ across membrane
high Na+ on outside, low Na+ on inside
SGLT Transporter
active transport protein
- transport of glucose to cytosol coupled with 2 Na+ transport from the intestinal lumen (low glucose) to cytosol (high glucose)
with the SGLT transporter, what happens when one of the solutes is at low concentration
the other solute will fail to bind to the transporter
what is the equation for deltaGtotal
deltaGtotal = deltaGc + deltaGm
how is the movement of Na+ ions across the PM membrane governed
two forces acting in the same direction:
- membrane potential electric potential (outside of PM more positive than inside (-70 mV)) so Na+ will move in to “balance” the - membrane potential
- ion concentration gradient (concentration of Na+ much higher outside)
where is SGLT found
- internal mucosa of small intestine
- proximal tubule of nephron
how can the amount of active transport be increased at the physiological level
microvilli (such as in small intestine) to increase SA so you can have a greater number of transporters
lactose permease LacY
symporter, active transport protein
- proton-driven cotransport of lactose from outside to inside
lactose permease LacY structure
12 TMS helices clustered into two symmetrical loves
- substrate binding site is a central cavity
how is the proton gradient re-established (move H+ back outside) for the lactose permease LacY
during fuel oxidation by a proton pump
lactose permease LacY mechanism
rocking motion between two domains, powered by contribution of charged amino acids
steps of lactose transport
1) loading of H+ to negative E269
2) binding favors binding of lactose
3) presence of both substrates favor interaction between (+)R144 and (-)E269
4) pairing of (-)E269 destabilizes H+ and releases lactose
5) H+ released too
6) no more substrates to stabilize the (+)R144 and (-)E269 interaction and lactose permease LacY returns to original conformation
can lactose escape the cell
yes, because the lactose permease LacY mechanism is fully reversible. BUT, lactose generally broken down by B-galactosidase in the cytosol
AE1
“Anion-exchanger” Antiporter active transporter in respiratory cells
- transports out HCO3- from inside cell to outside with import of Cl-
why is there HCO3- inside erythrocytes and why does it need to be moved
waste CO2 from tissue is released from resp tissues, enters plasma and then RBC
- CO2 converted to HCO3- because it is more soluble in plasma so it can be better carried to the lung
- in the lungs, HCO30 re-enters the RBC and converted to CO2, then exhaled
why is the import of Cl- into erythrocyte cytoplasm beneficial
1) transport HCO3- out for better transport of CO2 to lung
]2) acidifies RBC cytoplasm to allow for release of O2
describe AE1 transporter action in the lungs
Cl- from inside transported out while HCO3- imported back in
