Introduction to Homeostasis and G-Protein Signaling

Vocabulary and key concepts covering homeostasis, feedback mechanisms, and G-protein coupled signaling pathways from the introductory lecture.

Homeostasis

The ability to maintain stable internal conditions despite changes occurring outside or within the body.

Mechanism

A process, pathway, or function occurring at a physical or chemical level as a response to an environmental change.

Dynamic Equilibrium

A state of balance where there is a limited range of tolerated deviations from an average value.

Set Point

The average value around which a homeostatic mechanism maintains a physiological condition.

Negative Feedback

A corrective process where a stimulus elicits a response that decreases the initial stimulus, effectively turning off the loop.

Positive Feedback

A self-amplifying cycle where an initial stimulus evokes a response that increases the stimulus, causing rapid changes or switching conditions.

Hypoxia

A condition characterized by having low oxygen levels in the tissues.

Hypoxemia

A condition characterized by having low oxygen levels in the blood.

ATP ($Adenosine \text{ } Triphosphate$)

A nucleotide containing high-energy phosphate bonds that serves as the primary energy currency inside cells.

GTP ($Guanosine \text{ } Triphosphate$)

A molecule important for regulating cellular processes such as protein synthesis, microtubule assembly, and signaling through G-proteins.

Kinases

A group of enzymes that phosphorylate substrates by transferring a phosphate group from ATP to a protein.

Adenylyl Cyclase

An enzyme that catalyzes the conversion of ATP into cyclic AMP ($cAMP$).

Cyclic AMP ($cAMP$)

An intracellular second messenger that activates protein kinase A ($PKA$) and can open nucleotide-gated ion channels.

Guanylyl Cyclase

An enzyme that catalyzes the production of cyclic GMP ($cGMP$), which can activate protein kinase G ($PKG$).

GEF ($Guanine \text{ } nucleotide \text{ } exchange \text{ } factor$)

A protein that promotes the release of GDP from a G-protein to allow the binding of GTP for activation.

GAP ($GTPase \text{ } activating \text{ } protein$)

A protein that promotes the hydrolysis of GTP into GDP and inorganic phosphate, inactivating the G-protein.

GPCR ($G-protein \text{ } coupled \text{ } receptor$)

A seven-transmembrane receptor that acts as a GEF when a ligand binds to its extracellular site.

Heterotrimeric G-protein

A large signaling protein consisting of three different subunits: alpha ($\textstyle \boldsymbol{\textstyle \text{alpha}}$), beta ($\textstyle \boldsymbol{\textstyle \text{beta}}$), and gamma ($\textstyle \boldsymbol{\textstyle \text{gamma}}$).

$G_s$ Family

A family of G-alpha proteins that stimulates the activity of adenylyl cyclase.

$G_i$ Family

A family of G-alpha proteins that inhibits adenylyl cyclase and may also affect potassium channels via the beta-gamma subunit.

$G_q$ Family

A family of G-alpha proteins coupled to the enzyme phospholipase C ($PLC$), often involving calcium signaling.

Pertussis Toxin

A toxin produced by Bordetella pertussis that prevents $G_i$ from interacting with adenylyl cyclase, leading to whooping cough.

Cholera Toxin

A toxin from Vibrio cholerae that inhibits the GTPase activity of $G_s$, resulting in excessive cyclic AMP and severe diarrhea.

Signal Amplification

The process where the activation of a single receptor leads to the production of high quantities of second messengers and the activation of many enzymes.