Lecture 10 transmembrane transport Flashcards
Why do membranes exist in the cell?
- The plasmamembrane separates the cellular contents from the outside
- Internal membranes separate the contents of different organelles from the rest of the cell
- No membranes, no gradients, no energy!
What nutrients/molecules do cells need to take in/release?
Cells/organelles need to take-up/release these molecules:
+Ions
+Large uncharged polar molecules (glucose,fructose)
+Charged polar molecules (amino acids etc)
What are the kinetics of simple diffusion?
-Simple diffusion: non-saturable, linear kinetics
-Transport: protein mediated with saturable kinetics
+Consider the kinetics (rate of uptake) of oxygen and glucose. Simple diffusion is high for O2, low for glucose. But both are linear. The rate of uptake increases as you add more solute. However, the kinetics of glucose uptake into a cell are more like this. High rates of uptake at low concentrations and saturable to a velocity maximum of Vmax. This is transport and it’s mediated by membrane proteins.
What is the membrane protein function?
-It is the protein component of the membrane that is key to the “selective permeability” to most physiologically-important solutes. These are the gateways into cells!
-26% of the coding capacity of the human genome is for membrane proteins
-60% of drug targets are membrane proteins
+Inner Mx membrane is involved in ATP generation requiring proteins involved in the electron transport chain (more proteins than lipid)
+Myelin sheath provides electrical insulation for nerve axons (less proteins than lipids)
+The protein and lipid content of different cellular membranes can vary, dependent on the function of the membrane
What are some membrane protein functions?
- channels
- transporters
- integrins/adhesins/connexins/claudins
- receptors
- enzymes
Give some clinically relevant membrane protein function examples.
- CFTR: chloride ion channel; mutations either impair folding and trafficking to the apical membrane, or limit the open time of the channel. A new drug therapy, called a potentiator has been approved to increase the open channel probability; correctors to improve folding of those mutants that are impaired in this pathway are in the pipeline
- ABCB1 aka the multidrug resistance transporter or P-glycoprotein, effluxes drugs directly from the membrane preventing their intracellular accumulation – and thus resistance to chemotherapy
- Connexin 26 is a hexamer involved in cell-cell contact (a 6xconnexin 26 from one cell docks with a 6xconnexin 26 from an adjacent cell. This forms a gap junction allowing communication in the form of the ion (K+) flow between cells when the junction is open. Important for hearing in the cochlear cells of the ear (see funmed practical).
- FGFR3 (fibroblast growth factor receptor), a PBL topic in funmed is a tyrosine kinase receptor – only the extracellular domain of this receptor has been crystallized, so the transmembrane domains are missing, but it is co-crystallized with fibroblast growth factor. When FGF binds the receptor dimerises and autophosphorylates itself, this attracts and activates other kinases which signal thru various pathways including thru map-kinase to change gene expression and inhibit proliferation. The gain of function mutations acquired in achondroplasia stabillise the dimeric receptor and so it signals longer, and there are fewer chondrocytes required to calcify the cartilage skeleton.
How do channels mediate passive
movement of solute?
Channel: 1. Continuous pore through the membrane 2. Can be regulated 3. Can be selective 4. Only work down hill i.e solute moves down its electrochemical gradient to equilibrium 5. Bulk flow high \+Regulate by opening/closing pore \+Selective by narrowing the channel and providing weak interaction points for the solute, so for example, there may be negatively charged amino acids at the selectivity filter to repel chloride and attract sodium.
How can transporters also facilitate
passive movement of solute?
Passive Transporter:
- Specific solute binding sites alternately exposed on different sides of the membrane
- Only works down-hill
- Low capacity
What is the difference between
passive and active transport?
-Passive transporter:
1. solute binding sites randomly exposed on either side of the membrane.
2. Solute binding induces a conformational change, exposing the solute to the other side of the membrane
3. Net flux is dependent on the electrochemical/concentration gradient, and only to equilibrium
-Active (efflux) transporter:
1. High affinity solute binding site exposed to cytosol.
2. Solute binding induces ATP binding/lysis
3. Conformational change exposes low affinity binding site to extracellular space
4. Net flux is dependent on ATP (primary active pump), and can be uphill (against electrochemical/concentration gradient)
+(Dependence on ATP is indicative of primary active transport)
How do channels and some transporters
harness electrochemical gradients?
-electrochemical gradient with no membrane potential
-electrochemical gradient with membrane potential negative inside
+Most pertinent to the human plasma membrane: the voltage difference, the –ve charge on the inner leaflet phosphatidyl-serine, combined with the efflux of sodium ions by the Na+/K+ ATPase pump generates a powerful electrochemical gradient for Na+
-electrochemical gradient with membrane potential positive inside
How can transport be driven using
the energy from ion gradients?
- Strict coupling of the binding of both the transported solute and the co-transported ion
- The free energy released as the co-transported ion moves down its electrochemical gradient drives the solute up its electrochemical gradient
- The ion gradient is generated by an ATP driven (primary active) pump, so this secondary active transport!
- Symport if the ion moves in the same direction as the solute, antiport it moves in the opposite direction
What is the difference between
primary and secondary active transport?
Primary active transporter:
-Uphill solute translocation is possible if coupled to ATP hydrolysis.
+e.g. The Na+/K+ ATPase that effluxes sodium to generate a powerful gradient. SERCA, the Sarcoplasmic/Endoplasmic Reticulum Ca2+ P-type ATPase transporter
Secondary active transporter:
-Uphill solute translocation is possible if coupled to the downhill movement of an ion.
+e.g. SGLT1 the Sodium/Glucose Linked Transporter of the intestinal epithelium
How are multiple different transporters
harnessed for transcellular transport?
Example: Glucose uptake
- SGLT1 symporter (Na+ driven glucose symport)
- Glut2 facillitated diffusion, down hill transport
- Na+K+ ATPase, a primary active pump, keeps cellular Na+ low
What are a recessive liver diseases caused by
mutations in primary active transporters?
-Progressive familial intrahepatic cholestasis (PFIC)
+A rare, fatal disease of childhood caused by mutations in any of 3 primary-active transporters
What are the signs and symptoms of PFIC?
- jaundice (yellowing of skin and/or eyes; failure to excrete bilirubin)
- pruritis (severe itching)
- failure to thrive (lack of fat/vitamin uptake)
- hepatosplenomegaly (enlarged liver and/or spleen)
- loss of appetite with nausea and vomiting
- foul smelling fatty stool (inability to absorb dietary fat)
- dark urine (bilirubin excreted via kidneys)
