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WVSOM - Molecular-2
Transport Proteins, Receptor Biology, Signal Transduction
| Question | Answer |
|---|---|
| Receptor characteristics | Specificity (only recognize & bind one/few ligands), high binding affinity for ligand, saturable binding, reversible binding, tissue specific distribution, biological response |
| Characteristics of hydrophilic ligands | Cannot transverse membranes (bind extracellular domain of transmembrane receptor), transported free in blood plasma, easily stored in gland cell vesicles (preformed stock rapidly secreted as needed), rapid responses, short 1/2-life, rapidly cleared |
| Characteristics of hydrophobic ligands | Pass membranes to bind intracellular receptors, require binding proteins to be transported in blood, difficult to store in cells (synthesized as needed), slow responses, long 1/2-lives (hours to days) |
| Receptor regulation | Receptor number, affinity for ligand, affinity for intracellular targets, cellular localization, membrane fluidity |
| Receptor | A cellular factor that recognizes and binds a specific ligand to induce a response |
| Ligand | A molecule bound by another molecule such as a receptor |
| Isoreceptor | Different receptors bound and activated by the same ligand, often inducing distinct responses in different cells |
| Paracrine | For an intercellular signaling molecule to diffuse over a short distance, usually through interstitial spaces, to induce a response |
| Endocrine | For an intercellular signaling molecule to diffuse through the blood |
| Intercellular signaling molecule | A non-nutrient ligand secreted by one cell to induce a response in another cell (ex: hormone, growth factor, cytokine) |
| Hormone | An intercellular signaling molecule that controls metabolism or physiology |
| Growth factor | An intercellular signaling molecule that controls cell cycle progression, cellular differentiation, or morphogenesis during development |
| Cytokine | A growth factor involved with hematopoiesis (immune system) |
| Hydrophobic ligands | Steroids (cholesterol derivatives) and FAT derivatives (prostaglandins, retinoic acid) |
| Hydrophilic ligands | AA derivatives (catecholamine-tyrosine, histamine-histidine, serotonin-tryptophan), peptides and proteins (insulin, glucagon, FSH, TGF-beta, etc.), nucleotides (ADP for platelets, cAMP for slime molds), NO CARBS! |
| Kinetics | Receptor characteristics (binding rate, affinity, saturability, reversibility) quantified with formulae (Scatchard analysis) derived from Michaelis-Menten equation from enzyme kinetics |
| Nuclear receptors | "Steroid receptors", hydrophobic ligands, cyto-nucleoplasmic (NOT transmembrane), zinc-finger TF; constitute gene family (similar sequences, similar structures, same basic mechanism) |
| Nuclear receptor sub-families | Glucocorticoid family (glucocorticoid & testosterone receptors), estrogen receptor, non-steroid family (thryoid hormone, retinoic acid); members of subfamilies related to each other than to members of other 2 families |
| Types of intercellular signaling molecules | Nuclear receptors and transmembrane receptors |
| Transmembrane receptors | Binding for hydrophilic ligands; 3 domains: ligand binding domain (extracellular side), hydrophobic transmembrane domain (spans membrane), signal transduction domain (cytoplasmic side) |
| Extracellular ligand binding domain | Extracellular side of membrane, domain binds ligand or may be bound by ligand, ligands with binding activities are usually proteinaceous hormones or growth factors, receptor binding domains recognized by ligands = short oligosaccharides |
| Transmembrane domain | Able to propagate signal from ligand on extracellular side of membrane to cytoplasm; receptor must transverse lipid bilayer |
| Signal transduction domain | Receptor must be able to interact with signal transduction factors on cytoplasmic side; usually enzymatic reaction |
| Transport protein receptors | Receptors that act as transport molecules; ligand binding opens passageways, allows specific factors to pass through; passageway considered signal transduction domain (i.e. ligand-gated channels) |
| Ligand-gated channel | Neural transmitter = ligand; induce channel opening (closing), allowing specific ions to pass through membrane; action potential propagated from one cell to another |
| Passive diffusion | Spontaneous movement of molecules without expenditure of energy, down concentration gradient (high-low concentration), move with electric potential (to area with opposite charge) |
| Facilitated diffusion | Passive diffusion through specific transmembrane proteins (channels or transporters) |
