what are the classes of ion channels whose ability to coduct charges varies w/ membrane potential?
1) inward rectifiers, aka Kir, or inward-rectifying K+ channels
2) voltage-gated channels
what is the structure of inward rectifiers?
tetrameter of homologous subunits; each subunit has 2 TM helical regions (M1, M2)
4 subunits arrange in assembled channel so that M2 helices (inner helices) face each other in an inverted tepee structure, tapering intracellular end
M1 helices are more lateral
center of protein is water-contianing pore lined by M2 helices, extending thru the membrane
p-loop at extracellular mouth of channel projects into pore from each subunit
what is the p-loop of the inward rectifier?
where is it, what is its function?
loop structure projecting into pore from each of the M1/M2 subunits
creates a narrowing, allowing only K+ ions to pass
where the selectivity filter is
what is the selectivity filter of the inward rectifier?
its structure, its function?
signature sequence of 5-6 residues w/in the p-loop
carbonyl groups from protein backbone project into pore here
form rings of dipoles, w/ electronegative oxygen oriented toward center
is so narrow, only single-file dehydrated K+ ions can pass through: dehydrated K+ ion interacts w/ all 4 carbonyls within a ring, thereby compensating for cost of dehydration (b/c its energetically expensive)
why does the inward rectifier's selectivity filter discriminate against Na+ ions?
because dehydrated Na+ ions are too small to be stabilized by simultaneous interactions w/ all four carbonyl groups
whereas dehydrated K+ ions are optimally sized to interact w/ all 4 carbonyl groups protruding into selectivity filter
what does the high density of protein structure around the selectivity filter indicate?
indicates the importance of precise dimensions for the proper function of the selectivity filter
creates a rigid frame, tight structure
therefore carbonyl ions making up filter must be consistently spaced
highly selective for K+
how does an inward recitifer change its conductance according to a change in membrane potential?
inward rectifier's pore has acidic residues that're accessible from intracellular side of channel
cations in the cytoplasm - Mg2+, spermine (a polyamine) - cannot get past selectivity filter, but can enter pore from cytoplasmic side, & interact w/ negative residues
when cations enter pore, pore is blocked, conductance is low
normal conditions: K+ currents are net outward; some inward movement occurs, though;
if membrane is well polarized (hyperpolarized), K+ ions move inward, & displace the blocking cation from the pore
what gives inward rectifying channels their name?
"rectifiers" = conductorsthat carry current better in 1 direction than the other
under experimental conditions, when Vm is set negative to VK, observe these channels carry large inward currents better than large outward currents
the inward movement of K+ is what unblocks the blocked channel, when it's blocked w/ a cation like spermine or Mg2+
when do inward rectifiers have high/low conductance? re: membrane being polarized/depolarized
when membrane depolarizes, inward rectifiers are blocked and so they have low conductance
when membrane is hyperpolarized, get inward K+ movement, and inward rectifier has high average conductance
how do different Kir channels differ from each other?
w/ respect to the extent that they rectify:
strong rectifiers decrease in conductance upon depolarization
weak rectifiers may not show any change in conductance upon membrane depolarization
what is the structure of voltage-gated K+ channels?
each subunit has 6 TM helices (named S1-6)
S1-4 are mostly embedded in lipid bylayer later to channel pore; S5&6 have interted tepee structure, P-loop, K+ channel sequence w/ selectivirt filter; S6 lines pore
S4 has multiple positive charged residues
which helices are the voltage sensor of voltage gated K+ channels?
how do they respond to polarization to open/close channel? explain how voltage dependence occurs
S4 = voltage sensor b/c of its positively charged residues; on intracellular side of membrane b/c attracted to uncompensated (-) charges in cytoplasm of a polarized membrane
S5/S6 create activation gate that pinches off channel to ion flow
when membrane depolarizes, S4 helices move toward outer leaflet of bilayer b/c tehre are fewer uncompensated (-) charges near inner face of membrane to attract S4 helices
when all 4 S4 helices move to outer position, activation gate is pulled open, K+ ions flow through the pore
what does it mean if a voltage-gated K+ chanel deactivates?
a voltage-gated K+ channel that stays open for as long as membrane remains depolarized, and deactivates when the membrane potential returns to its resting value & voltage sensors return toward the inner leaflet of the membrane
they're prepared to activate again in response to next depolarization
what does it mean if a voltage-gated K+ channel inactivates?
how does this work?
if K+ channel exhibits a decrease in conductance (g), aka stops conducting soon after activation, despite sustained membrane depolarization
"ball and chain":
voltage-gated K+ channels carry own intracellular blocking cations, in form of basic amino acids at N-terminus of each subunit
when activate gate opens, binding sties w/in pore for positively charged ball are exposed, enable ball to lodge in pore, block ion flow
activation gate cannot close until ball and chain vacates pore; when it does, sometimes see flicker of current before activation gate closes
what is the process of de-inactivation for a voltage-gated K+ channel?? describe how/when/where it occurs, how fast it happens
when membrane potential returns to resting value following inactivation of a channel, inactivating "particle" - ball and chain - exits channel, returns to cytoplasm
this = de-inactivation
not instantaneous; so even after membrane repolarizes, there's a delay (milliseconds) before channel activates again
what inactivates a voltage-gated Na+ or Ca2+ channel?
positive charges on intarcellular loop between III and IV
upon depolarization, loop enters pore, clogs/blocks it the voltage-gated Na+/Ca2+ channels so inactivation occurs, even though channel's still open
what is the structure of a Na+/Ca2+ channel?
a single polypeptide w/ 24 transmembrane helices arranged in 4 domains (I-IV), each w/ 6 helices
are Na+/Ca2+ channels very selective?
how do they activate?
have selectivity filters on their p-loops, but details of ion selection not well understood
activate similarly to K+ channels