Membranes and Signal Transduction Flashcards
What are the two types of complex lipids?
Neutral lipids (storage fats and oils) Polar lipids (lipids found in cell membranes)
What is a phosphoglyceride?
A type of glycerolipid which is polar
Contains 2 fatty acids
What type of molecule is a fatty acid?
Amphipathic - has hydrophilic and hydrophobic areas
How many double bonds do unsaturated fatty acids have?
1-4
How do phosphoglycerides arrange themselves in the cell membrane?
Into a bilayer
How are different membrane fractions isolated?
Differential centrifugation - filter the homogenate and centrifuge until you have a second segment, which is a mixture of mitochondria, chloroplasts, lysosomes and peroxisomes
Then use equilibrium density gradient centrifugation, which separates the different layers of membranes due to their differing densities
Lipid composition is…
membrane-specific
How is the fluidity of a membrane determined?
By temperature and lipid composition
What happens to the lipid bilayer as temperature decreases?
It becomes a rigid crystalline structure, which is a big problem
How do sessile organisms maintain membrane fluidity at low temperatures?
They alter the lipid composition of the membrane
How is fluidity increased in the lipid bilayer
It is increased by the degree of unsaturation of the phosphoglycerides in the membrane, as unsaturated fatty acids put a kink in the hydrocarbon tail of the phospholipid
How do plants respond to reduced temperatures?
They decrease the ratio of saturated to unsaturated fatty acids esterified to membrane phosphoglycerides - there will be more phosphoglycerides containing polyunsaturated fatty acids. This together with sterols keep the membranes fluid
What are sterols?
Present to regulate membrane fluidity and modulate the activity of membrane-bound enzymes
What enzyme is responsible for inserting double bonds into fatty acids?
Fatty acid desaturase
Amino acids with polar side chains
Hydrophilic and tend to be on the outside of proteins
Non-polar amino acids
Hydrophobic and are found in the core of proteins, avoiding water
What are the three types of membrane protein?
Integral proteins
Lipid-anchored proteins
Peripheral proteins
Example of integral protein
Glycophorin A
Composed of two monomers
Each monomer contains 3 distinct segments called domains
The extracellular and cytosolic domains are largely named up of polar amino acids
The membrane spanning region by contract is made up largely of non-polar hydrophobic amino acids
What method was used to find evidence for the for the diffusion of proteins across the cell membrane’s surface?
Fluorescence recovery after photobleaching (FRAP)
What is the process of fluorescence recovery after photobleaching (FRAP)?
- Firstly, specific meme brand proteins are labelled with a fluorescent agent
- Bleach an area of the cell membrane with a laser, removing the fluorescent agent proteins
- Measure the intensity of the fluorescence in in the bleached area. If it begins to increase, it suggests that fluorescent proteins from other areas of the cell membrane have diffused into the bleached area
Roles of biological membranes
Cell-cell recognition
Semi-permeable barrier
Platform for linked reactions
Compartmentalisation
What is signal transduction?
The study of cellular events responsible for coupling an extracellular stimulus to its characteristic intracellular response
What is the signal transduction field of research called?
Intracellular signalling our stimulus-response coupling
What is a property of receptors?
They are highly specific to the ligand they bind to
Examples of luganda
Insulin
Acetylcholine
Plant hormones such as auxin
What is a property of ligand-receptor binding?
It is reversible
Receptors have different …. for their ligands
Affinities
A high affinity receptor will bind to its ligand at very low concentrations
What does ligand binding induce?
A conformational change of the receptor - ligand alters the tertiary/quaternary structure of the receptor, so the receptor can interact with other proteins
What are intracellular second messengers?
Simple molecules that amplify a signal
The concentration of the second messenger increases in the cytosol after cell stimulation
The concentration of the intracellular messenger decreases when the stimulus is removed
An increase in the concentration of the intracellular second messenger activates target proteins to relay the signal further into the cell
What are 2 examples of intracellular second messengers
Ca2+
Cyclic adenosine monophosphate (cAMP)
What is an example of protein modification?
Protein phosphorylation
- Transfer of a phosphate from ATP to an amino acid on a target protein is catalysed by the enzyme protein kinase
- Phosphorylation results in a change in the conformational of the substrate
- If the substrate is another enzyme, then protein phosphorylation could activate the enzyme to relay the signal
What enzyme dephosphorylates protein substrates?
Protein phosphatase
What are second messenger activated protein kinases?
Activated when the concentration of an intracellular second messenger is increased in the cytosol following cell stimulation
3 examples of second messenger activated protein kinases
- Protein kinase A (PKA) is activated by cAMP
- Calcium dependent protein kinase (CDPK) is activated by Ca2+
- Protein kinase C (PKC) is activated by Ca2+ and diacylglycerol
What are G protein coupled receptors?
A large family of receptors involved in many responses, present in plants animals and yeast
What is protein kinase?
A kinase enzyme that modifies other proteins by chemically adding phosphate groups to them
What is an example of signal transduction through G protein coupled receptors?
Cholecystokinin (CCK) Signalling
What is cholecystokinin?
CCK is a peptide secreted by the mucosal cells of the duodenum into the bloodstream. It’s secretion is increased when the mucosal cells encounter the products of digestion
What does CCK do when it reaches the pancreas?
