membrane transport Flashcards
what does a signal tranduction cacade lead to?
amplification of orignal signal
briefly describe active transport
Process where you can pump ions across a membrane using ATP. Can use a variety of transporters+channels
- co transporter
- ligand gated
- voltage gated
- mechanically gated
how are Na2+, Ca2+, K+ channels structurally similar?
have 6 transmembrane domain helixes
all have S4 voltage sensor subunit made of many +amino acids
around 70kD
what molecule is calcium (signalling) regulated by?
calmodulin!
which exposes the hydrophobic residues of calcium =changes conformation so signal cascade can continue
active site of receptors become active and can target effector protein (PKA)
where is calcium stored?
biggest store is ER
also cytosolic, golgi, secretory vesicles
mitch temporarily
describe structure of calmodulin
has 2 domains joined by a flexible linker and each domain has 2 EF hands that can bind to one calcium molecule each
EF hands found on many binding proteins
Describe the ‘‘off mechanisms’’ in calcium signalling
SERCA pump
NCX changer
Ca buffers and chaperone like calsequestrin
problems in these can lead to pathologies!
What does the SERCA pump do?
pumps the Ca2+ from cytosol to the ER/SR lumen using ATP
what does calsequestrin do?
buffer/chelatator protein within SR and binds to calcium
helps with the SERCA pump so it doesn’t have to pump against a v high concentration gradient
How can SERCA be regulated?
> both +/- control
phospholambin inhibit SERCA
Under b-adrenergic stimulation it gets phosphorylated so this is reversed
calsequestrin activates
how does the sodium-calcium exchanger work? (NCX)
3 sodium IN CELL, 1 calcium out of cell so helps with calcium efflux
this exchanger is electrogenic and creates a current/ membrane potential
why is NCX better than ATPases?
NCX works quicker than ATPases and at higher concentrations
what is the normal physiological range cytosolic and extracellular calcium
extracellular - 1.4mMol
intracellular 100nMol at rest
so calciuim has one of the largest ioninic gradients
Cells use transient changes in gradient to drive cellular processes
how can calcium be put into its stores?
SERCA pumps are located on the membrane of endo/sarcoplasmic reticulum to store calcium
what can too much mitch uptake of calcium lead to?
too much ROS generation and eventual apoptosis
and sig reduction in energy production and eventual necrosis
How do calcium spikes stay local to PAC?
secretory granules
are surrounded by mitch which have mitochondrial uniporters to uptake calcium/buffering a global rise of Ca2+
Ca+ used for energy production for secretion
how does calcium maintain its low intracellular/cytosolic conc? (100nmol)
- using pumps and exchangers
- sequester/store calcium in SR, ER, mitch stores
- chelating calcium, having it bound to proteins like calbindin sequestrin
which receptors are present on the ER?
Ip3 receptors and ryanodine receptors which mediate calcium release
how do non excitable cells become activated?
e.g Pancreatic Acinar Cells
ey do not have voltage gated ion channels so cannot generate an AP
rely on a second messenger allowing
calcium release from intracellular stores which can then activate processes like granular secretion as seen in PAC
how do excitable cells become activated?
through voltage gated calcium channels where membrane potential changes lead to depolarisation and conformational change of RyR and release of calcium from store
pulses of calcium
how can we replenish the ER store ?
via store operated calcium entry
use the STIM1 ER membrane proteins which respond to low calcium in ER and interact with plasma membrane to TO ACTIVATE ORAI proteins and activate the CRAC channels
calcium entry then moves through the ER pump to remain in ER
to terminate SOCE, de-oligomerisation of STIM1 occurs when cytosolic Ca2+ is increased
generally speaking how would elevated calcium levels result in pathology in AP?
high cytosolic calcium can detabilize zymogen granules and expand and rupture
this releases digestive enzymes like trypsin and result in cell death -> may spread to neighbouring cells = inflammation
what could be some causes of elevated calcium channels
bile acids
alcohol metabolites
drugs like aspariganases
Aspariganases used to treat leukemia
what is the issue with CRAC channels in AP?
CRAC channels on plasma membrane open to replenish the ER store
but this can EXACERBATE THE Problem and further contribute to Ca+ overload
How are cAMP concentrations established?
Can be compartmentalised in cell and slowly degraded by phosphodiesterase
Can be immobile at low concentrations and upon stimulus there is fast diffusion rate of cAMP in cell so we see an effect
Why do cell membranes require so many ion channels, transporters and atpase?
Because the lipid bilayer is impermeable to charged ions so transport mechanisms other than diffusion need to be developed
where can EF hands be found?
they able to sense calcium
> calmodulin has 2 EF hands so 4 binding sites
> STIM1/2 have EF too to sense ER store of calcium
