Plasma Membrane (B.B) Flashcards
What are the multiple purposes of the plasma membrane?
- Encloses cells/organelles –> defines boundaries –> which allows chemical processes to be separated –> allows for ideal conditions –> i.e. lysosome has a acidic pH 4.5-5.0.
- Mediates molecular traffic across boundary –> movement of ions or molecules (Na+/K+ pumps)
- An important role in setting up concentration gradients –> electrons transport chain and ATP synthesis.
- Membranes contain receptors –> react to signal molecules which results in changes within the cell (hormones)
- Organise complex enzyme sequences –> i.e. embedded enzymes in the electron transport chain.
Draw out the structure of the lipid bilayer + include labels.
What are the roles of lipids within an organism?
- Storage lipids (triacylglycerides)
- Signalling molecules (hormones –> testosterone)
- Cofactors
- Pigments
- Structural –> lipid bilayer –> 5-10% of the dry mass of most cells.
What are the lipids found in the lipid bilayer? What are their properties?
The cell membrane is composed of polar lipids –> polar end and non-polar end –> also known as amphipathic molecules.
Most common lipid –> phospholipid.
What is the structure of a phospholipid?
- Polar-head group –> variable head group followed by a phosphate followed by a glycerol molecule.
- Non-polar hydrophobic tails –> 2 fatty acid tails that are bonded to the glycerol molecule via an ester linkage –> tails range between 14-24 carbons –> one chain is normally saturated whereas the other is unsaturated.
What is the effect of saturation and carbon chain length on the fluidity of the membrane?
- Increased unsaturation (double bonds) –> creates kinks in the fatty acid tails –> increased kinks decreases the surface area for V.D.W interactions between adjacent chains —> results in decreased packing of the phospholipids –> increases fluidity.
- Increased carbon chain length –> increases the tendency of hydrocarbon tails to interact with one another in both the same and opposite monolayer.
What other factor apart of lipid composition influences the membrane fluidity?
The temperature
- Increased temperature –> more kinetic energy –> more movement/higher fluidity.
- Decreased temperature –> less kinetic energy –> less movement/lower fluidity.
What are the three main different variable terminal groups of phospholipids?
3 major types:
- Phosphatidylcholine (PC) –> choline group at the head group.
- Phosphatidylserine (PS) –> Serine group at the head group.
- Phosphatidylethanolamine –> Ethanolamine group at the head group.
Apart from the main phospholipids, what other phospholipid is present with a distinct structure?
Sphingomyelin –> also a phospholipid –> different structure.
- Uses sphingosine instead of glycerol –> long acyl chain with an amino group and two hydroxyl groups at the other end.
- Most commonly –> fatty acid attached to the amino and phosphocholine attached to one terminal -OH.
- This leaves one free -OH group that can form hydrogen bonds.
What is the structure of the sterol (cholesterol) found in the lipid bilayer?
Sterol - Cholesterol –> Note that the type of sterol changes depending on the organism –> mammals have cholesterol.
Composed of…
- Polar head group
- Rigid steroid ring structure –> induces rigidity in the bilayer
- Non-polar hydrocarbon tail (short)
How does cholesterols size impact membrane fluidity?
Sterol (cholesterol) is comparatively much smaller than a normal phospholipid –> disrupts the packing of the phospholipids in the bilayer –> less tight –> increases the fluidity.
Explain how the shape and nature of the phospholipids cause spontaneous bilayer formation in an aqueous environment.
Recap of hydrophilic and hydrophobic interactions:
- Hydrophilic molecules dissolve in water because charged/polar groups form favourable electrostatic interactions or H-bonds with H2O.
- Hydrophobic molecules can’t dissolve in water because they lack polarity or charge to form energetically favourable interactions with the H2O molecules –> when H-phobic molecules do interact –> water forms cage-like structures around them –> this increases the order relative to the free water around –> increases the free energy (energetically unfavourable) –> thus free energy cost is minimized if the H-Phobic molecules cluster together.
Hence…..
