Membrane transport

Question 1

GLUT1

Answer 1

• 12 transmembrane Ahelices
• moves glucose with its gradient. Either direction
  1. Outward open conf binds glucose. Dependant on Km and concentration
  2. Ligand-bound occluded
  3. Inward open release of glucose
  4. Ligand-free occluded

Question 2

ATP powered pumps

Answer 2

• P class: H+ and salt ion. Ion gradients. Phosphoprotein intermediate
• V/F classes: H+ ions
• ABC superfamily: toxins, drugs, phospholipids, proteins

Question 3

Ca+2 ATPase in Sarco Reticulum

Answer 3

• P pump
• establish low resting Ca+2 (10-7) in cytosol of most cells
• muscle relaxation = Ca+2 from cytosol –> SR
• 2 conformations
  E1: cytosolic face
  E2: exoplasmic (ER lumen) face
• 1 ATP for 2 Ca+2

Question 4

Ca+2 ATPase steps

Answer 4

1. 2Ca+2 and 1 ATP bind. E1
2. Hydrolysis + phosphorylation= conf change. E2. ADP release
3. Ca released into lumen
4. Dephosphorylation. Pi released. E1

Question 5

Na/K pump

Answer 5

• 3 Na out and 2 K in per ATP
• each ion moves against its gradient
• E1 conf: dephosp opens to cytosolic face (Na associates, K dissociates)
• E2: phos opens to exoplasmic face (Na dissociates, K associates)

Question 6

Na/K steps

Answer 6

1. Na and ATP bind. E1
2. Phos of aspartate, ADP released. E2
3. Na release and K binds. E2
4. Dephos. E1
5. K release. E1

Question 7

Multi-Drug Resistant Transporter (ABCB1)

Answer 7

• ABC superfamily. drugs,toxins, hydrophobic/philic
• 2 Transmembrane (T) and cytosolic ATP-binding domains (A)
• ATP hydrolysis= solute movement
• Ligand-binding cavity alternately exposed to exoplasmic/cytoplasmic faces
• high expression in liver

Question 8

Non-Gated Ion channels and Resting potential

Answer 8

a) impermeable to Cl, K, Na
- no electric potential
- osmotic potential is the same on either side of cell
- composition is different
b) only permeable to K
- potential = -59mV. Cytosolic face is negative (inside)
- K moves from cytosol to exoplasmic

Question 9

Animal cell resting potential

Answer 9

• generated by Na/K ATPase and non-gated K channels
• -70mV
• flutuates in axons/neurons. Dopolarization

Question 10

K+ channels

Answer 10

• tetramers of 4 identical subunits
• selectivity filter due to backbone of carbonyl oxygens of amino acids
• K fits perfectly, bound to oxygens
• Na imperfect fit, higher in energy. thermodynamically more stable for it to stay hydrated outside of channel
• Na/K are hydrated in solution, K dehydrated in channel pore

Question 11

2-Na/1-glucose symporter

Answer 11

• cotransporter: both symporter and antiporter
• electrochemical gradient
• 2 Na and 1 glucose outside –> inside
• against glucose gradient
• with Na gradient

Question 12

2-Na/1-glucose symporter steps

Answer 12

1. 2 Na and glu bind outward conf
2. conf change. occluded substrates
3. conf change, inward facing
4. dissociation of substrates
5. revert to outward facing