Transporters & Ion Channels 3 Flashcards
Sodium ion channel selectivity
Achieved using different selectivity filters:
- Oxygen backbone
- Side chains of Glu
- Beside P-helix contains a P2 half helix
- Short loop between P and P2 helices responsible for selectivity (same in Ca channels)
Voltage gated Sodium Channels (Nav) transports Na in a partly hydrated form
Calcium ion channel selectivity
Similar to Sodium
- can be interchanged by simple mutation
- Ca2+ is also transported in hydrated form
Voltage gated ion channels
structures of Kv1.2 and NavAb provide insight into voltage gated sodium ion channels
Voltage sensing domain (S1-S4) detect depolarisation/change in electric field
- arginines move in sliding mechanism promoting the movement of S5 & S6 via the S4-S5 helix
The Voltage Sensing Domain (VSD) causing channel to open
S1 to S4
- S4 contains positive residues (4x arginines) which respond to membrane potentials in a sliding helix model
VSD/S4 movement transferred to ion channel (S5-S6) via S4-S5 helix
- Causes kinking of S6 to open channel
- Opening/closing occurs at bottom half of protein (at the activating gate)
- Glycine residue in S6 acts as a hinge
Inactivating voltage gated Na ion channels
A second gate spontaneously inactivates after a few ms of opening
Ball and Chain inactivating mechanism
- Uses the channel inactivating segment to block the otherwise open Na+ channel
Mechanoreceptors: opening ion channels from mechanical sensing
When closed, channel is in part of membrane that’s dipped
~~~~~~~\___x___/~~~~~~~
Upon mechanical stimulation, membrane stretches and the dip reduces allowing channel inside to open
~~~~¬—–x—–¬~~~~
Pain receptors
Use Nociceptors (noxious stimuli detected) such as TrpV1
- Similar structure/organisation to voltage gated ion channels, despite lack of sequence homology
- S1-S4 don’t move, S6 controls opening/closing
Primary function to stimulate immune and pain response:
- Sensitized by other inflammatory components, leading to thermal hyperalgesia
- Responds to: Noxious temperatures (>43C), Acidic pH, Arachidonic acid metabolites and endocannabinoids (inflammatory mediators) and Capsaicin (chili peppers)
Opposite receptor to TrpV1-heat detector (detects cold)
TrpM8 detects cold, responds to:
- Gentle cooling (<23C)
- Menthol, icilin, linalool, geraniol, hydroxycitronella
- Low doses of menthol reduce threshold for cold detection
Co-transport by Symporters and Antiporters
Rely on secondary active transport (energy for active transport is acquired from transmembrane ion gradients of 1 of the 2 molecules)
- Cotransport = transporting 2 molecules simultaneously
- Symport = cotransport in one direction
- Antiport = cotransport with both molecules going in opposite directions
Highly substrate specific (substrate binding pocket analogous to enzymes)
Symport (SGLT1) mechanism
Uses energy of sodium and proton gradients
- Transports up to 3 sodium ions or protons
- Transports a wide range of (or multiple) substrates including: organic metabolites, nutrients, toxins (export), cations or anions (e.g. Galactose)
Inward facing conformation = Na+ release (down gradient)
- Na release changes Na binding pocket and galactose binding pocket
- Galactose (glucose) binding affinity reduces and galactose is released
Bacterial Symporter homologue: Leucine Symporters (LeuT)
Structure in occluded state, sodium can’t bind without Leucine - ensuring coupled transport
- Sodium bound in dehydrated form
- One of the sodiums coordinates Leucine substrate
vSGLT
Sodium/glucose symporters from Vibrio parahaemolytics
SGLT belongs to the sodium solute symporter (SSS) family and cotransports 2 sodium with 1 D-glucose
- Shares LeuT fold (alternating access mechanism)
Releases Na+ at inward-facing conformation due to lower intracellular Na concentration (down gradient)
- Na release changes Na and the Galactose (N64) binding pockets
- Altered binding pocket conformation has lower affinity causing galactose release
Multi-drug resistance from bacterial efflux pumps
causing antibiotic resistance?
5 families of efflux:
1) ATP binding cassette (ABC) superfamily
2) Major Facilitator Superfamily (MFS)
3) Multidrug and Toxic-compound Extrusion Family (MATE)
4) Small Multidrug Resistance (SMR) family
5) Resistance Nodulation Division (RND) family
Glucose uptake in Small Intestine
Relies on 4 transporters:
1) Uniporter (GLUT2)
2) Secondary Active Symporter (SGLT1)
3) Primary active ATPase
4) Potassium channel
Glucose levels are high in cytoplasm due to SGLT1 cotransporter
- Uniporter GLUT22 therefore needed to transport glucose into bloodstream
Lowering stomach pH
Action by P-type K+/H+ ATPase
- pH inside parietal cells (specialised epithelial cells) maintained by Cl-/HCO3- antiporter combined with passive CO2 diffusion across membrane
- Cl- & K+ homeostasis maintained by non-gated ion channels
- Net transfer of HCl into stomach (H+ and Cl- released into stomach)
