6. Ion Channels Flashcards
what are the three broad groups of ion channel
- voltage-gated
- neurotransmitter-gated
- second messenger-gated
what does gating refer to
how they open and close
- generally in the presence of a specific molecule that activates the channel
describe ion movement during depolarisation
- Na+ channels open = Na+ influx - membrane becomes more +
2. K+ channels open = K+ efflux - membrane voltage plummets (more -)
what is another name for K+ channels involved in depolarisation
delayed rectifiers
what does the term delayed rectifiers mean
there is a delay which fits with sodium channel activation and deactivation
where are neurotransmitter-gated channels typically found
in the nervous system
what happens when neurotransmitters bind to a neurotransmitter-gated channel?
can have either an excitatory or inhibitory effect
- e.g. result in action potential or inhibit it
name 2 cations
acetylcholine, glutamate
what happens when a cation binds to a neurotransmitter-gated channel?
results in action potential
what happens when a anion binds to a neurotransmitter-gated channel?
causes an inhibitory effect
name 2 anions
GABA
glycine
name 2 examples of second- messengers that can gate channels
cAMP
cGMP
give an example of second-messenger gated channels in the human body
in the retina: rod cells
- rod cells absorb photon activating a g-protein
- g-protein binds GTP activating phosphodiesterase
- phosphodiesterase works by hydrolysing cGMP into non-cyclin GMP
- ion channels open in the presence of cGMP, close in its absence
= hyperpolarisation as sodium can no longer enter
what is the structure of voltage-gated channels
6 transmembrane domains 1 por domain voltage sensor (+ charged AAs) N terminus forms ball and chain arrangement
what makes the transmembrane spanning domain lipid stable
hydrophobic side chains
what do the ions move through in voltage gated channels
the pore - also called the selectivity filter
what domain in voltage gated channels is the voltage sensor
S4 domain
how does the voltage sensor detect voltage change
have charged side chains that can detect voltage change in the membrane
what is the purpose of the ball and chain
block the pore causing inactivation
what is the structure of the voltage gated channel during deactivation
here the chain is not on but the pore is closed
what is the structure of potassium channels
built out of 4 identical transmembrane subunits symmetrically arranged around a central pore
what does each potassium transmembrane subunit contain
two membrane spanning alpha helices
what is a tetramer
a structure with 4 subunits
what do the potassium channel helices form, what does this generate
an inverted cone
- generates a water filled cavity called the vestibule
describe how potassium moves through the potassium channel
K+ enters the selectivity filter and loses its 8 waters
this allows it to bind to carbonyl oxygen atoms
the pore only allows dehydrated ions to fit through
potassium binds tightly to carbonyl oxygens, the ion electrostatically repulses the one in front pushing it further down to the next carbonyl oxygen binding site
why are potassium channels selective for potassium over sodium
sodium ions bind very strongly to water molecules (= hydration shells)
this makes it too big to fit through the pore
describe sodium channel structure (3)
4 domains - each connected (monomer structure)
larger than K+
amino acid motif swings into the pore causing inactivation
how does the sodium channel transport ions
transports hydrated ions
pores are wide enough to accommodate sodium and its tightly bound water
give an example of a plant that uses voltage gated channels to close leafs
mimosa plant
where is the voltage sensor located
in S4
what happens when a voltage is sensed
positive charges on S4 react and move stretching the protein open
- pulling S5 and S6 away from interacting on other subunits
