Intro to Ion Channels Flashcards
What is membrane transport?
The selective movement and redistribution of solutes across biological membranes.
What are examples of membranes involved?
Plasma membranes.
Vacuoles.
Mitochondria.
Chloroplasts.
Endoplasmic reticulum (ER).
Sarcoplasmic reticulum (SR).
Lysosomes.
What is the importance of membrane transport?
Osmoregulation / turgor control.
Nutrient acquisition.
Waste excretion.
Compartmentalisation of metabolism.
Energy transduction.
Signal transduction.
What are the 3 classes of transport system?
Primary pumps.
Carriers.
Ion channels.
How do primary pumps work?
Use a primary source of energy to pump ions against electrochemical gradients
Primary active transport
Ions = usually H+ or Na+
Pumps are electrogenic.
Establish electrochemical gradients for ‘driver’ ions.
How do carriers work? (general)
Energised by ‘driver’ ion electrochemical gradients
Secondary active transport.
What are the classes of carriers?
Symporters / co-transporters
= move solute against gradient
Antiporters / counter-transporters
= get rid of waste solute in opposite direction
Facilitators / uniporters
= cannot be energised, allow facilitated diffusion passively down electrochemical / chemical gradients
How do ion channels work?
Transport is always passive down electrochemical gradients.
Usually highly regulated with defined open and shut kinetics = gating.
Usually selective for specific ions.
What are the 3 distinct components of signalling networks?
Perception of signal.
Intracellular signalling.
Cellular responses.
= ion channels involved in all 3
What are the 4 basic properties of ion channels?
Permeation.
Selectivity.
Gating.
Modulation.
What is permeation?
When a channel opens = permeant ions move through.
Direction governed by electrochemical gradient.
Rate governed by electrochemical gradient / intrinsic properties of channel pore.
What is selectivity?
Simple = distinction between cations and anions.
Other channels more complex = can distinguish between size
Non-selective = permeable to anions and cations.
What is gating?
The process of transition between the open and closed states is gating.
What is the frequency or opening / closing directly influenced by?
Changes in membrane voltage (voltage gated).
Binding of intracellular or extracellular ligands (ligand gated).
Mechanical stress (mechano-sensitive).
Changes in temperature.
Which substances can modulate gating of channels?
Ca2+, H+, ATP, Fatty Acids, Phosphorylation, G proteins.
= they influence gating following activation (indirectly)
How to clone ion channels?
Size fractionate on a gel to form different pools of mRNA and inject into oocytes (e.g. from Frog)
Electrophysiology.
Generate cDNA library (used to generate large amounts of mRNA).
Repeat fractionation and expression to isolate single mRNA.
Sequence mRNA - gene identified
e.g. anoctamins
What is another example of expression cloning?
Heterologous expression and complementation of yeast mutant growth phenotype
e.g. trk1D, trk2D mutant
What are the 6 principles of hydropathy analysis? (Topology)
(1) Assume hydrophobic polypetides in a hydrophobic environment (membrane) form apha helices.
(2) Membrane span is 6nm (equivalent to 20 a.a. in a helix)
(3) Each a.a. is given hydropathy index (reflects hydrophic/hydrophilic nature)
(4) Stretches of a.a. sequence are analysed by computer using a moving segment approach (20 a.a.)
(5) Hydropathy index is plotted
(6) Values over +1 are scored as forming transmembrane spans
