Cell membranes and signalling Flashcards
functions of membranes
A selective barrier to the passage
of molecules
Detecting chemical signals from
other cells
Anchoring cells to adjacent cells
and to the extracellular matrix of
connective-tissue proteins
lipid rafts
Cholesterol, phospholipids, and specific proteins work together to curve the membrane and pinch off a section to form a vesicle.
This occurs through membrane bending, which is driven by proteins like clathrin, caveolin, or other coat proteins.
Cholesterol-rich lipid rafts help to stabilize this curvature due to their ordered nature.
types of junctions that can join cells
desmosomes
tight junctions
gap junctions
integrins
transmembrane proteins in plasma membrane that bind to specific proteins in extracellular matrix and link them to membrane proteins on adjacent cells
interstitial fluid
fills gaps between cells
desmosomes
Characterised by accumulations of
protein known as dense plaques
along the cytoplasmic surface of the plasma membrane.
These proteins serve as anchoring points
for cadherins.
function is to hold adjacent cells firmly
together in areas that are subject to
considerable stretching, such as the skin
cadherins
proteins that extend from the cell into the extracellular space, where they link up and bind with cadherins from an adjacent cell.
tight junctions
Form when the extracellular surfaces of
two adjacent plasma membranes join
together so that no extracellular space
remains between them
Tight junctions block the extracellular space so molecules can’t flow freely in the interstitial fluid
gap junctions
consist of protein channels linking the cytosols of adjacent cells.
connexins from the 2 membranes join to form protein-lined channels
only allows small molecules through
at any concentration difference, what does the magnitude of flux depend on?
temperature
mass of molecule
surface area
medium through which the molecules are moving
how is specificity of a protein channel determined
pore size
charge
binding sites
types of gated channels
ligand gated
voltage gated
mechanically gated
ligated gated ion channels (chemical messengers)
a specific molecule binds to the channel causing an allosteric or covalent change in the shape of the protein
voltage gated ion channels
changes in the membrane potential cause a movement of charged areas of the protein, altering its shape
mechanically gated ion channels
physically deforming (stretching) the protein changes its conformation
factors determining magnitude of solute flux through a mediated system
saturation of transport binding sites (influenced by solute concentration and affinity of the transporter to the solute)
number of transporters in the membrane
rate at which conformation change occurs
primary active transport
direct use of ATP which is hydrolysed by an ATPase protein transporter
transporter is phosphorylated
covalent modulation, causes conformational change that increases affinity of solute binding site
secondary active transport
use of an electrochemical gradient across a membrane against their concentration gradient
direction and magnitude of ion flux is dependent on
concentration and electrical difference
(electrochemical gradient)
sodium/potassium pump mechanism
ATP is associated to the transporter
binds 3Na+
binding sites for K+ are of low affinity
ATPase removes a phosphate and phosphorylates the transporter
conformational change reduces affinity for Na+ and exposes to extracellular fluid
new conformation increases K+ affinity and binding causes dephosphorylation, conformation reverts back to original so that K+ released in intracellular fluid
major primary active transport proteins found in most cells
Ca2+ ATPase
Na+/K+ ATPase
H+ ATPase
H+/K+ ATPase
secondary active transport mechanism
low Na+ inside cell, high solute
electrochemical gradient directs Na+ into cell
Na+ binds to one site, solute to another
both released into cell
osmosis
net diffusion of water across a
membrane, which is dependent on water
concentration
osmotic pressure
”force” required to prevent the flow of water into a solution
osmolarity (total solute concentration) of extracellular fluid
285-300 mOsm
ligand
molecule/ion bound to a protein by either electrical attractions between oppositely charged ionic or polarised groups on
the ligand and the protein or weaker attractions due to hydrophobic forces between nonpolar regions on
the two molecules
binding does not involve covalent bonds
reversible
saturation
fraction of total binding sites that are occupied at any given time
An equilibrium is rapidly reached between unbound ligands in solution and their corresponding protein-binding sites.
what 2 factors does % saturation of a binding site depend on
concentration of unbound ligand in solution
affinity of the binding site for the ligand
two mechanisms in cells that selectively alter protein shape and alter enzymes affinity for substrates
allosteric modulation
covalent modulation
doesnt increase maximum rate
allosteric modulation
occurs when a protein has two binding sites of a protein to one of the sites alters the shape of the other
one binding site is the functional site, the other is the regulatory site to which a modulator (ligand) binds
cooperativity
when a ligand binds to the first of several functional sites on a molecule, this induces a change that increases the affinity of other functional sites
covalent modulation
covalent bonding of charged chemical groups to some of the protein’s side chains.
eg phosphorylation by kinases (dephosphorylation by phosphatase)
catabolism
breakdown of organic molecules
anabolism
synthesis of organic molecules
calorie
amount of heat required to raise the temperature of 1g of water 1°C
law of mass action
the concentration of reactants
or products can determine the direction at which the net reaction proceeds
cofactors
magnesium, iron, zinc, copper
Binding of a metal to an enzyme alters conformation the substrate– allosteric regulation
coenzyme
organic molecule
It directly participates as one of the substrates in the reaction
usually derived from vitamins
how to change concentration of enzyme
increase protein synthesis
increase protein degradation
metabolic pathway
sequence of enzyme-mediated reactions leading to the formation of a particular product
rate limiting enzymes
often the sites of allosteric or covalent regulation
Control of enzymes through allosteric or covalent modulation can be important in
determining the direction
of reversible/irreversible reactions
neurotransmitters
function rapidly over short distances
hormones
function slower, usually over longer distances
antagonists
drugs that acts as competitors to receptor
dont cause response in cell
agonists
drugs that mimic the messenger
trigger response in cell
down regulation
a decrease in the total number of target cell receptors for a given messenger
increased sensitivity
increased responsiveness of a target cell to a given messenger
can be caused by upregulation
binding causes change in conformation of receptor, resulting in a change to…
permeability, transport properties or electrical state of the cell
metabolism
secretory activity
rate of proliferation/differentiation
contraction
protein kinases
activate other proteins by transferring a phosphate group to them
second messenger model with cAMP and kinases
first messenger binds to receptor
triggers the production of cAMP (cyclic adenosine monophosphate), which acts as a second messenger.
cAMP activates Protein Kinase A (PKA).
cascade where the signal is amplified:
Each activated Protein Kinase A phosphorylates multiple downstream enzymes.
