Membrane lecture 5

Question 1

Overview

Answer 1

• Evolution of alternating access model = transport protein consists of entity within membrane that either faces outward + binds substrate on outer face of membrane or undergoes conf change

Question 2

ATP binding cassette transporter

Answer 2

• 2 Tm region for transport, 2 NBD that hydrolyses ATP → free E to drive conf change
• 2 NBD engage in symmetric dimer w/ 2 ATP molecules sandwiched in the dimer
• e.g. type I ABC importers responsible for nutrient uptake, ATP hydrolysis drives conf change
• Export mechanism (inward facing, bind ATP, flips NBD away → Δ access of bs from inward to outward, ADP dissoc
• Import (closed → occluded, conformational Δ that moves accessibility of B-12 bound site to inward facing, catalytic region open to ATP, another Δ that releases B-12 → closed ATP free → outward open

Question 3

Secondary transporter

1. MFS

Answer 3

• Includes uni- sym- and antiporters
• Transport small solutes
• Lactose permeate = common structure of 12TM helices btw 2 domains
• Sugar switches to inward conf
• 2 domains rock back and forward
• Mechanism (FucP, proton binds Asp, sugar binds outward open → proton jumps + triggers conf change, sugar disc → reverse back to open)
• Alternating access mechanism = 2 major conf inward + outward facing, NTD + CTD change position relatively

Question 4

Secondary transporter

2. LeuT superfamily

Answer 4

• Sodium symporters
• Internal symmetry, 2x5TM + surrounding helices that x form part of mechanism
• bs for Na+ + solute = at interface btw 2 domains
• OUTWARD = water penetrates from outside, gate shut inside
• INWARD = opposite
• OCCLUDED = pockets can be occupied by ions/H20 but closed on both sides
• DAT

Question 5

Elevator-mechanism transporter

Answer 5

• Scaffold domain + transport domain
• Open state (gate open, occluded state up, gate moves + solute leaves
• Alternating access mechanism