| Facilitated diffusion v. passive diffusion | Facilitated diffusion = specific (only certain molecules or ions to pass), faster, saturable (limited number of channels or transporters) |
| Vmax | Maximal diffusion rate when all transport proteins are occupied |
| Active transport | Forced movement across a membrane, driven by expenditure of energy (usually ATP hydrolysis), against concentration gradient / electric potential |
| Transport proteins | Facilitated diffusion or active transport; only allow certain factors to pass; 1. transmembrane domain, 2. polar head domain (interact with membrane), 3. hydrophobic face (interact with lipid bilayer), 4. hydrophilic passageway |
| Types of transport proteins | Channels, transporters, ATPase pumps |
| Channels | Mediate facilitated diffusion (single file flow of factors through protein), selective (only allow one type of factor to pass), gated (close in response to stimuli) |
| Transporter | Pass one factor (or set of factors) at a time by conformational change of protein |
| ATPase pumps | Mediate active transport by directly hydrolyzing ATP; four types (P class, F class, V class, ABC class) |
| Types of transporters (CAREFUL: NOT transport proteins) | Uniporters, symporters, antiporters |
| Uniporter | Transport one factor across membrane |
| Symporter | Simultaneously transport two factors in same direction |
| Antiporter | Simultaneously transport factors in opposite directions; used to drive compounds down [ ] gradient (or electric potential) to provide energy to drive another compound against gradient; energy stored in membrane by separate ATPase that establish gradient |
| P class ATPase | Ion pumps (Na+/K+ ATPase); energy from ATP hydrolysis (phosphorylates residue on subunit of pump); induces conformational shift allowing factor to pass against gradient |
| F class ATPase | Proton pumps (mitochondria); produce ATP; H+ through pumps (high to low []); proton current drives ADP phosphorylation (i.e. reverse ATPase) |
| V class ATPase | Proton pumps (lysosomes, osteoclasts); lower pH; produce acidic lumens; separate transport proteins required to import neg. charge or export positive charge (required for lowering pH) |
| ABC class pumps | ATP binding cassette; largest and most diverse class of ATPase pumps; transport wide range of factors; examples = multidrug-resistance protein (MDR-1) and Cystic Fibrosis transmembrane conductance regulator (CFTR) |
| ABC ATPase: Multidrug-resistance protein (MDR-1) | Excretes hydrophobic compounds from tissues (ex: liver); toxins hydrophobic; MDR-1 detoxifies cells; drugs hydrophobic (also excreted); identified in tumor & tissue cultures (acq. resist. to hydrophobic chemo; mutations caused cell to overexpress pump) |
| ABC ATPase: CFTR | Functions as channel; ATP hydrolysis req. to open channel (once open, ions flow passively); regulatory domain (phosphorylated by cAMP signaling to open channel) |
| CFTR in lung | CFTR open=Cl- to flow into mucus lining airways; phosphorylated=Na+ channel repressor (few Na+ diffusing into epithelial cells); effect=raise [NaCl] in mucus, raise osmotic pressure, draw H2O into mucus, raise mucus fluidity, ciliary beating clears mucus |
| Cysic fibrosis | No functional CFTR; [Na+], [Cl-] not raised in mucus, collects in airways, too viscous for cilia to move; airway obstructions; growth of Pseudomonas aeruginosa (deteriorates lung tissue) |
| Cystic fibrosis secondary symptoms | CFTR channel locations = lung, pancreas, sweat glands, liver, large intestine, testes; secondary symptoms = salty sweat, male sterility, pancreatic hypoglycemia; multiple symptoms from single gene = plieotropy |
| Mechanism for transporters and P class ATPases | Two binding sites for translocated factors; positioned on each end of transmembrane passage; one = high affinity, one = low affinity; two conformational states (E1, E2) |
| E1 and E2 | E1=Resting conformation; high affinity (E1) binding site open, low affinity (E2) closed; E1 site binds factor to be transported; causes conformational shift in E2 (E1 closes, E2 opens); factor translocated through passageway to low affinity (E2) site |
| Symporter transporter mechanism | Multiple binding sites (low affinity on one side, high affinity on the other) |
| Antiporter transporter mechanism | High and low affinity sites on opposite side of the membrane |
| P class ATPase transporter mechanism | Similar to high and low affinity binding (E1, E2); additional ATPase domain for binding & hydrolyzing ATP; phosphate (from ATP) forces transport protein to shift from E1 to E2; factor translocated to low affinity site |
| Higher concentrations on exoplasmic side of membranes | Na+, Ca2+, Cl- |
| Higher concentrations in cytoplasm | K+, HCO3 |
