WEEK 9 (Enzyme-coupled receptors) Flashcards
What are receptors?
- Receptors are proteins associated with cell membranes or located within the cell
- Receptors “recognise” signalling molecules by binding to them
- Binding of receptors by signalling molecules -> Cell behaviour changes
What are the three main classes of Cell-surface Receptors?
- Ion-channel-coupled receptors
- G-Protein-coupled receptors
- Enzyme-coupled receptors
What is the function of Ion-channel-coupled receptors?
They change the permeability of the plasma membrane to selected ions, altering the membrane potential and producing an electrical current
What are Transmitter-gated ion channels?
Ion-channel-coupled receptors that open in response to the binding of an extracellular signal molecule
Describe how Ion-channel-coupled receptors function in synapses
The released neurotransmitter binds to and opens the TRANSMITTER-GATED ION CHANNELS in the plasma membrane of the postsynaptic cell -> Resulting ion flows alter the MEMBRANE POTENTIAL of the postsynaptic cell -> Converts the chemical signal back into an electrical one
Where are Ion-channel-coupled and G-Protein-coupled receptors important in the body?
- ION-CHANNEL-COUPLED RECEPTORS = electrically excitable cells e.g muscle cells
- G-PROTEIN-COUPLED RECEPTORS = every cell type in the body
How do G-Protein-coupled receptors function?
G-protein-coupled receptor binds its extracellular signal molecule -> Activated receptor signals to a G protein on the opposite side of the plasma membrane -> Turns on/off an enzyme in the same membrane
What are G-proteins?
G proteins (GUANINE NUCLEOTIDE-BINDING PROTEINS) are a family of proteins that act as molecular switches inside cells and are involved in transmitting signals from a variety of stimuli outside a cell to its interior
[BOUND TO GTP = ON; BOUND TO GDP = OFF]
Describe the structure of G-Protein complexes
- Made up of alpha (α), beta (β) and gamma (γ) subunits
- Beta and gamma subunits can form a stable dimeric complex called the BETA-GAMMA COMPLEX
- The polypeptide chain traverses the membrane as seven α helices. The cytoplasmic portions of the receptor bind to a G-protein inside the cell.v
How many GPCRs are there in humans?
More than 700
What are G-protein-coupled receptors also called?
Seven-transmembrane receptors
What are the two most frequent target enzymes for G-proteins?
- ADENYLYL CYCLASE = produces the small intracellular signalling molecule CYCLIC AMP
- PHOSPHOLIPASE C = generates the small intracellular signalling molecules INOSITOL TRIPHOSPHATE and DIACYLGLYCEROL
How do G-proteins become activated?
An activated GPCR activates G proteins by promoting the α subunit to eject its GDP and pick up GTP
How is the G-protein anchored to the plasma membrane?
Both the α and γ subunits of the G protein have covalently attached lipid molecules that anchor it to the plasma membrane
How do G-proteins become inactivated?
Activated α subunit interacts with its target protein -> It activates that target protein for as long as the two remain in contact -> The G-protein α subunit switches itself off by hydrolysing its bound GTP to GDP