When amphipathic phospholipids are exposed to an aqueous environment –> hydrophobic ends bury themselves away from the H2O whereas, the hydrophilic molecules are attracted –> minimizes energetic cost.
What are the two way that phospholipids can arrange themselves?
- Spherical –> micelles –> single layer of tails pointing inwards.
- Bilayer –> double-layered sheets
The bilayer arrangement is preferred because…
- Most energetically favourable when there are no free edges –> minimizes the contact of the H-phobic region with H2O —> forms sealed compartment –> leads to the formation of the cell.
What are the general properties of the lipid bilayer?
- 5-8 nm thick –> equivalent to 50-80Å –> 1nm = 10 Å
- Bilayer appears trilaminar (having 3 layers) under a microscope.
- Fluid
- Impermeable to most polar/charged particles
- Permeable to some non-polar compounds
4/5. Which makes the membrane semi-permeable.
How are liposomes created under laboratory conditions?
Liposome –> artificial membrane
- Add phospholipids to water –> form multilamellar vesicles with onion like arrangement.
- Use sonication (sound energy) –> changes the structure of the vesicles to form sealed compartments surrounded by a bilayer –> known as a liposome.
Liposomes are useful to study lipids/proteins –> you are able to manipulate the environment in order to test things.
How can fluorescent molecules/gold particles be used to support the theory of membrane fluidity?
Fluorescent labels/gold particles attached to the lipid molecules in the membrane –> phospholipid emits fluorescence that can be traced –> Green fluorescent protein (GFP) emits green fluorescence when exposed to blue light.
If all phospholipids in the liposome are tagged –> use a laser to bleach a particular region (no longer fluorescent) –> over time the surrounding fluorescent molecules will migrate into the bleach.
This migration can be tracked and supports membrane fluidity.
How can the spin label be used to track membrane fluidity?
A spin label can be added –> nitroxide (N-O) –> has unpaired electrons –> spin creates a paramagnetic signal –> can be detected with electron spin resonance (ESR) which is like NMR. Hence, one can trace the movement of phospholipids using this technique.
What are the different types of movement that can be performed by phospholipids?
- Lateral diffusion –> movement within a single monolayer –> extremely quick –> diffusion coefficient (D) –> 10-8cm-2s-1. This is possible because the Van der Waals forces exist and not covalent.
- Flexion –> phospholipid tails flexing and bending.
- Rotation –> rapid rotation of phospholipids about their long axis.
- Flip-flop –> movement between monolayers —> slow process –> except for cholesterol which is able to flip-flop rapidly.
Why can phospholipids NOT flip-flop? How is it made possible?
- It is highly energetically unfavourable for hydrophilic heads to move through the hydrophobic region.
Solution?
Phospholipid translocators/flippases –> enzymes that catalyze the flip-flop of phospholipids.
What two factors influence the fluidity of the membrane?
- Temperature
- Composition –> types of lipids
The bilayer changes state to a 2D rigid crystalline gel state at a specific temperature –> known as a phase transition –> changes depending on the phospholipids present.
The more packed it is –> the lower the temperature under which the layer will undergo a phase transition.
The less packed it is –> the higher the temperature under which the layer will undergo a phase transition.
This is influenced by lipid composition.
- Chain length –> increased chain length –> more interactions –> increased packing
- The number of double bonds –> increased number of D.bonds –> increased kinks –> less interactions –> less tighlty packed.
Can bacteria change the composition of their plasma membrane in order to adjust to the environment?
Bacteria exposed to fluctuating temperatures can change the phospholipid composition (i.e. the number of double bonds/chain length) –> to ensure that the layer does not undergo a phase transition.
How does cholesterol affect the bilayer?
- Cholesterol inserts itself near the polar head group of the phospholipids –> hydroxyl group is located next to the polar head group –> consequently the rigid ring structure interacts and partly immobilizes the region near the polar head group –> this decreases the mobility of the first few -CH2 groups of the tail –> makes the layer less deformable –> decreases the permeability to small water-soluble molecules.