| Electric potential difference across membrane | 30-70 mV (~200,000 V/cm) |
| Term for membrane in reference to electric potential | Capacitor (thin barrier separating opposite charges) |
| Na+/K+ ATPase | Establish electric potentials across membranes (also Na, K gradients); pump exports 3 Na+, imports 2 K+ for every ATP hydrolyzed; exoplasmic greater (+) than cytoplasmic (-); potential amplified due to lots of K+ channels (diffuse out of cell) |
| Na+/K+ ATPase equilibrium | Established between diffusion of K+ out of cell and electrical force driving it in |
| Function of Na+/K+ ATPase | Store energy across membrane for other transport proteins to utilize to drive factors against their concentration gradients |
| Erythrocyte energy expenditure to maintain electric potential/concentration gradient | 50% |
| Kidney cell energy expenditure to maintain electric potential/concentration gradient | 25% |
| Signal transduction pathways | Regulatory pathways that transduce signals into cells; signals begin w/ intercellular signaling molecules (hormones/growth factors), interact w/ transmembrane receptors to transduce signal thru plasma membrane to intracellular effectors & 2nd messengers |
| Effectors | Components of signal transduction pathways; signal propagated from upstream to downstream effectors |
| 2nd messenger | Effectors that change concentration (increase or decrease) in response to ligand-receptor binding (i.e. cAMP, DAG, IP3, Ca2+) |
| Signal transduction pathways | TGF-beta, Ras-MAPK, cAMP, phosphoinositide |
| Properties that explain diversity of transduction responses | 1. different versions of each pathway (homologous factors for each step), 2. response to pathway depends on cellular context (different downstream targets), 3. crosstalk between pathways (regulate each other) |
| Receptor: Tyrosine kinase | Activates Ras-MAPK > phosphoinositide |
| Receptor: G protein coupled | Activates cAMP = phosphoinositide |
| Receptor: G protein coupled > tyrosine kinase | Activates phosphoinositide |
| TGF-beta signaling | Family of proteinaceous intercellular signaling molecules controlling numerous developmental events from embryo to adult (i.e. mesoderm induction, tissue polarity, cellular proliferation, bone morphogenesis, immunosuppression) |
| Serine/threonine kinase receptors | Receptors that bind TGF-beta homologs; heterotetrameric (type II, I); II phosphorylates serine/threonine residues on I, activates kinase activity; I phosphorylates specific serine residue of effector = Smads |
| Smads (Sma and mothers against decapentaplegic) | Effector molecules for TGF-beta signaling (two forms - receptor regulated Smads, coSmads) |
| Receptor regulated Smads | Specific to particular TGF-beta like pathways; phosphorylated by activated receptors; phosphorylation induces them to associate with coSmads |
| coSmads | Common factors for all TGF-beta pathways; different receptor regulated Smads bind same coSmad (Smad4 in vertebrates); not phosphorylated; bind with receptor regulated Smads that have been phosphorylated |
| Heteromeric Smad complex | Receptor regulated Smads + coSmads: translocate to nucleus, function as transcription factors, control gene expression by binding recognition sequences to activate / inhibit transcription of specific genes |
| Pathway conservation | Homologous versions of same transduction pathways used to induce diff. responses; entire pathways duplicated & diverged to fulfull distinct roles; common characteristic of all; helps account for complex responses controlled by few pathways |
| Ras-MAPK Signaling | Pathway activated by tyrosine kinases; controls blood glucose levels, metabolism, cell cycle progression, differentiation, apoptosis (i.e. insulin - activates hepatic RasMAPK that dephosphorylates glycogen synthase to induce glycogen synthesis) |
| Tyrosine kinase receptors | Form dimers; subunits associate in response to ligand binding; one class where subunits covalently linked |
| Transautophosphorylation | Ligand binding & dimerization induces receptors to phosphorylate themselves (auto); each subunit phoshorylates other member of dimer (trans); induces complex of 3 effectors (adaptor, GEF, Ras) to form around signal transduction domain |
| Ras-MAPK: Adaptor protein (ex: GRB2) | First factor recruited into complex around phosphorylated tyrosine kinase receptor |
| Ras-MAPK: GEF | Guanine nucleotide exchange factor (ex: son of sevenless-SOS); factor recruits final component of complex, Ras; functions to dislodge GDP bound to Ras; Ras auto. refills binding site w/ guanine; usually grabs GTP b/c it's much more conc. in cell than GDP |
| Ras-MAPK: Ras | Activated by GTP binding, allowing it to serve as cofactor for downstream kinases, such as Raf |
