L8: Carriers

Question 1

Valinomycin structure

Answer 1

• Ring w/ trimeric repeat cycle of D- and L- valine, lactic acid and hydroxyvaleric acid
• K+ bound to centre (hydrophilic interior w/ carbonyl ligands coordinating M+, lipophilic exterior; diffuses free in membrane)
• High selectivity for K+ over Na+ (lower free energy of dehydration)

Question 2

Valinomycin mechanism, drawback as a model

Answer 2

• Naked K+ ion binds to pocket
• Moves across membrane by diffusion (random)
• Electrochemical gradient outside results in K+ released at other side, rehydrates
• Carrier diffuses back
• Doesn’t provide basis for molecular mechanistic model (just kinetic), its too small (gene cloning reveals that carriers are usually large proteins)

Question 3

The two mechanism types for K+ uptake

Answer 3

1. Operates at low [K+]ext
   Km: 10 to 40 microM
   High affinity uptake/low capactiy
2. Operates at greater [K+]ext
   Km: mM conc. rang
   Low affinity uptake/high capacity

Question 4

Proposed model of uniport transport by GLUT1

Answer 4

• Gluc binding site is facing outside, poised to bind gluc
• Undergoes conformational change after binding gluc; BS faces inward
• Gluc released, GLUT1 re-orientates

Question 5

Neurotransmitter sodium symporters (NSSs) - background

Answer 5

e.g. Dopamine..
- major route for neurotransmitter clearance
- Requires Na+ and Cl- to drive uphill transport
- Dysregulation of NSS function associated w/ disorders inc. depression, epilepsy, Parkinson’s
- Inhibition of neurotransmitter uptake is target for therapy
- Primary targets for psychostimulants and amphetamines

Question 6

NSSs structure + function

Answer 6

• 2 hairpin loops form ‘gates’ which restrict access to substrate binding sites
• ‘Alternating access model’ w/ rocking motion
• Neurotransmitter binding is Na+ dependent (Na+ binding increases affinity of BS for NT)
• Whether NT is bound or released depends on Na+ conc.

Question 7

Facilitators examples

Answer 7

GLUT2:
- Main transporter of glucose btwn liver and blood, and glucose absorbance from gut
- Also forms part of ‘glucose sensor’ in pancreatic beta cells
- Insulin recruits more GLUT2 protein to the PM
- Dysregulation -> diabetes
GLUT1:
- Main transporter of glucose to the brain
- GLUT1 deficiency syndrome
-> epilepsy
- Overexpression of GLUT1
-> prognostic indicator for cancer

Question 8

Yeast glucose transport

Answer 8

• Only has facilitated glucose transport (evolved in glucose-rich environment)
• Normally, fungi are evolved in nutrient-depleted environment, uptake of glucose energised by coupling to H+ gradient

Question 9

Na+-Ca2+ antiporter in cardiac muscle PM

Answer 9

• Maintains low cytosolic Ca2+ (3Na+:1Ca2+)
• Na+/K+ ATPase maintains a Na+ gradient enabling Ca2+ efflux via the antiporter

Question 10

Secondary active transporters (examples in brief)

Answer 10

• Na+ coupled sugar transport in mammalian cells (glucose uptake from intestinal lumen and into blood)
  -Na+-Ca2+ antiporter
• H+ pumping proteins (integrated system of transporters
• Nutrient transport in plants (coupled to H+ gradient)