3.1 Proteins & Biosignaling Flashcards
Biosignalling
process in which cells receive and act on signals
Ion channels
transmembrane proteins that provide a pathway for ions to enter the cell
facilitated diffusion of molecules down a concentration gradient
3 main groups of ion channels
ungated
voltage gated
ligand-gated
Ungated ion channels
ion channels that are always open
Voltage-gated channels
open or close based on membrane potential charge near the channel
Ligand-gated Channels
binding of a specific ligand to the channel causes it to open or close
ex. neurotransmitters bind to post-synaptic channels (ex. GABA opens chloride channels)
Enzyme-linked receptors
membrane receptors that also display catalytic activity in response to ligand binding
often participate in cell signaling through initiation of second messenger cascades
3 primary protein domains of enzyme-linked receptors
membrane-spanning domain
ligand-binding domain
catalytic domain
Membrane-spanning domain of enzyme-linked receptors
anchors the receptor in the cell membrane
Ligand-binding domain of enzyme-linked receptors
stimulated by the appropriate ligand and induces a conformational change that activates the catalytic domain
Catalytic domain of enzyme-linked receptors
activates cellular enzymes; initiates second messenger cascade
One of the most common enzyme-linked (catalytic) receptors
receptor tyrosine kinases (RTKs)
composed of a monomer that dimerizes upon ligand binding
the dimer is the active form that phosphorylates additional cellular enzymes (included the receptor itself)
G protein-coupled receptors (GPCRs)
3transmemebrane integral proteins that interact with G proteins to transmit signals to effector cells
how do GPCR’s work?
a ligand binds to the extracellular portion of the receptor and triggers and intercellular signalling cascade
G protein 3 subunits
alpha, beta, and gamma
3 main types of G proteins:
Gs - stimulates adenylate cyclase which i_ncreases levels of cAMP_ in the cell
Gi - inhibits adenylate cyclase which decreases levels of cAMP in the cell
Gq - activates phospholipase C which cleaves a phospholipid from the membrane to form PIP2; PIP2 is cleaved to DAG and IP3; IP3 opens calcium channels in the ER to increase calcium levels in the cell
“resting state” of G protein
all three subunits are attached (alpha, beta, gamma)
GDP is bound to alpha subunit
what happens to G protein followin ligand binding to GPCR?
GDP dissociates and GTP binds to alpha subunit
the alpha subunits separates from the beta-gamma dimer
what happens after ATP binds to the alpha subunit of G protein?
the alpha-GTP subunit and beta-gamma subunits bind to target proteins to initiate the signalling cascade
once the target protein is activated, it can relay a signal
