Key Steps in Signal Transduction Pathways

Primary messenger

A signaling molecule (ligand) that binds to specific receptor on cell surface or inside cell.

Reception

The process where the receptor accepts and then undergoes conformation changes and activates intracellular signaling molecules.

Secondary messenger

Molecules like cAMP, DAG, IP3, and CA2+ that relay information to result in protein kinase A or C activation.

Amplification

The process where a small number of signaling molecules can produce a large cellular response.

Termination

The process of turning off the signal through mechanisms like ligand degradation, receptor internalization, or deactivation of proteins.

GPCR

G Protein Coupled Receptors, a class of membrane receptors.

RTKs

Receptor Tyrosine Kinase, a class of membrane receptors.

Ligand-Gated Ion Channels

A class of membrane receptors that open in response to binding of a ligand.

Epinephrine

The chemical signal that activates the β-adrenergic receptor.

G protein

A Guanine nucleotide-binding protein that acts as a molecular switch inside cells.

Activation of G protein

Occurs when a ligand binds to G Protein Coupled Receptor, causing the receptor to activate the G protein by exchanging GDP to GTP on the alpha-subunit.

Result of G protein activation

The alpha subunit with GTP and the Beta subunit + Gamma subunit dimer can interact with other proteins like AC (Adenyl Cyclase) to produce secondary messengers.

cAMP

Cyclic AMP, a second messenger activated by epinephrine binding and converted from ATP.

Kinase

An enzyme protein that adds a phosphate group (PO4^3-) to target molecules, usually proteins, in the process of phosphorylation.

PKA

Protein Kinase A, which is activated by cAMP.

Degradation of cAMP

Phosphodiesterase enzyme breaks down cAMP into AMP, stopping the activation of PKA.

Deactivation of G Protein

The G-alpha-subunit hydrolyzes GTP to GDP, making the G protein inactive.

Signal amplification

When a single signaling molecule leads to a huge response inside the cell due to cascading activation.

Steps of signal amplification

1 receptor activates many G proteins; each G protein activates adenylyl cyclase; adenylyl cyclase makes many cAMP molecules; each cAMP activates multiple PKA enzymes; each PKA phosphorylates multiple targets.

Receptor dimerization

The process where two RTKs come together to form a dimer upon ligand binding.

Autophosphorylation

Each RTK phosphorylates tyrosine residue on the other receptor's intracellular domain (cross-phosphorylation).

Signal relay by RTKs

Phosphorylated tyrosines act as docking sites for downstream signaling proteins (Grb2, SOS, or PI3K) which propagate the signal inside the cell.

Cellular response from RTKs

Can lead to cell growth, division, differentiation, or survival depending on the context.