- The fatty acid tail of cholesterol doesn’t influence the rigidity.
- Cholesterol is small –> disrupts the packing of phospholipids –> High conc. of cholesterol prevents hydrocarbons chains from joining together and crystallizing –> results in looser packing –> maintains fluidity at low temp –> cholesterol level can be varied in order to manipulate fluidity.
How can liposome be used to investigate permeability?
- Prepare liposomes in a solution with the substance of interest.
- Separate liposomes into a solution lacking the substance of interest.
- Measure the rate of movement of the substance into the external solution.
- Data will be consistent in showing that some non-polar can diffuse whereas polar/ionic can’t transverse.
How many different lipids are there in plasma membranes?
Phospholipid bilayer –> lipid composition is very complex.
- Membranes –> 500-2000 different lipids
- Simple red blood cell –> well over 150 lipids
- Complexity reflects the variation in head groups, hydrocarbons, desaturation of major phospholipid classes and minor lipid with important functions.
What are lipid domains? Why are they needed?
Domains
- WIth certain lipid mixtures, we can observe phase segregation in which different lipids come together to form domains.
- Domains called lipid rafts –> permanent large scale segregation is uncommon –> specific proteins/lipids concentrate in a more temporary dynamic fashion –> facilitated by protein-protein interactions.
Why?
Good for organising and concentrating membrane proteins for transport (vesicles) or for protein assemblies (important for conversion of extracellular to intracellular signals).
Give an example of a lipid raft?
Lipid raft composed of sphingomyelin and cholesterol –> thicker than the rest of the membrane.
Useful –> acomodates membrane proteins –> i.e. GPI anchor proteins.
Is the distribution of lipid between monolayer symmetrical?
Example using Phosphatidylserine (PS)
No!
Lipid composition between monolayers varies greatly –> composition changes a lot –> not absolute.
Example:
- > Platelets can only form blood clots when PS is expressed in the outer leaflet.
- > Animal cells undergo apoptosis (a form of programmed cell death, discussed in Chapter 18), phosphatidylserine, which is normally confined to the cytosolic (or inner) monolayer of the plasma membrane lipid bilayer, rapidly translocates to the extracellular (or outer) monolayer –> signals neighbouring cells, such as macrophages, to phagocytose the dead cell and digest it.
How does this translocation occur of PS?
- The phospholipid translocator that normally transports this lipid from the outer monolayer to the inner monolayer is inactivated.
- “scramblase” that transfers phospholipids nonspecifically in both directions between the two monolayers is activated.
What is the importance of lipid asymmetry?
Lipid asymmetry is functionally –> especially in converting extracellular signals into intracellular ones.
- Many cytosolic proteins bind to specific lipid head groups found in the cytosolic monolayer of the lipid bilayer –> hence these heads groups are required for the function of the proteins.
The enzyme protein kinase C (PKC), for example, which is activated in response to various extracellular signals, binds to the cytosolic face of the plasma membrane, where phosphatidylserine is concentrated and requires this negatively charged phospholipid for its activity.
- In other cases, specific lipid head groups must first be modified to create protein-binding sites at a particular time and place.
One example is phosphatidylinositol (PI), one of the minor phospholipids that are concentrated in the cytosolic monolayer of cell membranes (see Figure 13–10A–C). Various lipid kinases can add phosphate groups at distinct positions on the inositol ring, creating binding sites that recruit specific proteins from the cytosol to the membrane. An important example of such a lipid kinase is phosphoinositide 3-kinase (PI 3-kinase), which is activated in response to extracellular signals –> recruits specific intracellular signaling proteins to the cytosolic face of the plasma membrane
Role of lipid droplets within cells?
- Lipid droplets act as a storage of excess lipids.
- They can be retrieved for membrane synthesis or as a food source.
- Adipocytes are specialized for this purpose whereas other cells have less and smaller droplets.
- Lipid droplets store neutral lipids –> (triglycerides/cholesterol esters) –> no hydrophilic group –> form droplets –> droplets are surrounded by a monolayer with a large number of proteins –> formed in ER.
