Membrane lecture 4

Question 1

Overview

Answer 1

• Channel + pore = small once Δ during transport
• Transporter = TM conf change
• Pump = catalytic events drive Δ

Question 2

Aquaporins

Answer 2

• Water selective, high H20 permeability
• Structure = tetramer, each subunit has 6TM connected w/ 5 loops
• 3 helices where 3rd = re-entrant, then 6,5,4 where 6 = re-entrant too
• Loop B + E have conserved NPA motif needed to maintain proton gradient
• ar/R site = selectivity filter
• NPA orient water
• x allow protons through, main barrier = NPA, 2nd = ar/R
• Large solutes excluded
• Experiment
• Glpf = glycerol selective (↑ glycerol permeability, ↓ H20 than Aqp)

Question 3

Water conduction

Answer 3

• hAqp4 1.8A structure
• 2 1/2 helices w/ NPA motif
• Breaks H bond chain of H20 in centre, prevents H+ conduction against column of water
• Can adopt alternative conformation + break H bonded chain
• Glpf = also tetramer, central constriction pore
• Polar region of pore interacts w/ OH of glycerol, hydrophobic interacts w/ hydrophobic

Question 4

Transporter

Answer 4

• Uniporter, symporter or antiporter
• P type ATPase (gradient of key cation like Na/K+,
• Conserved DKTGTLT + TGES motives
• 10 TM helices, region in cytoplasmic side responsible for ATPase catalytic machinery
• Ca2a+ ATPase
• ATP bound to catalytic site opens TM region to Ca2+ from inside cell