Tema 4

Question 1

general properties of ionic channels

Answer 1

as they are proteins they are made of amino acids
the conducting pore allows polar molecules to go through the membrane (non-polar)

they can be highly selective- thanks to oxygen of the polarised carboxyl group
less selective are wider and molecules pass binded to water (IONS NEED H20)

Question 2

types of ionic channels

Answer 2

GATED: voltage, ligand, stress

NON-GATED (leak channels)
*Na+ and K+ channels (more for K+)

Question 3

models of gated channels

Answer 3

they can be:
- DISCRETE
- GENERALIZED
- BLOCKING PARTICLE

Question 4

parts of an ionic channel (structure)

Answer 4

channels domains (normally 4)
outer vestibule
selectivity filter
diameter of the filter
phosphoralisation site

Question 5

K+ leak channels

Answer 5

shape of a cone; extracellular side is wider
made by 4 identical subunits and each has:
* 2 transmembrane helix domains
* 1 shorter helix that forms the pore

In the inner ad outer membrane there are -ve charges amino acids
It binds to a H20 molecule (as its an ion)
In the selectivity filter the K ion is forced to separate form the water molecuole

Question 6

process functioning of ionic channel

Answer 6

In the inner ad outer membrane there are -ve charged amino acids
The ion binds to a H20 molecule, as the inner surface of the channel is full of water
The shorter helix provides satibilsation // the -ve charge of the carboxyl group of the a.a allows the ion to move through the channel

In the selectivity filter the K ion is forced to separate form the water molecuole
The oxygen from the carboxyl group replaces the water

Question 7

VOLTAGE OPERATED CHANNELS+STATES

Answer 7

They open, when there are changes in the membrane potential (depolarisation)
They can be highly selective to an ion (not always): Na+, K+, Ca2+

3 states:
- CLOSED: They can open, but need a stimulus.
Activation gate is closed + inactivation gate is open

-OPEN: after the stimulus, the activation gate opens and the ion goes through

-INACTIVE: inactivation gate activates and the channel can’t active EVEN if there is a new stimulus

Question 8

Na+, K+, Ca2+ VOLTAGE GATED CHANNEL

Answer 8

1 single polypeptide with 4 homologous, transmembrane domains.
Each domain has 6 alpha helix transmembrane proteins (S1-S6)
The domains are bound to short chains with non-homologous residues

*S4 acts as a voltage sensor, as it has a +ve charged a.a
*S5 + S6 are relates with the selectivity filter
*H5 forms the pore of the channel
*N segment that inactivates the channel

Na, Ca and K CHANNELS ARE VERY SIMILAR IN STRUCTURE

Question 9

LIGAND OPERATED CHANNELS

Answer 9

They open with the presence of specific chemical substances in the binding site, such as acetylcholine - NEUROTRANSMITTERS

There are pentamers and tetramers

Question 10

MECHANICALLY/ STRESS OPERATED CHANNELS

Answer 10

They respond to the distortion of the membrane
In sensorial receptors (touch, vibration, pressure…)

The vibration causes the stereo-cilia to incline, stretching the binding filaments which open the ionic channels