Module 4 - Water, ions, and membranes Flashcards
What are the two classes of ions?
Physiologically useful ions:
Carry a charge (Na⁺, K⁺, Cl⁻)
Biochemically useful ions:
Take part in enzymic reactions or make parts of proteins (Mg²⁺)
Some may be physiologically and biochemically useful (Ca²⁺)
Hydration shell: what is it and how do different ions affect it?
The cluster of water molecules around an ion in water
The effective size of an ion affects it - higher charge density causes a bigger hydration shell, causing more water molecules to surround the ion, reducing mobility (meaning smaller ions have a larger)
Membrane permeability
Ions and moderately sized polar molecules find it challenging to pass through the lipid bilayer
Small molecules like water and ethanol pass through the bilayer readily
LGIC: what do they do, where are they located, what are examples, and what are the key characteristics?
Ligand-gated ionic channels change the membrane potential after two agonists bind to two receptor sites and open the channel
Transmembrane
Nicotinic-acetylcholine receptors, Cys loop receptors
- They are all multisubunit receptors
- Can allow fast signalling
- Several structurally different types
- Desensitisation mechanism - if the ligand is bound for too long then the channel closes
nAChR: what is it, what is its frequency in mammals, what subunits does it have, and how has its effect been investigated?
Pentamic ligand-gated ion channel which allows fast neurotransmission
There are 17 types in mammals but only 16 are present in humans and large amounts of it are in electric organs in fish like the Torpedo fish
Two α, one β, one ε, and one δ, (instead a γ in the foetus)
By mutating genes, the effect of certain amino acids within the sequence can be investigated
Removal of acetylcholine in snails
As well as acetylcholinesterase removing acetylcholine from synapses, acetylcholine binding protein (AChBP) is released from nearby glial cells to ‘sponge’ up ACh
AChBP
Acetylcholine binding protein which is structurally identical to the N-terminal extracellular domain of nAChR (both are pentameric and have the same structure)
nAChR and GABAa name changes
Referred to as ligand-gated ion channels until the 1980s when tetrameric, structurally dissimilar to nicotinic receptors ionotropic glutamate receptors were discovered
To get around this, a new classification method was used: Cys loop receptors. Cys loop receptors involve a disulfide bridge between two cysteine amino acids forming a loop
How are receptors that do not have Cys loops classified?
Pentameric ligand-gated ion channels (pLGIC)
This is used to classify nicotinic receptor relatives that do not have a Cys loop
ELIC: what is it and how does its gating work?
Erwini chrysanthemi ligand-gated ion channels that are cation channels gated by amines
GLIC: what is it and how does its gating work?
Gloeobacter violaceus ligand-gated ion channels that are cation channels gated by protons
How are receptors investigated?
Before - mutations (long and difficult to assess direct cause-and-effect)
Now - crystallisation
What organisms do not have ligand-gated ion channels?
Plants - it was believed that prokaryotic organisms also did not; however, recent evidence shows that some do.
How do ion channels have high specificity?
The amino acid ring forming the gate has the opposite charge to the ion, drawing it through, and there are specific size restrictions that assure that only the correct ion passes through
Ion channel naming
Classified by gating and ion selectivity (Voltage-gated potassium channels)
If a natural ligand is present, then it is named after that (GABAa receptor)
