Cell Signalling and G-Protein Coupled Receptors

Comprehensive vocabulary flashcards covering the components, mechanisms, and pathways of cell signalling, protein phosphorylation, and G-Protein Coupled Receptors (GPCRs).

First messenger

An extracellular signaling molecule (such as a hormone) that initiates a transduction pathway by binding to a specific cell-surface receptor.

Effector protein

A membrane-bound enzyme stimulated by a relay protein that produces secondary messengers to elicit metabolic or structural changes within the cell.

Second messenger

An intracellular signaling molecule whose concentration changes in response to the binding of an extracellular first messenger to a cell-surface receptor; examples include cAMP ($3',5'\text{-cyclic AMP}$), cGMP ($3',5'\text{-cyclic GMP}$), DAG ($1,2\text{-diacylglycerol}$), and $\text{IP}_3$ (inositol $1,4,5\text{-trisphosphate}$).

Signal transduction pathway

A chain of specific molecular interactions leading to various responses within the cell, including changes in enzyme activity, protein synthesis, and cellular movement across the nucleus, cytoplasm, or plasma membrane.

Signal amplification

A cascade process involving a series of enzymes (typically protein kinases) that do not get consumed in the reaction, enabling a minute concentration of ligand (e.g., $10^{-10}\text{ M}$ epinephrine) to exponentially produce large concentrations of active intracellular enzymes and products.

Protein kinase

An enzyme that transfers the terminal phosphate group of ATP to a side-chain hydroxyl group of a target protein (or autophosphorylates in the case of receptor tyrosine kinases).

Tyrosine kinase

A specialized protein kinase that specifically phosphorylates tyrosine amino acid residues situated on target proteins.

Protein phosphatase

An enzyme that catalyzes the hydrolytic removal of a phosphate group ($\text{P}_i$) from a phosphorylated protein, thereby providing reversibility to kinase-mediated regulatory cascades.

Serine (Ser)

An amino acid with a hydroxyl side chain ($-\text{CH}_2\text{OH}$) that serves as a common site for regulatory covalent phosphorylation via ester linkage.

Threonine (Thr)

An amino acid containing a secondary alcohol group on its side chain ($-\text{CH(OH)CH}_3$) that can covalently accept a phosphate group in an ester linkage during protein phosphorylation.

G-Protein Coupled Receptor (GPCR)

The largest class of cell-surface receptors, also known as Serpentine or $7\text{-TM}$ receptors, featuring an invariable 7-transmembrane domain, variable extracellular ligand-binding regions, and cytoplasmic loops that couple to intracellular G proteins.

Heterotrimeric G protein

A membrane-anchored trimeric protein complex composed of $\text{G}_\beta$, $\text{G}_\beta$, and $\text{G}_\beta$ subunits that transduces extracellular signals received by GPCRs to downstream cellular effectors.

$$\text{G}_\beta\text{ subunit}$$

The membrane-anchored subunit of a heterotrimeric G protein that possesses intrinsic GTPase activity, holds GDP in its resting state, and dissociates from the $\text{G}_{\beta\beta}$ complex upon conformational activation by GTP.

$$\text{G}_{\beta\beta}\text{ complex}$$

A dimeric G-protein regulatory unit in which the $\beta$ subunit binds the GTPase domain of the $\beta$ subunit to preserve its GDP-bound conformation, and the lipid-anchored $\beta$ subunit stabilizes the complex at the plasma membrane.

G-protein activation cycle

The pathway in which ligand binding triggers a GPCR intracellular loop to promote exchange of GDP for GTP on the $\beta\text{ subunit}$, causing dissociation into active $\text{G}_\beta\text{-GTP}$ and an active $\text{G}_{\beta\beta}$ complex until GTP hydrolysis terminates the signal.