Human Physiology, Vander Ch.5

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Receptors

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(in sensory system) Specialized peripheral ending of afferent neuron, or separate cell intimately associated with it, that detects changes in some aspect of environment; (in intercellular chemical communication) specific binding site in plasma membrane or interior of target cell with which a chemical messenger combines to exert its effects

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Specificity

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

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Affinity

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Strength with which ligand binds to its binding site

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Saturation

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Occupation of all available binding sites by their ligand

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Competition

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Ability of similar molecules to combine with the same binding site or receptor.

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Antagonists

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(drug) molecule that competes with another for a receptor and binds to the receptor but does not trigger the cell’s response. (muscle) muscle whose action opposes intended movement

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Agonists

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Chemical messenger that binds to receptor and triggers cell’s response; often refers to drug that mimics action of chemical normally in the body.

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Down-regulation

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Decrease in number of target-cell receptors for a given messenger in response to a chronic high concentration of that messenger

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Up-regulation

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Increase in number of target-cell receptors for given messenger in response to chronic low extracellular concentration of that messenger.

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Supersensitivity

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Increased response to a ligand due to receptor up-regulation.

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Receptor activation

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Change in receptor conformation caused by combination of messenger with receptor.

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Signal transduction pathways

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

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Transcription factor

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One of a class of proteins that act as gene switches, regulating the transcription of a particular gene by activating or repressing the initiation process.

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First messengers

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Extracellular chemical messenger

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Second messengers

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Intracellular substance that serves as relay from plasma membrane to intracellular biochemical machinery, where it alters some aspect of cell’s function.

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Protein kinase

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

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Tyrosine kinases

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Protein kinase that phosphorylates tyrosine portion of proteins; may be part of plasma membrane receptor

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Guanylyl cyclase

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Enzyme that catalyzes transformation of GTP to cyclic GMP

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Cyclic GMP (cGMP)

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

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cGMP-dependent protein kinase

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

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JAK kinases

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Cytoplasmic kinase bound to a receptor but not intrinsic to it

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G proteins

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Family of regulatory proteins that reversibly bind guanosine nucleotides; plasma membrane G proteins interact with membrane ion channels or enzymes.

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Plasma membrane effector proteins

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Plasma-membrane protein that serves as ion channel or enzyme in signal transduction sequence

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Adenylyl cyclase

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Enzyme that catalyzes transformation of ATP to cyclic AMP

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Cyclic AMP (cAMP)

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

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Phosphodiesterase

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Enzyme that catalyzes cyclic AMP breakdown to AMP

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cAMP-dependent protein kinase

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

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Phospholipase C

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

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Diacylglycerol (DAG)

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

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Inositol trisphosphate (IP3)

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

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Protein kinase C

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Enzyme that phosphorylates certain intracellular proteins when activated by diacylglycerol

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Calmodulin

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

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Calmodulin-dependent protein kinases

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An intracellular enzyme that, when activated by calcium and the protein calmodulin, phosphorylates many protein substrates within cells; it is a component of many intracellular signaling mechanisms.

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Eicosanoids

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

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Arachidonic acid

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Polyunsaturated fatty acid precursor of eicosanoids

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Cyclic endoperoxides

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Eicosanoid formed from arachidonic acid by cyclooxygenase

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Prostaglandins

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One class of a group of modified unsaturated fatty acids (eicosanoids); function mainly as paracrine or autocrine agents

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Thromboxane A2

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Thromboxane that, among other effects, stimulates platelet aggregation in blood clotting

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Leukotrienes

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

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Phospholipase A2

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Enzyme that splits arachidonic acid from plasma membrane phospholipids

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Cyclooxygenase (COX)

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Enzyme that acts on arachidonic acid and initiates production of cyclic endo peroxides, prostalandins, and thromboxanes

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Lipoxygenase

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Enzyme that acts on arachidonic acid and leads to leukotriene formation

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Primary response genes (PRGs)

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Gene influenced by transcription factors generated in response to first messengers