General info/facts about membrane proteins?
- Most specific tasks are performed by membrane proteins –> respiration, photosynthesis, cellular response to environmental change (hormones).
- Proteins make up roughly 50% of membrane mass –> varies i.e. myelin on axon is 25% protein by mass whereas, membranes in mitochondria/chloroplasts are roughly 75% protein by mass.
- Since lipids are tiny compared to proteins –> roughly 50 lipid molecules for every protein.
- At least 20% of all active DNA that can be used for protein synthesis encodes for membrane proteins.
What are the different types of integral membrane proteins?
Note –> the region that crosses through the hydrophobic bilayer (hydrophobic region of protein) –> called the transmembrane domain –> the TM domains are linked by soluble regions (pass through the aqueous environment)
- /2. –> alpha helices –> 1 is a single pass helix whereas, 2 has multiple helices passing through the membrane
- Beta-barrel –> made from curved Beta strands.
What are the different types of peripheral membrane proteins?
- Monotropic membrane protein –> passes through a single monolayer NOT the entire membrane,
- /6. Proteins associated with phospholipids –> covalently attached –> known as lipid modification.
- Is a GPI anchor –> found on exterior leaflet always –> linked to phospholipids via oligosaccharides linker.
- /8. associated with other proteins (non-covalently) –> can be released easily –> due to environmental change.
Note –> protein 5 is made in the cytosol and anchored to the membrane via a covalent bond whereas, protein 6 is made in the ER –> TM protein is cleaved off and GPI anchor added –> then transported to the membrane in a vesicle.
What are the two main types of membrane proteins?
1, Alpha helix
- Beta-barrel –> arrows show the direction from the N (Start) to the C terminus (end)
When shown in diagram –> polypeptide backbone is shown –> side chains are dismissed (convention).
Describe the structure of the alpha helix.
Alpha helix structure
- Alpha helix starts from the amino terminus and ends at the carboxyl terminus with the R-groups sticking out away from the helix.
- Peptide bonds are polar/hydrophilic and cause there is no H2O –> A.A is forced to form H-bonds.
- Helix is held together by a network of hydrogen bonds between the Carboxyl group (N residue) and the primary amine group (-N-H) (N+4 residue)
- All hydrogen bonds are intrahelical.
- Structure maximizes H-bonds –> H-bonds stabilise helix –> helix is a very stable conformation.
- Normally there are 20-30 residues within one helix.
- In the TM domain –> the residues need a hydrophobic R-group.
How to interpret a TMHMM (hydrophobicity analysis)?
- Large red peaks –> indicate a high probability of those residues being located within the membrane.
- The top layer of the graph –> indicates the topology –> Top line = aq/soluble region (extracellular) –> Thick block = TM domains –> Bottom line = aq/soluble region (intracellular).
How are TM domains inserted into the membrane?
The strong drive to maximize hydrogen-bonding in the absence of water means that a polypeptide chain that enters the lipid bilayer is likely to pass entirely through it before changing direction since chain bending requires a loss of reg- ular hydrogen-bonding interactions.
Describe the structure of Beta-barrels?
Beta-barrels
- Formed by multiple curved Beta-strands –> creates rigid structures –> used for porins
- Found in bacteria, chloroplasts and mitochondrial membranes.
- Beta-barrel –> Hydrophobic residues point towards the lipids –> Hydrophilic residues point towards the core fo the barrel.
- Loops of the polypeptide often protrude into the lumen of the channel –> narrows the channel –> controls the solutes that can pass –> allows porins to be highly selective.
- Can also be used as an anchor for cytosolic loops that act as a binding site.
Why are alpha helices often preferred over Beta-barrels?
Alpha helices are able to slide against each other/allow for conformational change –> useful characteristic for membrane proteins (channels, carrier, hormone receptors, etc.
Whereas, Beta-barrels to rigid for this.
An important point concerning protein topology within the membrane.
The topology/orientation of proteins relative to the membrane doesn’t change —> basically the proteins orientation within membranes stays constant –> essential for function.
