- Signaling Molecules: small molecules that bind to larger receptors of specific target cells
- Hydrophilic ligands: cannot cross membrane and bind to membrane receptors
- Ex: proteins that are both large and polar
- Hydrophobic ligands: non-polar signaling molecules that can cross membrane and bind to intracellular receptors
- Receptors: specific proteins that receive ligands and trigger transduction
- Membrane receptors: transmembrane & amphipathic; consists of an extracellular ligand-binding site and an intracellular domain that initiates transduction pathway
- Intracellular receptors: proteins in cytoplasm or nucleus
- When a ligand binds to the cell-surface receptor, the inside part of the receptor “changes”
- Usually means it changes shape, which may make it active as an enzyme or let it bind to other molecules
G Protein-Coupled Receptors
- GPCRs: amphipathic, transmembrane protein that activates a G protein → which activates another membrane protein → triggers cellular response or activates second messenger
- G protein inactive with GDP and activated when GDP is replaced with GTP
Process Summary
- GPCR receives signal: specific messenger ligand binds to outward surface of receptor
- GPCR activates G protein: ligand binding activates GPCR → GPCR exchanges a GTP for the GDP on a nearby G protein → activates G protein
- G protein binds to and activities [membrane] effector protein:
- Effector protein initiates cellular response:
- Enzymatic activity: effector protein may be enzyme that catalyzes specific substrate → ex: protein kinase and initiate kinase cascade
- Produce second messenger cAMP:
- If effector protein is adenylyl cyclase, enzyme makes cAMP
- Pathway activates cytoplasmic protein (ex: protein kinase)
- Response may be stimulatory or inhibitory
- Produce second messenger IP3 and DAG:
- Produce second messenger CA2+:
- If effector protein is adenylyl cyclase, enzyme makes cAMP
- GPCR signaling/pathway is deactivated when GTP is hydrolyzed:
Receptor Tyrosine Kinases (RTKs)
- Kinases: Enzyme activated through phosphorylation and can activate a protein by catalyzing transfer of terminal phosphate from ATP to amino acid — tyrosine with this kind
- Protein Kinase A: ser/thr kinase activated by elevated cAMP levels
- Can receive growth factor → cell division → malfunctions = cancer
Process summary
- RTK receives signal: ligand binds to its outer surface
- RTK forms a dimer: two RTKs associate → form a pair (dimer)
- RTK is activated by autophosphorylation: many phosphates can attach
- Relay proteins are phosphorylated by RTK
- Relay proteins initiate transduction pathway: activated relay proteins are released → each relay protein can activate cellular response or initiate protein kinase transduction pathway → each cause different response
- RTK pathway deactivated by dephosphorylation or receptor protein packaged in vesicle (endocytosis)
GPCRs vs RTKs Pathways
- RTK directly responsible for initiating transduction pathway; GPCR indirectly activates transduction pathway via G protein and effector molecule
- RTK can trigger multiple transduction pathway → direct lots of coordinated responses;
GPCR triggers single pathway → single response
Ligand-Gated Ion Channels
- Gated Ion receptor: transmembrane channel protein that opens/closes in response to ligand binding to allow ions to pass thru
- Ligand-gated ion receptor receives signal: ligand binds to outward face
- Receptor channel opens and ions pass through: Ligand binding caused 3D shape of receptor to change → open or close channel → allows a specific ion to pass thru
- Ions initiate chemical response:
- Ligand-gated ion receptor deactivated when ligand detaches or enzymatically degraded: Ligand binding site can be blocked by allosteric ligand or channel blocker
- There are also voltage-gated ion receptors that open or close in response to voltage differences across the membrane
- Transmission of nerve impulse along neuron; Na+ enters → cell more + → if strong enough stimulates voltage-gated Na+ channel and then a voltage-gated K+ channel to open
- Some controlled by electrical signals, some in organelle membrane (ER)
- Ligand can block binding to stop diseases or open channel to allow flow of CA+, NA+, or K+
Example:
- Acetylcholine: neurotransmitter that transmits nerve impulses between nerve cells (neurons)
- Does not enter cytoplasm
- Acetylcholine binds to ligand-gated receptor molecules → opens gated channel → allows Na+ to enter cell → cells become more positive → change in membrane voltage (active potential) initiates nerve impulse → can stimulate muscle contraction