Ion Channels and Co-transporters Flashcards
Give an overview of ion channels?
Found in bacteria, archaea and eukaryotes
Ion channels enable facilitated transport of ions
Some are very selective (potassium, sodium, calcium) and some are only cation selective
Non-gated, voltage-gated or ligand-gated (and also mechano, temperature or light gated)
Describe K+ channels?
Potassium channels are tetramers
Most K+ channels have 6 TM helices, but an important bacterial homologue KcsA has only 2
TM helices 5 and 6 (S5 and S6) from the channel
In voltage-gated K+ channels, helices 1-4 form the Voltage Sensing Domain (VSD)
What is specific K+ ion channel?
KcsA was first K+ channel for which a structure was solved and showed how potassium selectivity is achieved
A shorter tilted helix (P-helix) fills the top of the ‘cone’ structure
The P-helix is part of the P-segment containing the selectivity filter
Describe the structure of the KcsA?
The ‘bottom’ half of the cavity (or vestibule) is hydrated (contains water)
In the crystal structure 1 hydrated potassium was observed in the ‘bottom’ cavity or vestibule
Below the vestibule (cytoplasmic side) is the activation gate - which opens and closes in voltage-gate Kv channels
The ‘top’ of the channels, surrounded by a stretch of amino acids held in place by the shorter P-segment provides the potassium selectivity -> Selectivity filter
This stretch of amino acids is conserved in potassium channels
In potassium channels, the carbonyl oxygens line the channel = perfect for the K+ ions
Describe the KcsA selectivity?
In the crystal structure, 4 dehydrated potassium ions are observed in the ‘top’
It is believe that not all 4 potassium sites are occupied at the same time
The radius of the ion and ‘provision’ of 8 ligands provides selectivity of the channel
Electrostatic repulsion of neighbouring potassium ions weakens the bonding
Describe Na2+ and Ca2+ channels?
Eukaryotic Na+ and Ca2+ channels have single-chain pseudo-tetrameric structure
This structure mirrors the tetrameric K+ channels
They share similarities - selectivity can be interchanged through simple mutation in the filter
There isn’t an easy way to distinguish between CaV and NaV from the primary sequence
Ion selectivity dictated by the amino acid sidechains
Describe the structure of the Na2+ channels?
Sodium selectivity is achieved using different selectivity filters, with a similar pore architecture
The first structure of a Na2+ channel was solved for NaVAb, (voltage-gated Na2+ channel from a proteobacterium)
Besides the P-helix, it contains a P2 half helix that is not present in KV channels
A short loop between the P and P2 helix is responsible for Sodium and Calcium selectivity
Give a comparison of the selectivity filters of Na2+ and K+?
The channel lined by the selectivity filter of NaV is larger than KV - despite the fact that Na+ is smaller than K+
NaV transports Na+ in partly hydrated form and the hydration might change during transport
It may only get dehydrated with aspartate/glutamate
Besides the backbone oxygens, the Na selectivity filter contains side chains of Glu
The filter is less conserved in NaV (compared to KV)
Describe Na2+ voltage gated channels?
S1-S4 form the voltage-sensing domain (VSD)
Helix 4 contains positive residues (typically/canonically four arginine’s) that respond to membrane potentials in a ‘sliding helix’ model
This is due to electrostatic repulsions of charges
Movement of the VSD and S4, is transferred to ion channel (S5 and S6) via the S4-S5 helix
Forces by the VSD result in the ‘kinking’ of S6, opening the channels (S6 is loosely packed)
Opening and closing occurs at the bottom of the protein (below the vestibule), known as the activating gate or gating region
A glycine residue in the TM (S6) helix acts a hinge
What feature do NaV and KV channels have?
NaV and KV channels have a ‘second’ gate - which are spontaneously inactivated after a few ms
This mechanism is due to the channel inactivating segment and inactivation occurs according to a “ball-and-chain” mechanism
- Initial depolarisation, movement of voltage-sensing a helices, opening of channel
- Movement of channel-inactivating segment, inactivation of channel
Channel inactivating segment in KV channels located at the N-terminus
Channel inactivating segment in NaV located in on of the loops
Give an overview of ligand-gated ion channels?
There are many different ligand-gated channels (also mechanosensors, light-activated)
Example - transient receptor potential (TRP) channels
Describe transcient receptor potential (TRP) channels?
There are 7 TRP channel families, TRPC, TRPV, TRPM, TRPA, TRPN, TRPML, TRPP
Cryo-EM showed similar structural organisation with KV and NaV
Unlike voltage-gated channels - many Trp channels can sense multiple stimuli (within one channel), including physical and chemical stimuli
The S1-S4 domain does not ‘move’, but opening /closing is still gated by S6
Almost all Trp channels are cation selective and transport both monovalent and divalent cations (e.g. Na+ and Mg2+)
Describe the example TRPV1 channel?
Primary function of TRPV1 (and V2, V3 and V4) is as a nociceptor (including heat) to stimulate immune and pain response
TRPV1 is sensitised by other inflammatory components, leading to thermal hyperalgesia
TRPV1 responds to Noxious temperatures (> 43o C) Acidic pH Arachidonic acid metabolites and endocannabinoids (inflammation mediators) Capsaicin (chili peppers)
Describe the example TRPM8 channel?
Primary function of TRPM8 is as a cold detector (details are debated)
TRPM8 responds to
Gentle cooling (<23o C)
Menthol, icilin, eucalyptol, linalool, geraniol, hydroxycitronellal
Low doses of menthol lower the threshold for cold detection
What are the definitions of uniporters, symporters and anitporters?
Energy for active transport is acquired from transmembrane ion gradients
Uniport - one moving, passive-mediated diffusion or facilitated transport
Symport - two moving in the same direction, secondary active transport
Antiport - two moving in opposite directions, secondary active transport
