8. Carrier-mediated transport

Question 1

how can valinomycin be used as a antibiotic

Answer 1

makes the membrane leaky to potassium

Question 2

valinomycin:

• what is the structure
• where does K+ bind

Answer 2

ring structure due to trimeric repeating cycle

K+ binds in the centre of the ring in a hydrophilic pocket

Question 3

what is the mechanism of transport for valinomycin

Answer 3

1. valinomycin binds to potassium
2. potassium is dehydrated so it can freely diffuse
3. leaves the membrane and become hydrated again

Question 4

what are the kinetic properties of carriers (3)

Answer 4

1. binding of substrate from outside and formation of carrier-substrate complex
2. re-orientation of the carrier-substrate complex to align the binding site to the inside
3. empty carrier re-orientates itself to align its binding site with the outside

Question 5

define Vmax

Answer 5

the maximum possible rate of the reaction at that enzyme concentration

Question 6

define Km

Answer 6

the substrate concentration at which the velocity of the reaction is HALF Vmax

Question 7

what does a low Km indicate

Answer 7

a high affinity of the enzyme for the substrate

Question 8

what does a high Km indicate

Answer 8

indicates low enzyme affinity for a substrate = a higher substrate conc. would fix this

Question 9

give an example of an inophore

Answer 9

valinomycin

Question 10

define inophore

Answer 10

a small lipid molecule that can bind a specific ion

Question 11

describe how glucose is transported across the membrane using GLUT2

Answer 11

1. glucose binding site faces outwards and binds glucose

substrate binding = extra contact = high affinity between TMS 7 & 1

2. = conformational change, orientates the binding site inwards
3. glucose dissociates
4. GLUT2 re-orientates to expose the binding site to the outside

Question 12

what are neurotransmitter sodium symporters involved in

Answer 12

removing dopamine from the synaptic cleft

Question 13

how do sodium symporters work to remove dopamine

Answer 13

sodium and chloride is required to drive transport of dopamine against its concentration gradient

removing dopamine from the synaptic cleft

Question 14

what drugs target sodium symporters - what is the effect

Answer 14

anti-depressant drugs

• increasing the amount of dopamine present in the synaptic cleft

Question 15

what is the alternate access model

Answer 15

hairpin loop is open and outward facing = binding occurs

then the inward facing binding site opens and substrates are released

BOTH GATES ARE NEVER OPEN AT THE SAME TIME

Question 16

what is the structure of neurotransmitter sodium symporters

Answer 16

two hairpin loops form gates (alternate access model)

8 TMS

Question 17

what is the difference between GLUT1 AND GLUT2

Answer 17

GLUT1 -main transporter of glucose to the brain (via the blood brain barrier)
GLUT2 - main transporter of glucose between the liver and the blood

Question 18

the disregulation of which GLUT transporter leads to diabetes

Answer 18

GLUT2

Question 19

name 2 examples of physiologically important carriers

Answer 19

sodium-coupled sugar transport in mammalian cells

sodium-calcium antiporter in cardiac cell membranes

Question 20

how does Na+-coupled sugar transport work in mammalian cells

Answer 20

uses sodium/potassium ATPase to transport glucose against its concentration gradient via GLUT2 symporter

Question 21

why is Na+Ca2+ antiporters important in cardiac muscle cells

Answer 21

maintains low cytosolic calcium - which allows diastole following contraction

Question 22

how do Na+Ca2+ antiporters work in cardiac muscle cells

Answer 22

Na+/K+ ATPase which maintains sodium gradient - enabling calcium efflux via antiporter

Question 23

what is the benefit of coupling sodium transport to glucose transport

Answer 23

free energy produced from moving NA+ down its conc gradient, permits the transporter to move glucose against its conc gradient (energetically costly)

Question 24

how does sodium-calcium antiporters work to remove calcium from the heart muscle following systole

Answer 24

Na/K+ pump maintains Na+ gradient

antiporter brings 3 Na+ into the cytosol and removes 1 calcium molecule per rotation - this uses the free energy favouring the efflux of calcium