5.2 Signal Transduction Pathways

Adenylyl Cyclase

Enzyme that catalyzes transformation of ATP to cyclic AMP

Aspirin

Inhibits cyclooxygenase, therefore blocking the synthesis of the endoperoxides, prostaglandins, and thromboxanes

Calmodulin

Intracellular calcium-binding protein that mediates many of calcium’s second-messenger functions

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 mechanisms

cAMP-Dependent Protein Kinase (Protein Kinase A)

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

cAMP Phosphodiesterase

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

cGMP-Dependent Protein Kinase

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

Cyclic AMP (cAMP)

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

Cyclic Endoperoxides

Eicosanoids formed from arachidonic acid by cyclooxygenase

Cyclic GMP (cGMP)

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

Cyclooxygenase (COX)

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

Diacylglycerol (DAG)

Second messenger that activates Protein Kinase C, which then phosphorylates a large number of other proteins

Eicosanoids

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

First Messengers

Extracellular chemical messengers such as hormones

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

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

Guanylyl Cyclase

Enzyme that catalyzes transformation of GTP to cyclic GMP

Inositol Triphosphate (IP3)

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

Janus Kinases (JAKs)

Cytoplasmic kinases bound to a receptor but not intrinsic to it

Leukotrienes

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

Lipoxygenase

Enzyme that acts on arachidonic acid and leads to leukotriene formation

Nonsteroidal Anti-Inflammatory Drugs (NSAIDs)

Drugs that block cyclooxygenase; major uses are to reduce pain, fever, and inflammation, inducing expression of a protein that inhibits phospholipase A2

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

Phospholipase A2

Enzyme that splits arachidonic acid from plasma membrane phospholipid

Phospholipase C

Receptor-controlled plasma membrane enzyme that catalyzes phosphatidylinositol biphosphate (PIP2) breakdown to inositol triphosphate (IP3) and diacylglycerol (DAG)

Prostaglandins

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

Protein Kinase

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

Protein Kinase C

Enzyme that phosphorylates certain intracellular proteins when activated by diacylglycerol

Receptor Activation

Change in receptor conformation caused by combination of messenger with receptor

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

Second Messengers

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

Signal Transduction Pathways

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

Thromboxanes

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