Unit 1 - Membrane Proteins and Signalling Flashcards
Cyclic GMP (cGMP)
Binds to ligand gated sodium channels and keeps them open. When hydrolysed to GMP by the enzyme PDE the sodium channels close. This causes a build up of sodium ions ouside the cell resulting in hyperpolarisation and the generation of a nerve impulse.
Transducin
A G-protein that is activated by photoexcited rhodopsin. One rhodopsin activates many G-proteins, creating a cascade. Each transduction activates one molecule of the enzyme phosphodiesterase (PDE).
Rhodopsin
A retinal-opsin complex embedded in photoreceptor membranes (rods and cones) in the retina. Changes conformation to become photoexcited rhodopsin when retinal absorbs light, triggering a cascade that amplifies the photon signal.
Retinal
A light sensitive molecule derived from vitamin A that captures light energy. It is a prosthetic group which binds to the protein opsin.
Transporter proteins
Bind to the ions or molecules to be transported and undergo a conformational change. Can be facilitated diffusion or active transport.
Voltage gated channels
Change conformation as a result of a change in ion concentration (voltage) across a membrane eg. during nerve impulse transmission
Ligand gated channels
Specific channels that change conformation and open when a signal molecule bonds to it. eg. sodium channels in the retina and at synapses
Channel proteins
Selective proteins that allow facilitated diffusion of ions and polar molecules through the membrane without changing their conformation.
Specific transmembrane proteins
Channels or transporter proteins that span the membrane and allow ions or polar molecules through.
Small non-polar molecules
Include oxygen and carbon dioxide. They are able to pass freely through the phospholipid bilayer. Ions and polar molecules cannot.
Peripheral proteins
Are not embedded in the phospholipid bilayer but are bound to the membrane surface by ionic and hydrogen bonding with hydrophilic R groups on their surface.
Integral proteins
Penetrate the hydrophobic interior of the membrane and are held there by strong hydrophobic interactions between the phospholipid tails and hydrophobic R groups of the amino acids.
Phospholipid bilayer
A double layer of phospholipids with the hydrophobic tails pointing inwards and the hydrophilic heads pointing towards the aqueous cytoplasm and external fluid surrounding the cell.
Fluid mosaic model
Cell membranes have this structure - a phospholipid bilayer containing integral or peripheral globular proteins arranged in a ‘patchwork’ or ‘mosaic’
Sodium-potassium pump (Na+/K+ATPase)
Transmembrane transporter found in most animal cells.
Actively transports sodium ions out of cells and potassium ions into cells along steep concentration gradients, using energy from hydrolysis of ATP.
3 sodium go out, 2 potassium come in (3-S-O-P-I-2)
The pump has 2 stable conformational states with differing affinities for the 2 ions.
Glucose symport
Found in the small intestine. The sodium-potassium pump generates a sodium ion gradient across the membrane, which drives the active transport of glucose.
The glucose transporter transports sodium ions and glucose at the same time, in the same direction (from outside the cell, back in) but sodium ions are moving down a concentration gradient, glucose against.
Receptor
Globular protein with a binding site for a specific signaling molecule.
Found on target cells. Binding of the signalling molecule changes the conformation of the receptor, and initiates a response in the target cell.
Hormone
Extracellular signalling molecule secreted by an endocrine gland into the blood.
The hormone circulates in the blood until it reaches the receptor of its target cell, or is broken down.
Hydrophobic hormones
Hydrophobic hormones are steroid hormones such as oestrogen and testosterone.
Hydrophilic hormones
Peptide hormones such as insulin, glucagon, growth hormone (somatotrophin) and ADH (anti-diuretic hormone).
Neurotransmitters
Substances released into the synaptic cleft (the gap between nerve cells).
They are hydrophilic signalling molecules that allow transmission of a nerve impulse. eg acetylcholine and noradrenaline.
Hydrophobic signals
They are lipid soluble and pass through membranes, binding to intracellular receptors in the cytosol or nucleus of the target cell. eg steroid hormones
The receptors bind to their specific hormone and act as transcription factors, binding to DNA and stimulating or inhibiting transcription.
The hormone-receptor complex binds to specific DNA sequences - hormone response elements (HRES) which influences the rate of transcription.
Hydrophilic signals
Peptide hormones and neurotransmitters.
They are unable to cross the hydrophobic part of the plasma membrane, so bind instead to transmembrane receptor molecules.
This results in signal transduction.
Signal transduction
Transmembrane receptor molecules for specific hydrophilic signalling molecules change conformation when the signal ligand binds.
The signal is transduced - the behaviour of the target cell changes in response to the binding of the signalling molecule. The signal does not enter the cell.
Transduced signals may involve G-proteins or cascades of phosphorylation by kinases.
G-proteins
Relay signals from activated receptors (receptors with a signalling molecule attached) to target proteins such as enzymes and ion channels.
This activates a signalling pathway inside the cell.
Phosphorylation cascade
Allows more than one signalling pathway to be activated inside the cell.
One kinase activates the next one in the sequence and so on, resulting in the phosphorylation of many proteins.
Insulin receptor
A kinase linked receptor in the plasma membrane of fat and muscle cells.
Once insulin binds, the signal is transduced triggering a phosphorylation cascade that triggers the recruitment of GLUT4 glucose transporters to the plasma membrane.
The transporters allow glucose to enter the cell, reducing blood glucose level.
Diabetes mellitus - type 1
Loss of control of blood glucose level caused by a failure to produce insulin.
Type 1 is treated with insulin injections.
Diabetes mellitus - type 2
Caused by loss of receptor function (type 2).
Associated with obesity - cells are no longer sensitive to insulin.
Exercise triggers recruitment of GLUT4, therefore improving uptake of glucose to fat and muscle cells, thus reducing blood glucose levels.
Resting membrane potential
A state where there is no net flow of ions across a membrane.
The sodium-potassium pump moves 3 sodium ions out, and 2 potassium ions into a neuron.
Potassium channels allow some of the potassium to leak back out.
This results in a positive charge outside the cell, compared to the inside.
Resting potential = -70mV
Action potential
A wave of electrical excitation along the plasma membrane of a neuron.
Depolarisation
A sudden reversal of the resting potential, caused by an influx of sodium ions through a ligand gated ion channel.
The ligand gated channel opens due to the binding of a neurotransmitter.
If there is sufficient ion movement, it triggers voltage gated sodium channels in the next section of membrane to open, causing it to depolarise. The effect travels like a wave along the neuron.
Repolarisation
Once a critical voltage is reached, the voltage gated sodium ion channels close, voltage gated potassium ion channels open and potassium ions diffuse out of the neuron restoring resting potential.
Once resting potential is reached, the potassium channels close, and the sodium-potassium pump (which keeps working throughout) resets the ion gradients.
Synapse
The gap between neurons. The wave of depolarisation triggers voltage gated calcium ion channels to open.
The influx of calcium ions triggers the release of hydrophilic neurotransmitters into the gap, via the secretory pathway.
The neurotransmitters bind to ligand gated sodium channels in the membrane of the target cell, opening them and causing depolarisation and transmission of the nerve impulse.
Photopsin
Retinal combines with different forms of opsin, so absorb light of different wavelengths. This results in colour vision in cone cells.
