basics of membrane transport- Flashcards
what do kinases and phosphatases do?
kinase - phosphorylate, activate signal transuction pathways
phosphatase - remove phosphate, dephosphorylate, shut down signalling pathway
what does a signal tranduction cacde leas to?
amplification of orignal signal
what adaption would a cell have to detect a signal at LOW CONC?
the cell surface receptors would be HIGH AFFININTY and coupled to an amplification system to activate the signalling systems
also SPECIFICITY, reversible on/off non covalent bond , able to saturate
name some examples of how cell can respond to cell signals
- alter metabolism e.g glygcogen metabolism
- excitation to propogate nerve impulse
- grow + divide e.g. GF response, mitogenesis
- cell death (programmed) apoptosis
- alter GENE expression e.g IgM synthesis
outline a simple signal cascade
cortisol is able to freely cross the plasma membrane (hydrophobic) to the glucocorticoid cytosolic receptor.
GR is a TF so is able to move to the nucleus and alter gene transcription.
what would a ligand bind to if it was hydrophilic?
If hydrophilic then molecule can’t cross lipid membrane
So require cell SURFACE receptors with a extracellular ‘ligand’ domain to couple
describe ways to turn receptor on/off
use GTP and GDP proteins
use phosphorylation/kinases
name some examples of secondary receptors
2nd messengers allow amplification of signals
cyclic AMP -> PKA
PLC -> IP3
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 around 70kD
how would a ligand enter a cell if it was charged?
well the lipid bilayer is IMPERMEABLE to charged ions
therefore there is a need for ion channels (and pumps/transporters)
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 strucutre 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
off mechanisms in calcium signalling
SERCA pump
NCX changer
Ca buffers and chaperones
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?
it is a buffer protein within SR and bind to calcium
helps to pump calcium out against a concentration gradient
what inhibits SERCA
b-adrenergic stimulation results in phosphorylation of phospholambin
which inhibits SERCA
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 in response membrane potential
why is NCX better than ATPases?
NCX works quicker than ATPases and at higher concentrations
what is an agonist and an antagonist
agonist ACTIVATES receptor
antagonist INHIBITS the receptor
what is likely to happen if a mitogenic signal is not turned off
mitogenic signal = growth and divide
if not switched off -> uncontrolled cell division -> cancer/tumour
how do specific calcium codes differ?
the frequency, amplitude and duration of caclium osscilations determine the end result of selective intracellular activation processes
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
name some regulatory machinery for calciuim
Okay but how regulate
calcium pumps
ATP ases and exchangers like PMCA, SERCA
na/ca exchanger
STIM 1 and Orai1 CRAC channels as well for SOCE to replenish ouur stores of calcium
how can intracellular calcium levels be induced
stimuli bind to receptors on cell surface, leads to second messenger levels rising of
inositol triphosphate -ip3
cyclic adp robdoe
naadp (Nicotinic acid adenine dinucleotide phosphate)????????
how does calcium maintain its low intracellular/cytocsolic conc?> (100nmol)
- using pumps and exchangers
- sequeste/store calcium in SR, ER, mitch
- chelating calcium, having it bound to proteins
how can calcium be put into its stors?
SERCA pumps are located on the membrane of endo/sarcoplasmic reticulum to store calcium
there are also mitochondrial uniporters (although mitch store is temporary)
which receptors are present on the ER?
Ip3 receptors and ryanodine receptors which mediate calcium release from the Stores
how do non excitable cells become activted
e.g Pancreatic Acinar Cells
they do not have voltage gated ion channels so cannot generate an AP, rely on a second messenger
= calcium release from intracellular stores, which can then activate processes like granular secretion as seen in PAC
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
in very toxic high intracellular calcium, we can inhibit this oligomerisation of STIM1
what can a mutation in PMCA2 result in
is linked to deafness, ataxia (lack of coordination), motor imbalance
what is function of serca pump?
pumps calcium from cytosol into lumen of ER/SR using atp
> removes cytosolic calcium
how is serca pump regulated
phosphorylation of phospholambin inhibits SERCA and is under beta-adrenergic stimulation
what does the sodium calcium exchanger do? (NXC)
3 sodium inside cell
1 calcium outside of cell
> allows calcium efflux to occur
where are NXC usually present? What is the advantage
in cells with lot of calcium channels like cardiac myocytes, smooth muscle, renal, pancreatic
they work quicker than ATP-ases especiallly at higher concentrations
generally speaking how would elevated calcium levels result in pathology
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
what is the issue with CRAC channels
CRAC channels on plasma membrane open to replenish the ER store
but this can EXACERBATE THE PROBLEM
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
