Ion channel structure Flashcards
WHat do ion channels look like?
What do they differ by?
- Large polypeptides with 2 or more membrane-spaning subunits (all ion channels are dimer or tetramers), surrounding a pore (aqueous so ions can pass), homomeric or heteromeric
- Differ by ion selectivity + gating mechanism
Potassium channels (4)
Who + structure + gated by + mechanism
- 1998: Roderick MacKinnon first solved the structure of an ion channel with KcsA from Streptomyces Lividans (bacteria in soil)
- Homotetramer (made of 4 of the same subunits)
- Not VG but PH gated
- When it opens, a bunch of ions will flow into the inner chamber but selectivity filter make sure only K+ goes through.
KcsA (3)
selectivity filter + structure/subunits + p-region
- Selectivity filter there are 4 potential binding sites that can bind potassium but only 2 happen at a time due to electrostatic repulsion. You cannot cram that many + charge in a small space.
- Made of 2 transmembrane subunits and a loop helix loop on the extracellular region known as the P-region or p-loop. You need 4 of those for a channel. So 8 transmembrane domain in 1 KCSA channel.
- P region has the pore helix that contributes to shape of pore.
KCSA vs Shaker
KCSA is prokaryotes and shaker is eukaryotic. Shaker is VG but highly resembles each other at the selectivity filter.
Ions in solution are surrounded by ——–.
Selectivity is not based solely on ion diameter explain this:
- waters of hydration after the ionic bond breaks
- The K+ crystal radius is 0.133nm and Na+ is 0.095nm. However, sodium is smaller so the positive charges is more concentrated in a tighter space and holds onto water tighter. This attracts more H2O and acts as a bigger ion then K+ and has a bigger H2O shell and less likely to lose H2O.
Selectivity filter (3)
What + what determines selectivity + Need to be what to fit thru?
- Narrow regions in ion channel pores that act as molecular sieves
- Pore size and chemical interactions with residues lining the pore determine selectivity
- Need to be partially or compltely dehydrated to fit through
Sodium selectivity pore:
Na+ is partially dehydrated to fit through. K+ is too big to fit through.
K+ selectivity sequence (2)
What + highly….
- Thr- X- Gly- Tyr- Gly
- Highly conserved from prokaryotes and eukaryotes
K+ Selectivity how does it work:
Potassium selectivity in K⁺ channels is achieved through a narrow selectivity filter lined with carbonyl oxygen atoms from the protein backbone. These oxygen atoms create a cage-like coordination environment that mimics the hydration shell of K⁺, allowing the ion to be stabilized after dehydration (O is electronegative functional group and has - charge to attaract K+). The filter consists of four subunits arranged in a square, with oxygen atoms positioned at each vertex to precisely fit K⁺ but not smaller ions like Na⁺, which cannot form optimal interactions. This precise geometric and electrostatic arrangement ensures that only K⁺ ions can pass through efficiently.
What propels K+ through the SF and how are they arranged in the SF?
Propelling through is concentration gradient and repulsion between K+ ions.
K+ can be at either 4 site but never seen irl becayse the + charge repel eachother so water molecule is inbetween.
Selectivity filter is at the —- not —–.
- at the exit not entrance (extracellular side)
K+ must be —- to fit through the filter before being ——
- dehydrated
- rehydrated
Vestibule
Inner chamber of channel
Most VG channels form —– shape so transmembrane segment cross at the bottom when close and when activation gate open they ——.
- Inverted cone
- tilt slightly
Potassium channel selectivity filters are remakably percise and 10K times more selective for K+ then Na+. Why?
- All because of the structure arrangement of the carbonyl protein backbone at the filter. Carbonyl groups lining the selectivity filter form favourable electrostatic interactions with K+ but are too widely spaced to interact effectively with Na+. Na+ thus favour their shell of water and do not pass through filter due to being too big.
There are 3 major family of K+ channel:
- 2TM K Channels
- 4TM K channels
- 6TM K Channels
The 2TM K+ channels (5)
subunits + aka/bc +structure subunits that make it up + pore forming region contains + block
- 2 transmembrane helices per subunit(simplest)
- Inward rectifiers: Pass current more easily in the inward direction then out (pass K+ into cell)
- Homotetramers (4 of the same subunit making 8 transmembrane subunit for 1 2TM channel)
- Pore forming region contains pore helix and K+ selectivity sequence
- There is a voltage dependent block of channel pore by Mg2+ and polyamines when membrane is depolarized. If you hyperpolarize the cell, you pull out the + cation and relieve the block.
2TM channels conduct more current when (2):
potential/range +how
When the membrane is hyperpolarized (more negative inside), the channel opens, allowing K⁺ to flow. At membrane potentials more positive than -40 mV, the channel remains shut. These channels only conduct when the voltage steps in the negative direction (hyperpolarizing), meaning the electrical gradient can overshoot the chemical gradient, further hyperpolarizing the cell. Since the inside of the cell is more negative, it pulls K⁺ ions inward against their usual outward concentration gradient.
4TM K+ channel (4)
AKA + subunit + structure/subunits that make up it + P-region
- AKA leak channels which are always open K2P
- 4 transmembrane domains and 2 pore regions per subunit
- Homo or heterodimers (2 subunit form a complete channel and so a 4TM channel has 8 transmembrane segment)
- 2 extracelullar loop that makes up P-region
4TM channels current + IV curve (2):
- Pass currents in both direction because they are open at all voltage
- The IV curve has an outward current (K+ is leaving cell) at voltages above -90mV aka Eq potential of K+. At lower voltage then that, K+ influx.
Membrane is leaky to K+ means interior negative as K+ take positive away
6TM K+ channels (7)
AKA + subunit + structure/subunits that make up it + # of channel/subfamilies in humans + T1 + Beta + voltage sensor
- AKA Kv channels/ Voltage gated
- 6 transmembrane segments labelled S1-6. Hairpin between S5 and S6 known as H5 or P (selectivity filter)
- Homo or heterotetramers (4 subunit = 24 transmembrane in VG-K+ channels)
- 40 channels, 12 subfamilies in humans
- The T1 (tetramerization domain): Match up subunit to make homo/heterotetramers
- B: Not all K+ has B subunit (modify function)
- S4 is the voltage sensor and active at depolarization voltages
6TM K⁺ channels have six transmembrane (TM) segments per subunit. These channels typically include:
volatge sensing domain + pore forming
S1-S4: Form the voltage-sensing domain (VSD), with S4 containing positively charged residues that detect changes in membrane voltage.
S5-S6: Form the pore domain, with the selectivity filter between them allowing K⁺ selectivity.
K+ VG channel types (2):
- A-types
- Delayed rectifiers (majority): Repolarizes AP
6TM K+ channels are voltage sensitive and activated at ———. They regulate ———-. There are 2 type, explain what they do:
- depolarized voltages (+ to -50mv) and close at negative potentials.
- shape and firing pattern of AP
- Fast KA vs classical delayed (KVdr) channels. KVdr is important to AP repolarization (recovery phase) and take longer to open, current is sustanined overtime. KA is a fast current rise and shuts off quick.
