Ch. 5 - Cell Signaling

Receptor

(for messengers) protein either on cell surface, in the cytosol, or in the nucleus that binds a chemical messenger such as a hormone or neurotransmitter and mediates its actions

(in sensory system) specialized peripheral ending of afferent neuron, or separate cell intimately associated with it, that detects changes in some aspect of environment

Signal transduction

The process by which a messenger molecule initiates a sequence of intracellular events that leads to a cell’s

Specificity

Selectivity; ability of binding site to react with only one, or a limited number of, types of molecules

Affinity

The strength with which a chemical messenger binds to its receptor

Saturation

The degree to which receptors are occupied by messengers (ex. if all are occupied, the receptors are FULLY saturated)

Competition

The ability of different molecules to compete with a ligand for binding to its receptor; competitors are generally similar in structure to the natural ligand

Antagonist

A molecule that competes with a ligand for binding to its receptor but does NOT activate signaling normally; prevents the action of the natural ligand (ex. certain types of antihistamines)

Agonist

A chemical messenger that binds to a receptor and triggers the cell’s response; mimics a normal messenger’s action (ex. decongestants)

Down-regulation

A decrease in total number of target-cell receptors for a given messenger; may occur in response to chronically high extracellular concentration of the messenger

Up-regulation

An increase in total number of target-cell receptors for a given messenger; may occur in response to chronically low extracellular concentration of the messenger

Increased sensitivity

Increased responsiveness of a target cell to a given messenger; may result from up-regulation of receptors

Internalization

Down-regulation of plasma membrane receptors by receptor-mediated endocytosis

Receptor activation

Change in receptor conformation caused by combination of messenger with receptor

Signal transduction pathways

Sequences of mechanisms that relay information from plasma membrane receptor to cell’s response mechanism

Nuclear receptors

Members of a family of receptor proteins that are localized in cell nuclei, or which are transported to the nucleus upon activation; include the steroid and thyroid hormone receptors

First messengers

Extracellular chemical messengers such as hormones

Second messengers

Intracellular substances that serve as relays from plasma membrane to intracellular biochemical machinery, where they alter some aspect of cell’s function

Protein kinase

Any enzyme that phosphorylates other proteins by transferring to them a phosphate group from ATP

Receptor tyrosine kinases

The major types of receptor proteins that are themselves enzymes; these receptors are on plasma membranes and respond to many different water-soluble chemical messengers

Cyclic GMP (cGMP)

Cyclic 3′,5′-guanosine monophosphate; cyclic nucleotide that acts as second messenger in some cells

Guanylyl cyclase

Enzyme that catalyzes transformation of GTP to cyclic GMP

cGMP-dependent protein kinase

Enzyme that is activated by cyclic GMP and then phosphorylates specific proteins, thereby altering their activity

janus kinases (JAKs)

Cytoplasmic kinases bound to a receptor but not intrinsic to it

G proteins

Proteins from a family of regulatory proteins that reversibly binds guanosine nucleotides; plasma membrane G proteins interact with membrane ion channels or enzymes

G-protein-coupled receptors

Cell membrane proteins that bind an extracellular signal and then activate an associated G protein, leading to activation of another protein such as adenylyl cyclase

Adenylyl cyclase

Enzyme that catalyzes transformation of ATP to cyclic AMP

Cyclic AMP (cAMP)

Cyclic 3′,5′-adenosine monophosphate; cyclic nucleotide that serves as a second messenger for many “first” chemical messengers

cAMP phosphodiesterase

An enzyme in all cells that converts cAMP into an inactive molecule of AMP

cAMP-dependent protein kinase

Enzyme that is activated by cyclic AMP and then phosphorylates specific proteins, thereby altering their activity; also called protein kinase A

phospholipase C

Receptor-controlled plasma membrane enzyme that catalyzes phosphatidylinositol bisphosphate breakdown to inositol trisphosphate and diacylglycerol

diacylglycerol (DAG)

Second messenger that activates protein kinase C, which then phosphorylates a large number of other proteins.

inositol trisphosphate (IP3)

Second messenger that causes release of calcium from endoplasmic reticulum into cytosol

protein kinase C

Enzyme that phosphorylates certain intracellular proteins when activated by diacylglycerol

calmodulin-dependent protein kinases

Intracellular enzymes that, when activated by calcium and the protein calmodulin, phosphorylate many protein substrates within cells; they are components of many intracellular signaling mechanism

eicosanoids

General term for modified fatty acids that are products of arachidonic acid metabolism (cyclic endoperoxides, prostaglandins, thromboxanes, and leukotrienes); function as paracrine or autocrine substances

cyclic endoperoxides

Eicosanoids formed from arachidonic acid by cyclooxygenase.

prostaglandins

Members of one class of a group of modified unsaturated fatty acids (eicosanoids) that function mainly as paracrine or autocrine factors

thromboxanes

Eicosanoids derived from arachidonic acid by the action of cyclooxygenase; among other functions, involved in platelet aggregation

leukotrienes

Type of eicosanoid that is generated by lipoxygenase pathway and functions as inflammatory mediator

phospholipase A2

Enzyme that splits arachidonic acid from plasma membrane phospholipid

cyclooxygenase (COX)

Enzyme that acts on arachidonic acid and initiates production of cyclic endoperoxides, prostaglandins, and thromboxanes

lipoxygenase

Enzyme that acts on arachidonic acid and leads to leukotriene formation