G-Protein Signaling and Second Messenger Systems

Cyclic AMP (cAMP)

A second messenger discovered by Earl W. Sutherland (Nobel Prize 1971) that transmits signals from membrane receptors to intracellular targets.

Earl W. Sutherland (1971)

Discovered cyclic AMP as a second messenger and won the Nobel Prize in Physiology or Medicine in 1971.

Adenylyl cyclase (AC)

A membrane-bound enzyme that converts ATP into cyclic AMP (cAMP).

Phosphodiesterase (PDE)

Enzymes that break down cyclic nucleotides (e.g., cAMP → AMP), terminating second-messenger signals; multiple families exist.

Sutherland–Rall criteria

Four experimental criteria used to identify cAMP is being used as a second messenger (AC present, cAMP changes precede response, concentration–response, PDE inhibition exaggerates response).

G-protein

Trimeric cytoplasmic protein (α, β, γ) that transduces signals from GPCRs to effectors like adenylyl cyclase.

Martin Rodbell & Alfred Gilman (1994)

Scientists awarded the 1994 Nobel Prize for elucidating the role of G-proteins in signal transduction.

GPCR (G-protein coupled receptor)

A seven-transmembrane receptor that activates heterotrimeric G-proteins upon ligand binding.

Ternary complex model

Model where agonist, receptor, and G-protein form a three-component complex to produce signaling.

GDP (guanine diphosphate)

The inactive guanine nucleotide bound to the α subunit of G-protein in its resting state.

GTP (guanine triphosphate)

The active nucleotide that binds the G-protein α-subunit after GDP dissociates, enabling effector activation.

Gα-GTP

The active α subunit of a G-protein (with GTP) that interacts with effectors like AC or PLC.

GTPase activity

The intrinsic enzymatic activity of the Gα subunit that hydrolyzes GTP → GDP, terminating signaling.

Gs protein

The stimulatory G-protein α subunit that activates adenylyl cyclase to increase cAMP.

Gi protein

The inhibitory G-protein α subunit that inhibits adenylyl cyclase, decreasing cAMP.

Gq protein

The G-protein that typically activates phospholipase C (PLC), leading to IP3 and DAG production.

Cholera toxin

A bacterial toxin that inhibits Gα GTPase activity (prevents GTP hydrolysis), causing persistent activation of AC and elevated cAMP.

Protein Kinase A (PKA)

A cAMP-dependent protein kinase (discovered contextually with Krebs & Fischer) that phosphorylates many cellular targets.

Edwin Krebs & Edmond Fischer (1992)

Scientists awarded the 1992 Nobel Prize for discovering reversible protein phosphorylation and kinase cascades.

Phosphorylation

Addition of a phosphate group (usually by kinases) to a protein, often modulating activity or localization.

Phosphatase

Enzyme that removes phosphate groups, reversing kinase actions (dephosphorylation).

Amplification cascade

Sequential activation where one upstream event (e.g., ligand binding) produces a large downstream effect via enzyme cascades.

IP3 (Inositol 1,4,5-trisphosphate)

A soluble second messenger generated by PLC that triggers Ca²⁺ release from the endoplasmic reticulum.

DAG (Diacylglycerol)

A membrane-bound lipid second messenger generated by PLC that activates protein kinase C (PKC).

Phospholipase C (PLC)

An enzyme activated by Gq that cleaves PIP2 into IP3 and DAG.

Protein kinase C (PKC)

A family of kinases activated by DAG (and Ca²⁺ in some isoforms) to phosphorylate target proteins.

Constitutive activity

The phenomenon where a receptor has baseline activity in the absence of ligand.

Inverse agonist

A ligand that preferentially stabilizes the inactive receptor conformation and reduces constitutive activity.

Partial agonist

A ligand that binds receptors and produces a less-than-full response even at full receptor occupancy.

Biased agonism

When different ligands stabilize different receptor conformations, selectively activating specific downstream pathways.

Orthosteric antagonist

An antagonist that binds the same active site as the endogenous ligand (competitive antagonism).

Noncompetitive/irreversible antagonist

An antagonist that binds irreversibly or prevents agonist action even with increasing agonist concentration.

Allosteric modulator

A ligand that binds a site distinct from the orthosteric site and modifies affinity or efficacy of the orthosteric ligand.

PAM (Positive Allosteric Modulator)

An allosteric ligand that enhances the effect (affinity or efficacy) of an orthosteric agonist.

NAM (Negative Allosteric Modulator)

An allosteric ligand that reduces the effect (affinity or efficacy) of an orthosteric agonist.

Probe dependence

A property of allosteric modulators where they affect different orthosteric ligands differently.

Saturability (allosteric)

Allosteric modulation can reach a ceiling effect because the allosteric site itself can be saturated.

Compartmentalization

Spatial restriction of signaling components within the cell creating microdomains of signaling (local cAMP or Ca²⁺ pools).

Stochastic binding

Random, probabilistic binding/unbinding of ligands to receptors at the single-molecule level.

Subcellular fractionation

A method to separate cellular components (membranes, cytosol, organelles) — key in discovering soluble second messengers.

Robert Lefkowitz & Brian Kobilka (2012)

Scientists awarded the 2012 Nobel Prize for GPCR structure and how receptors activate G-proteins.

Western blot

A technique to separate and detect proteins using gel electrophoresis and antibodies.

ELISA (Enzyme-Linked Immunosorbent Assay)

An assay to detect and quantify antigens or antibodies using enzyme-linked detection.

Immunohistochemistry

Technique to visualize protein localization within tissue sections using labelled antibodies.

PCR (Polymerase Chain Reaction)

Technique to amplify specific DNA sequences; foundation for many molecular assays (Kary Mullis, 1993 Nobel in Chemistry).

Patch clamp

Electrophysiological method to measure ion currents across membranes (Neher & Sakmann, Nobel 1991).

BLAST (Basic Local Alignment Search Tool)

Bioinformatic tool for comparing DNA or protein sequences to database entries to infer identity/function.

Sutherland experiment (fractionation)

Sutherland's approach of separating membrane and cytosolic fractions to identify a transferable mediator (cAMP).

Ceiling effect (allosteric)

The limit to modulation produced by an allosteric ligand due to saturation of the allosteric site.

Physiological antagonism

Opposing physiological effects produced by two agonists acting at different receptors or pathways.

Good signal characteristics

Specificity, small size, rapid synthesis & termination, and compartmentalization, enabling precise cellular information transfer.