How can membrane proteins be studied?
- Functional and structural analysis requires soluble proteins –> but the lipid environment must be maintained –> essential for structure and function of the membrane protein.
- Detergent –> sodium dodecyl sulphate is used –> much more soluble in water than lipids.
1. Use detergent monomers (at high concentrations –> monomers join together to form micelles –> point called critical micelle concentration) to extract membrane proteins (displace lipids) –> micelles have a hydrophilic head + hydrophobic tail –> hydrophobic ends of detergents bind to the hydrophobic regions of the membrane proteins.
2. Forms water soluble protein-lipid detergent complex which is stable in solution
Problem? –> strong detergents, however, unfold (denature) proteins by binding to their internal “hydrophobic cores,”
Solution? —> some cases, removal of the SDS allows the purified protein to renature, with the recovery of functional activity.
- This can be used for testing –> when using mild detergents the protein can be solubilized and then purified in an active –> detergent concentration decreased in presence of phospholipid –> membrane protein reincorporates into small liposomes.
Explain the general way the transporters work to move molecules across the membrane.
Transporters often called carriers or permeases
- Process –> solute binds to a protein on one side of the membrane (binding site) –> leads to a reversible conformational change –> at one point the substrate will be in an occluded state –> eventually results in the release of the solute on the other side.
- Protein has a high-affinity solute binding site (only one binding site exposed at one time) –> this is important as the molecule needs to remain bonded throughout the conformational change.
- Systems work for export and import.
- Two key characteristics –> High-affinity binding and a radical conformational change. \
- Transportation can be blocked by a competitive and non-competitive inhibitors.
What are the different types of gating within channels?
Gating refers to the opening of the channel in response to a specific stimulus.
- Voltage-gated channels –> channel opens when there is a change in membrane potential.
- Ligand-gated –> an intracellular or extracellular mediator (different types of ligands –> transmitter, ion, nucleotide) binds to the binding site –> results in a conformational change –> opens channel –> when the ligand is released –> protein reverts to its original shape.
- Mechanically gated –> responsive to changes in pressure (osmotic pressure) —> aquaporin
Explain the structure of K+ channels.
K+ channels –> one of the most common channels.
- Made of 4 identical polypeptide chains –> each chain is made of 2 long helices that cross the membrane and 1 smaller helix (known as the pore helix) –> example from bacteria (S.Lividas) –> they are the first K+ channels that has its structure studied.
- The 2 long helices are TM domains whereas the shorter one is not.
- The opening of the channel –> is filled with many negative residues to attract positively charged K + .
- The selectivity loop/filter allows K+ (1.33Å) but nothing else i.e. Na+ (0.95Å) through.
- The short helix is polar with a positive end and a negative end closer to the channel.
Explain the structure of voltage-gated channels.
Voltage-gated channel - voltage-gated K+ channels –> Senses membrane potential –> changes conformation in response to changes in potential.
- Composed of a single polypeptide with 4 domains.
- Each polypeptide contains two transmembrane alpha helices –> that surround the central ion conducting pore.
- There are four additional alpha helices –> one of which is a voltage sensor (shown in red on the diagram).
- Voltage-gated senors –> abundant in arginine residues –> positively charged.
Explain how the voltage-gated channels works
- S4 helix (positively charged) is attracted to negatively charge on the inside of the membrane.
- As S4 is connected to the S5 subunit –> so this ensures that the pore remains closed.
- Membrane potential reverse –> outside is negative and inside is positive.
- S4 helix is attracted to the negative charge on the outside now –> moves up
- This then pulls down on the S5 subunit –> changes the conformation of the protein.
Note —> reverse process for closing.
- This then opens the pore allowing for the movement of K+ ions.
What are aquaporins and what is their structure?
Aquaporin –> water channels
- Responsible for water secretion in tears, sweat, water uptake (kidney –> collecting duct) and saliva production.
- Flow rate –> 109 S-1 –> low activation energy.
- The direction of flow depends on the osmotic gradient.