Each enzyme activates more molecules (e.g., producing 100 end products).
This exponential amplification allows a single signal to produce a large cellular response.
outcomes of kinase activation within a cell
ctive transport: Phosphorylation drives ion movement across the membrane.
Microtubule function: Changes in transport, secretion, and cell shape.
Enzyme activation:
Enzyme 1 leads to lipid breakdown.
Enzyme 2 leads to glycogen breakdown.
Protein synthesis: Phosphorylation influences processes in the endoplasmic reticulum, such as calcium transport and protein production.
Gene expression: Protein kinases can activate transcription (DNA → mRNA), leading to long-term cellular changes.
lipid soluble messengers
generally act by binding to intracellular receptor
steroid hormones, thyroxine, sex hormones
once inside the nucleus, a lipid soluble messenger would acts as a transcription factor
slower response than with membrane receptors
water soluble messengers
binding to the extracellular portion of
receptor proteins embedded in the plasma membrane (e.g. dopamine, adrenalin, melatonin)
critical points for water soluble messengers
broad range of receptors
activate intracellular signalling cascades
can activate downstream mediators
faster response
Type A
receptors that are ligand-gated ion channels.
Activation of the receptor by a first messenger (ligand) results in a conformational change to the receptor so it forms an open channel through the plasma membrane.
Type B
receptors that function as enzymes
intrinsic enzyme activity and most are protein kinases that specifically
phosphorylate the amino acid tyrosine
(receptor tyrosine kinases)
Type C
Receptors that interact with cytoplasmic Janus Kinases (JAKs)
dont have intrinsic kinase activity
Type D
G-Protein-Coupled receptors
Bound to the inactive receptor is a protein complex located on the cytosolic surface of the plasma membrane and belonging to the family of heterotrimeric proteins known as G proteins.
type c mechanism
binding of ligand to receptor causes conformational change in receptor that leads to activation of cytoplasmic kinase
type b mechanism
Binding of messenger changes the conformation
Receptor “autophosphorylates” its
tyrosine groups
Phosphotyrosines on the cytoplaasmic
portion are docking sites for cytoplasmic
proteins.
Docking proteins bind and activate
other proteins, which in turn activate
other signaling pathways
cytoplasmic proteins activated by phosphorylation
janus kinases
cytoplasmic kinase
a family of 4 kinases that are all tyrosine kinases
3 subunits of G proteins
alpha: can bind Guanosine-diphosphate (GDP) “OFF” or Guanosine-triphosphate (GTP) “ON”
beta
gamma
type d mechanism
The binding of a ligand to the receptor
changes the conformation of the receptor.
This activated receptor increases the
affinity of the alpha subunit for GTP.
When bound to GTP, the alpha subunit
dissociates from beta and gamma subunits.
Activated alpha subunit links with another
plasma membrane protein (effector protein, eg ion channels and enzymes)
signalling via adenyl cyclase and cyclic AMP:downstream signaling from G-Protein-Coupled receptors
The effector protein that is activated: the membrane enzyme adenylyl cyclase
This catalyses the conversion of cytosolic ATP molecules to cyclic 3´,5´-adenosine
monophosphate, or cyclic AMP (cAMP).
Cyclic AMP then acts as a second messenger. Inside the cell, cAMP binds to and activates an enzyme known as cAMP
dependent protein kinase. This is also called Protein Kinase A (PKA). PKA then phosphorylates downstream targets
Ca2+ in cells
maintained at very low concentration in cytosol so when first messenger binds to receptor and it opens, influx of Ca2+ into cytosol, also released from ER
Ca2+ as second messenger
binds to various cytosolic proteins, eg calmodulin. On binding with Ca2+ calmodulin changes shape, and
this allows active calcium-calmodulin to activate or inhibit kinases such as calmodulin-dependent kinases which in turn can activate other proteins using
ATP to phosphorylate them
cessation of receptor activation
decrease in concentration of first messenger molecules
enzymes in vicinity metabolise the first messengers, or it is taken up by other cells or just diffuses away
how can receptors be inactivated
chemical alteration, phosphorylation, to lower affinity for first messenger
plasma membrane receptors can be removed when the combination of first
messenger and receptor is taken into the cell by endocytosis
Arachidonic acid
polyunsaturated fatty acid derived from phospholipids in the plasma membrane
is a second messenger
arachidonic acid mechanism
first messenger binds to receptor
phospholipase A2 causes arachidonic acid to split off from membrane
can be metabolised by either cyclooxygenase pathway or lipoxygenase pathway
interfering with the cyclooxygenase pathway
aspirin inhibits it
reduces inflammation and blood clotting
Corticosteroids
inhibit phospholipase A2
effects downstream signalling pathways
eicosanoids
prostaglandis
cyclic endoperoxides
thromboxanes
leukotrines
can act as intracellular
messengers but are often released locally
acting in a paracrine
or autocrine
manner to
stimulate a variety of physiological responses
