Lecture 7 Flashcards
What is cell signaling?
The process by which cells communicate to regulate behavior and function using signaling molecules.
Why do cells in multicellular organisms need to be regulated?
To maintain homeostasis, coordinate functions, and respond to environmental changes.
What are molecular switches in signaling?
Proteins that toggle between active and inactive states in response to signals, such as phosphorylation and GTP-binding proteins.
How do kinases function in cell signaling?
Kinases transfer phosphate groups from ATP to specific amino acids (serine, threonine, or tyrosine) on proteins, modifying their activity.
What are the five main types of cell signaling?
Endocrine – Long-distance signaling via hormones in the bloodstream.
Paracrine – Local signaling affecting nearby cells.
Autocrine – A cell signals itself.
Neuronal – Specific target signaling via neurotransmitters in synapses.
Contact-dependent (Juxtacrine) – Direct cell-to-cell contact through membrane-bound signals.
How do signaling responses vary?
Fast response – Alters protein activity already present in the cell.
Slow response – Requires changes in gene expression and protein synthesis.
What are the three main types of extracellular receptors?
Ligand-gated ion channels – Change membrane permeability to ions.
G-protein-coupled receptors (GPCRs) – Activate G-proteins to initiate intracellular signaling.
Enzyme-linked receptors – Activate enzymes or associate with them to trigger responses.
How do GPCRs function?
GPCRs bind extracellular ligands, triggering a conformational change.
This activates G-proteins, which then regulate ion channels or enzymes inside the cell.
How do receptor tyrosine kinases (RTKs) work?
RTKs dimerize upon ligand binding and autophosphorylate their tyrosine residues.
Phosphorylated tyrosines serve as docking sites for intracellular signaling proteins.
What are second messengers in signaling?
Small molecules that relay signals inside the cell, such as:
cAMP (cyclic AMP)
IP3 (inositol triphosphate)
Ca²⁺ (calcium ions)
How does the cAMP pathway work?
GPCR activation → Adenylyl cyclase converts ATP into cAMP.
cAMP activates Protein Kinase A (PKA).
PKA phosphorylates target proteins to regulate gene transcription and enzyme activity.
What is the role of nitric oxide (NO) in signaling?
NO is a small gas that diffuses across membranes and activates enzymes (e.g., guanylyl cyclase), leading to smooth muscle relaxation (vasodilation).
How does the PI3K-Akt pathway promote cell survival?
PI3K phosphorylates inositol phospholipids, activating Akt (PKB).
Akt phosphorylates Bad, inhibiting apoptosis and promoting cell survival.
What is the MAP Kinase (MAPK) pathway?
RTKs activate Ras, which triggers a kinase cascade.
MAP kinase (ERK) phosphorylates transcription factors, influencing cell proliferation and differentiation.
How do cells integrate multiple signals?
Through feedback mechanisms:
Positive feedback – Enhances signal strength (e.g., all-or-none responses).
Negative feedback – Reduces signal intensity (e.g., oscillatory responses).
How do G-proteins regulate ion channels?
Example: The acetylcholine GPCR pathway in heart muscle cells.
GPCR activation releases the βγ subunit of the G-protein.
The βγ subunit binds to a K⁺ channel, opening it and slowing the heartbeat.
How is signal duration controlled?
Dephosphorylation by phosphatases.
GTP hydrolysis in G-proteins (via RGS proteins).
Receptor internalization and degradation.
What is the role of nerve growth factor (NGF)?
NGF is a polypeptide growth factor that promotes neuron survival and branching.
It activates Ras/MAPK and PI3K/Akt pathways.
How does epinephrine regulate metabolism?
Epinephrine binds to GPCRs, increasing intracellular cAMP.
cAMP activates PKA, which promotes glycogen breakdown and inhibits glycogen synthesis.
How do steroid hormones (e.g., cortisol) act as nuclear receptors?
Hydrophobic steroid hormones cross the plasma membrane.
They bind intracellular receptors that act as transcription factors, altering gene expression.