- Aquaporin is a tetramer made up of 4 identical polypeptide chains –> each monomer contains 6 transmembrane alpha helices + 2 half helices.
- Asparagine-proline-alanine residues are key in preventing the passage of hydronium ions (H3O+) –> positive charges repel –> act as a selectivity filter plus pore is 2.8Å –> prevents larger molecules from passing.
How do aquaporins prevent the transport of OH- and H3O+?
Aquaporin objective –> transport of neutral water
- Positively charged Arg residues prevent the passage of + charged ions (H3O+)
- Hydronium ions diffuse through water extremely rapidly, using a molecular relay mechanism that requires the making and breaking of hydrogen bonds between adjacent water molecules –> strategically placed asparagines –> bind to the oxygen atom of the central water molecule in the line of water molecules traversing the pore, imposing a bipolarity on the entire column of water molecules –> impossible for the “making and breaking” sequence of hydrogen bonds central Oxygen in H2O can’t form H-bonds –> H3O+ can’t get past central asparagine.
- OH- cannot pass because it is always associated with other molecules/ions.
What three types of receptors?
Three types of receptors
- Ion-channel coupled receptors –> ligand-gated ion channel which acts as a receptor + transport protein.
- Enzyme coupled reactions –> this is when one signal molecule binds two receptors (inactive) bring them together to form an active catalytic domain or active complex.
- G-protein coupled receptor –> receptor that has an exposed ligand binding site on the extracellular membrane –> active receptor binds to g-protein –> activated receptor and G protein activate an enzyme which initiates a cascade of reactions.
Describe the characteristics and structure of receptor of G-protein coupled receptors.
G-protein coupled receptors
There are many types –> all share a similar architecture
- Always 7 T.M.D
- Specific membrane topology
- N terminus is extracellular / C terminus is intracellular
- Within the helical bundle, one can find the ligand binding site.
- Ligand binds which induces a conformational change –> activate the G-protein
- Ligand has a high affinity for receptor –> affinity measured using molar constant –> measurement that measures the minimum concentration needed to see the desired response in the cell.
Describe the characteristics and structure of G-protein of G-protein coupled receptors.
G-protein –> protein modified to lipids in the membrane (receptors can pull specific G-proteins from the cytosol).
- Note there are more receptors than G-proteins –> G-proteins can bind to multiple receptors.
- They are heterotrimeric proteins (made of 3 polypeptides that are different –> denoted as α, β, γ)
- α and γ is modified so that they are attached to lipids –> β subunit is very important as it holds the two other subunits together and also mediates all interactions conducted by the other proteins.
Explain the process by which G-proteins coupled receptors are activated?
G-proteins coupled receptors
- In the unactivated form, the alpha subunit of the G-protein binds to GDP.
- However, when the ligand binds –> it causes a conformational change in the receptor.
- This conformational change catalyzes the exchange from GDP to GTP
- This results in the alpha subunit dissociating from the beta and gamma subunits.
- The released alpha subunit is free to interact with other proteins –> adenylate cyclase –> results in the activation of downstream signalling
For example: In response to epinephrine –> Adenylate cyclase –> converts ATP into cAMP –> cAMP activates protein kinase –> results in increased ATP production, degradation of food stores, etc.. –> effects also extend to the nucleus –> increases/decreased protein expression.
How is the G-protein deactivated?
Important to shut down system –> many metabolic pathways involve a signalling cascade which amplifies the response –> hence it is important not to waste resources.
Alpha subunits have GTPase activity –> Overtime the GTP bound to the alpha subunit breaks down into GDP –> once broken down it spontaneously reassociates with the Beta gamma dimer –> reforms the inactive heterotrimer.
This breakdown process is quick enough to minimise waste but slow enough that the Alpha subunit can impact the effector before breaking down.
What is an important consequence of the reducing environment in the
Another important difference between proteins on the two sides of the membrane results from the reducing environment of the extracellular environment.
- The cytosolic environment decreases the likelihood that intrachain or interchain disulfide (S–S) bonds will form between cysteines on the cytosolic side of membranes.
