3.1 Transporters (Passive and Active Transport)

Question 1

why do we study channels, transporters and receptors

Answer 1

30% of total human genome encodes for membrane proteins and 90% of all pharmaceuticals target membrane proteins

Question 2

passive transport

Answer 2

facilitated diffusion

• down concentration gradient
• use of a channel or a transporter

Question 3

active transport

Answer 3

• uphill against electrochemical gradient

- coupled to source of metabolic energy (either chemical reaction of downhill transport of another molecule)

Question 4

simple diffusion

Answer 4

passive transport:

diffusion of nonpolar compounds down concentration gradient

Question 5

facilitated diffusion

Answer 5

passive

• for: large molecules (sugar, AA, vitamins)
• depends on: transporter, [substrate]
• transport is saturable and specific
• relatively slow

Question 6

in facilitated diffusion, are any chemical bonds made/broken?

Answer 6

no

Question 7

in facilitated diffusion, which has more affinity for the transporter: substrate inside or outside

Answer 7

neither, they have the same affinity to the binding site of the passive transporter

Question 8

graph of GLUT1 activity (label axises and line)

Answer 8

slide 8

Question 9

why would you want a low Km

Answer 9

ensures that the transporter works even at low substrate concentrations, especially important if it’s something that the cell really needs

Question 10

what kind of transporter is the glucose transporter

Answer 10

passive transporter (facilitated diffusion)

Question 11

steps of glucose transport (4)

Answer 11

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

Question 12

how many conformations does the glucose transporter exist in

Answer 12

2

• T1: glucose-binding site on outside
• T2: glucose-binding site on inside

Question 13

what is Km

Answer 13

concentration of solute when the transport rate is half its maximum

Question 14

what does Vmax measure

Answer 14

rate which the carrier can flip its two conformations

Question 15

GLUT1

Answer 15

PM of RBC

- Km = 1.5 mM (blood [glucose]=5mM)

Question 16

why does glucose not get transport back out even though it is a passive transport system

Answer 16

as soon as glucose transported into the cell, it is phosphorylated to glucose-6P which has no affinity to GLUT1

Question 17

GLUT2

Answer 17

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)

Question 18

GLUT3

Answer 18

neuronal cells

• Km = 0.15 mM
• needs constant influx of glucose so very high affinity

Question 19

GLUT4

Answer 19

skeletal muscle

- Km = 5 mM

Question 20

draw the activity graphs for GLUT1 with GLUT2,3,4

Answer 20

slide 13?

Question 21

which GLUT is regulated by insulin

Answer 21

GLUT4 (found in skeletal muscle, fat, heart)

Question 22

how is GLUT4 regulated by insulin

Answer 22

• 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

Question 23

what is type 1 diabetes and what happens?

Answer 23

diabetes mellitus

• insulin not released
• GLUT4 stays in vesicles
• glucose in the blood not transported into the cell so blood sugar remains high

Question 24

active transport types

Answer 24

1) coupled: uphill transport of one solute coupled to downhill transport of another
2) ATP-driven: couples uphill transport to hydrolysis of ATP

Question 25

coupled transport energy source

Answer 25

electrochemical gradient of another solute

Question 26

symporters

Answer 26

coupled transporter | - transport in same direction (co-exchangers)

Question 27

antiporters

Answer 27

coupled transporter | - coupled transport in opposite direction (exchangers)

Question 28

what is the main driving force of active transport

Answer 28

Na+

Question 29

concentration gradient of Na+ across membrane

Answer 29

high Na+ on outside, low Na+ on inside

Question 30

SGLT Transporter

Answer 30

active transport protein - transport of glucose to cytosol coupled with 2 Na+ transport from the intestinal lumen (low glucose) to cytosol (high glucose)

Question 31

with the SGLT transporter, what happens when one of the solutes is at low concentration

Answer 31

the other solute will fail to bind to the transporter

Question 32

what is the equation for deltaGtotal

Answer 32

deltaGtotal = deltaGc + deltaGm

Question 33

how is the movement of Na+ ions across the PM membrane governed

Answer 33

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)

Question 34

where is SGLT found

Answer 34

- internal mucosa of small intestine | - proximal tubule of nephron

Question 35

how can the amount of active transport be increased at the physiological level

Answer 35

microvilli (such as in small intestine) to increase SA so you can have a greater number of transporters

Question 36

lactose permease LacY

Answer 36

symporter, active transport protein | - proton-driven cotransport of lactose from outside to inside

Question 37

lactose permease LacY structure

Answer 37

12 TMS helices clustered into two symmetrical loves | - substrate binding site is a central cavity

Question 38

how is the proton gradient re-established (move H+ back outside) for the lactose permease LacY

Answer 38

during fuel oxidation by a proton pump

Question 39

lactose permease LacY mechanism

Answer 39

rocking motion between two domains, powered by contribution of charged amino acids

Question 40

steps of lactose transport

Answer 40

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

Question 41

can lactose escape the cell

Answer 41

yes, because the lactose permease LacY mechanism is fully reversible. BUT, lactose generally broken down by B-galactosidase in the cytosol

Question 42

AE1

Answer 42

"Anion-exchanger" Antiporter active transporter in respiratory cells - transports out HCO3- from inside cell to outside with import of Cl-

Question 43

why is there HCO3- inside erythrocytes and why does it need to be moved

Answer 43

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

Question 44

why is the import of Cl- into erythrocyte cytoplasm beneficial

Answer 44

1) transport HCO3- out for better transport of CO2 to lung | ]2) acidifies RBC cytoplasm to allow for release of O2

Question 45

describe AE1 transporter action in the lungs

Answer 45

Cl- from inside transported out while HCO3- imported back in