It causes pancreatic acinar cells to secrete digestive enzymes including alpha amylase into the duodenum (via the bile duct), which helps with the digestion of food
What does binding of CCK to its receptor result in?
In increase in the concentration of free calcium ions in the cytosol of the acinar cell. This triggers the secretion of alpha amylase into the common bile duct
In the acinar cell, what are the calcium ions acting as?
Intracellular second messengers
What is the structure of the CCK receptor of acinar cells?
Single polypeptide
7 transmembrane helices
Has a domain on the cytosolic side which allows it to interact with the G protein
What is the structure of the G protein?
3 subunits (alpha, beta and gamma) Gamma and alpha subunits are lipid-associates proteins attached to the plasma membrane by a fatty acid modification
How do G proteins get their name?
Because they bind GTP (guanosine triphosphate) and GDP
What happens to the CCK receptor and the G protein once CCK binds to its receptor?
This activated the G protein associated with the CCK receptor. The G protein diffuses along the plane of the plasma membrane until it encounters the next component in the CCK signal transduction pathway
What is the next component that the G protein diffuses to?
Phospholipase C (PLC) When the G protein interacts with the PLC it becomes activated
What does phospholipase C do?
PLC hydrolyses a rare membrane phosphoglyceride called PIP2, producing IP3 and DAG. These are also intracellular second messengers
What is PIP2?
phosphatidyl inositol 4,5-biphosphate
What is IP3?
inositol 1,4,5-triphosphate
What is DAG?
1,2-diacylglycerol
What does IP3 do after it is activated?
It diffuses into the cytoplasm and binds to a receptor on the endoplasmic reticulum
What does the ER act as?
A calcium ion store
What happens as a result of IP3 binding to its receptor on the endoplasmic reticulum?
Calcium ions are released into the cytosol
What happens as a result of IP3 releasing Ca2+ into the cytosol and increasing its concentration?
Protein kinase C (PKC) migrates to the plasma membrane where it is activated by DAG
What does activated PKC do?
Participates in the reactions that control alpha amylase secretion
Therefore, what is the result of an increase of PLC activity?
The activation of PKC and an increase in alpha amylase secretion
How is stimulation stopped, reducing alpha amylase secretion?
ATPase enzymes pump Ca2+ out of the cytosol and back into the endoplasmic reticulum
What is a second example of signal transduction through G protein coupled receptors?
The action of adrenaline on skeletal muscle
What happens when an animal is frightened?
The adrenal gland secretes adrenaline into the bloodstream
What are the 2 major roles of adrenaline in skeletal muscle?
- It promotes the breakdown of glycogen to glucose-1-phosphate. This is then oxidised through glycolysis to provide the ATP required for sustained muscle contraction
- It inhibits the synthesis of glycogen
What enzyme catalysed the synthesis of cAMP from ATP?
Andenylyl cyclase (AC)
What enzyme catalyses the degradation of cAMP to AMP
cAMP phosphodiesterase (cAMP-PDE)
What is the target for cAMP?
Protein kinase A
through a series of steps ATP is produced for immediate muscle contraction
What bacteria causes cholera?
Vibrio cholera
Infected water supplies
What does the cholera bacterium do once ingested?
Secretes a protein called cholera toxin
What does the cholera toxin do?
The beta subunits of the toxin bind to a gut receptor on the intestinal mucosal cells lining the gut. The alpha subunit enters the cell
What does the alpha subunit do once in the cytoplasm of the mucosal cell?
The alpha subunit is an enzyme so it chemically modifies the G protein that activates adenylyl cyclase
What is a result of the G protein being activated by the subunit of the cholera toxin?
It is locked in its active state, so adenylyl cyclase is constantly activated and can’t be switched off. As a result there is a constant rise in the concentration of cAMP in the cells lining the gut
What is the result of the rising concentration of cAMP in the cells lining the gut?
It causes a continuous activation of PKA that phosphorylates a chloride ion channel and inhibits the Na+/H+ exchanger
What is the net result of the PKA’s affect on the mucosal cell?
A massive loss of water and electrolytes from the blood through the mucosal cells to the gut
What is the treatment for cholera?
Oral rehydration with a glucose electrolyte solution
What bacteria causes whooping cough?
Bordetella pertussis
What is secretes by the whooping cough bacterium?
Pertussis toxin
What does the pertussis toxin do?
Chemically modifies a G protein, locking it in its inactive state. This results in a massive activation of adenylyl cyclase because a G protein usually inhibits the action of adenylyl cyclase
What is the structure of the nicotinic acetylcholine receptor?
5 subunits (two alpha, one beta, one gamma and one delta) Each subunit is a single polypeptide containing 4 transmembrane helices (M1-M4) The 5 subunits form a metameric structure with a pore in the centre The M2 helix from each subunit faces into and lines the pore, forming the gate Each helix has a characteristic kink which forms a constriction in the channel
What happens when acetylcholine binds to the alpha subunits?
The gates open and Na+ enters the muscle cell. This is because binding of the acetylcholine to the alpha subunits causes all the subunits to rotate very slightly and this is sufficient to swing the kinked M2 helices outwards, opening the channel
What plant hormone builds up in drought and causes stomata closure?
Abscisic avid (ABA)