- Due to the reducing environments –> intrachain or interchain disulfide (S–S) bonds form on the noncytosolic side, where they can help stabilize either the folded structure of the polypeptide chain or its association with other polypeptide chains
What are the three main types of glycosylation?
- Protein –> glycoproteins –> oligosaccharide chain covalently bonded to the membrane protein.
- Protein –> proteoglycan –> long chain polysaccharide chain linked covalently to a protein core. Note –> protein core either extends across the lipid bilayer or is attached to the bilayer by a GPI anchor.
- Lipids –> glycolipids –> oligosaccharide chain covalently bonded to the membrane lipid.
Structure of bacteriorhodopsin?
Bacteriorhodopsin molecule is folded into seven closely packed transmembrane α helices and contains a single light-absorbing group or chromophore (retinal).
Retinal is covalently linked to a lysine side chain of the bacteriorhodopsin protein.
How does the bacteriorhodopsin protein transport H+ using light?
When activated by a single photon of light, the excited chromophore changes its shape and causes a series of small conformational changes in the protein, resulting in the transfer of one H+ from the inside to the outside of the cell.
Light-driven proton transfer establishes an H+ gradient across the plasma membrane, which in turn drives the production of ATP by a second protein in the cell’s plasma membrane.
Explain the mechanism of glucose transport fueled by a Na+ gradient.
What are the three different types of ATP driven pumps?
- P-type pumps are structurally and functionally related multipass transmembrane proteins. They are called “P-type” because they phosphorylate themselves during the pumping cycle. This class includes many of the ion pumps that are responsible for setting up and maintaining gradients of Na+, K+, H+, and Ca2+ across cell membranes.
- ABC transporters (ATP-Binding Cassette transporters) differ structurally from P-type ATPases and primarily pump small molecules across cell membranes.
- V-type pumps are turbine-like protein machines, constructed from multiple different subunits. The V-type proton pump transfers H+ into organelles such as lysosomes, synaptic vesicles, and plant or yeast vacuoles (V = vacuolar), to acidify the interior of these organelles.
- Extra –> F-type ATPases, more commonly called ATP synthases because they normally work in reverse –> use the H+ gradient across the membrane to drive the synthesis of ATP from ADP and phosphate
Explain the structure and function of Ca2+ pump in the sarcoplasmic reticulum (SR)
After the release of calcium stores from the SR –> Ca2+ pump moves Ca2+ from the cytosol back into the SR.
Structure
- 10 transmembrane α helices connected to three cytosolic domains.
- Amino acid side chains protruding from the transmembrane helices form two centrally positioned binding sites for Ca2+ –> normally available on the cytosolic side.
Function
- Ca2+ binding triggers a series of conformational changes that close the passageway to the cytosol.
- Activate a phosphotransfer reaction in which the terminal phosphate of the ATP is transferred to an aspartate.
- The ADP then dissociates and is replaced with a fresh ATP –> causing another conformational change that opens a passageway to the SR lumen through which the two Ca2+ ions exit.
- Ca2+ Binding sites stabilized by two H+ ions and a water molecule –> closes passageway.
- Hydrolysis of the phosphoryl-aspartate bond returns the pump to the initial conformation
What is the concentration of Na+ and K+ relative to each other? How is this maintained?
- The concentration of K+ is typically 10–30 times higher inside cells than outside, whereas the reverse is true of Na+. (High K+ conc. inside and high Na+ outside)
- Na+-K+ pump, or Na+- K+ ATPase maintains this concentration gradient –> Like the Ca2+ pump, the Na+-K+ pump belongs to the family of P-type ATPases -> functions as an ATP-driven antiporter, actively pumping Na+ out of the cell against the electrochemical gradient and pumping K+ in.
- Na+-K+ pump drives three positively charged ions out of the cell for every two it pumps in –> makes it electrogenic –> it drives a net electric current across the membrane, tending to create an electrical potential –> contributes more than 10% to the membrane potential
