ELM4: Water, membranes and ions Flashcards
What is meant by the polarity of water?
The molecule has two different charges
Oxygen more electronegative so has more electrons so more negative
What is an ion?
Any atom or molecule that has gained or lost an electron which gives them charge
What are three uses of ions?
Carry signals in body
Act as energy store
Interact biochemically with proteins and other molecules
What are the two ways that ions can be biologically important?
Physiologically
Biochemically
What happens when a positive ion is placed in water?
Attracts oxygen from water molecules
Forms shell of water molecules around the ion
What happens when you put a negative ion in water?
Attracts all the hydrogen from water molecules
What is the relationship between the size of an ion and the hydration shell?
Small ions have larger hydration shells
What is the structure of membranes?
Lipid bilayers
Hydrophilic head and hydrophobic tail drives the formation
What is active transport?
Movement against concentration gradient using energy
What is passive transport?
Movement with the concentration gradient not using energy
How do pumps work?
Primary active transport using energy from ATP
What are some features of pumps?
Couple to ATP
Slow
Usually move cations
What is an example of a pump?
Sodium potassium ATPase
Generates calcium and potassium gradient
How do cotransporters work?
Secondary active transport
Use movement of ions to power transporters
How do ion gradients act as a source of energy?
Used to transmit information
Used to power cellular processes
What is a carrier/transporter protein?
Bind substrate to one side of the membrane
Conformational change to move it inside
What are some features of carrier/transporter proteins?
Maximum rate is 10000/s
Active or passive
What is an ion channel?
Continuous pathway to allow ions to pass through membrane
What are some features of an ion channel?
Rate of 1000000/s
Passive transport
All transmembrane proteins
Selectively permeable using selectivity filter
Controlled opening using gating
What are the two ways used to characterise and name an ion channel?
Gating
Ion selectivity
What are the two main groups of ligand gated ion channels?
Cys loop receptors
Ionotropic glutamate receptors
What are cys loop receptors?
All have the cys loop structure
Protein loop in extracellular domain linked by bond between two cysteine
S-S
What are some common features of cys loop receptors?
Pore to let ions through
Ligand binding side
Coupling mechanism
Desensitisation mechanism
How did Ca and Na channels evolve?
Originally there was one gene for K channels which replicated
Created copies of the gene strung together
What is the pore forming subunit in Ca and Na channels?
The alpha subunit
What is the structure of alpha 2 delta subunits in Ca and Na channels
- Gene makes a single polypeptide
- Disulphide bond formed in extracellular domain
- Extracellular domain cleaved to yield two linked peptides
What is the structure of a nicotinic acetylcholine receptors nAChR?
Pentamer
2 alpha subunits
1 beta
1 delta
1 epsilon
Arranged around central ion channels
How were studies performed to see which part of nAChR did different things?
- Subunits put into a plasmid expression vector
- Mutate single sections
- Introduce plasmid into mammalian cell to compare mutant receptor to wild type receptor
What is the synaptic role of acetylcholine binding protein AChBP?
- ACh released into synapse and binds to nAChRs and glial cells
- Glial cells release AChBP into synapse
- Acts as molecular sponge for acetylcholine and lowers levels in synapse
How is AChBP used as a model of other receptors (Homology modelling)?
- Amino acid sequences aligned to give best match
- Alignment used to map unknown protein onto 3D structure of known protein
- Gives structure of unknown protein
Why can AChBP be used as a model of other receptors?
Strong relationship with real nicotinic receptors
Acts as a template for homology modelling
What are the two types of ligand gated ion channels in prokaryotes?
ELIC and GLIC
Homopentamers
Unknown function
No cys loop structure
What is ELIC?
Cation selective channel
Activated by amines like GABA
What is GLIC?
Cation channel
Gated by protons