| Ras-MAPK: GAP (GTPase activating protein) | Deactivates Ras; serves as cofactor for intrinsic GTPase activity (Ras); induced-Ras dephosphorylates GTP, convert to GDP; prevents Ras from activ. downstream kinases; always present=always inactiv. Ras; TK receptor repeatedly bound to perpetuate response |
| Ras-MAPK: MAP (mitogen-activated protein) kinase cascade | Downstream leg of Ras-MAPK; Ras-GTP activates Raf; phosphorylates & activates MEK (MAP & ERK kinase); MEK phosphorylates & activates MAP kinase; phosphorylates numerous proteins (enzymes, transcription factors, microtubule associated proteins) |
| Activation cascade | Every kinase molecule activates goes on to phosphorylate 100's of downstream molecules, so that signal is amplified exponentially from step to step; robusst responses induced by minute signals |
| cAMP signaling | Pathways controls many processes; ex: glucagon in liver, adrenaline w/ liver & adipose; activate glycogenolysis (phosphorylates glycogen phosphorylase), inhibit glycogen synthesis ( phosphorylates glycogen synthase) |
| G protein coupled receptors | Activates cAMP, phosphoinositides; transmembrane=7 alpha-helices, loops in cytoplasm & exoplasm; ligand binding=helix 7 (ligand specificity),3,5,6 (bind ligand); signal transduction=C terminus+loop btw helix 5,6 (regions interact with G protein effectors) |
| G proteins | 1st effectors of cAMP; 3 forms (G-alpha, G-beta, G-gamma); assoc. into trimeric complex, bound by G protein coupled receptors; ligand binding induces conformation change (receptor); receptor exchanges GDP (at G-alpha) for GTP; G-alpha dissoc. from trimer |
| G-alpha-GTP binds what after dissociation from trimer | Adenylate cyclase |
| Gs-alpha | Activate adenylate cyclase; controlled by separate ligands and receptors |
| Gi-alpha | Inhibit adenylate cyclase; controlled by separate ligands and receptors |
| G-beta,gamma controlling activity | Some brain adenylate cyclases, heart K+ channels, yeast mating type |
| Adenylate cyclase | Converts ATP to cAMP, forming internal phosphodiester bond between 5` phosphate and 3` -OH; done so when bound by Gs-alpha |
| cAMP | Serves as 2nd messenger, activating protein kinase A |
| cAMP phosphodiesterase | Deactivates cAMP signal, converts it back to AMP |
| Protein kinase A | Enzyme activated by cAMP, inactive form exists as tetramer (2 regulatory - R; 2 catalytic - C) subunits; cAMP bind R, inducing conformational shift (releases & activates C); C phosphorylate serine & threonine of specific proteins |
| IP3 (inosital triphosphate) | Six carbon ring with three phosphates |
| DAG (diacylglycerol) | 2 fatty acyl side chains esterified to glycerol; 1 FA saturated, other is arachidonic acid (precursor for prostaglandins, aspirin prevents inflammation by blocking pathway converting arachidonic acid to prostaglandins) |
| PIP2 (phosphatidylinositol biphosphate) | Substrate of PLC; products = IP3, DAG; PIP2 & DAG are integral membrane factors b/c of fatty acyl side chains; IPs released into cytosol |
| PLC (phospholipase C) | Enzyme is 1st unique effector in phosphoinositide pathway; activated by Gq-alpha-GTP (PLC-gamma = activated by tyrosine kinase receptors) to catalyze cleavage of PIP2 into IP3 & DAG |
| Phosphoinositide signaling | Use G protein coupled receptors with G protein homologs = Gq (Gq-alpha, Gq-beta, Gq-gamma); control metabolism, cell cycle progession, hormonal secretion, transport protein activity, cardiac & neuronal electric signal propagation, mental health |
| Phosphoinositide pathway: PLC-gamma | Activated by tyrosine kinase receptor |
| Pertussis toxin | Prevents release of GDP from Gq-alpha, blocking PLC activation; prevents GDP release from Gi-alpha protein, preventing adenylate cyclase inhibition; leads to whooping cough |
| Ca2+ | IP3 binds & opens Ca2+ gated channels (vesicles, ER); ion in cytosol = 2nd messenger; secreted in waves, believed to concentrate signal at peaks; serves as cofactor (controls activity of transport proteins, enzymes); induces muscle contraction |
| Ca2+ in crosstalk | Stimulates phosphodiesterase (reduces duration of cAMP signaling) |
| Protein kinase C (PKC) | Target of Ca2+; central enzyme; Ca2+ causes PKC to assoc. w/ plasma membrane; binds DAG (functions as cofactor - activating kinase activity); PKC phosphorylates various proteins (TF, enzymes); signal inactivated by rejoining IP3 & DAG = PIP2 |
| Lithium | Treats manic depressant patients; dampens mood swings; blocks phosphoinositide signaling by inhibiting one enzyme involved w/ rejoining IP3 & DAG; proposed to inhibit Gq |
| In cAMP signaling, who exchanges GDP for GTP? | Receptors; G proteins do not have GEF-like factors that can exchange GDP for GTP |
| Cholera toxin | Inhibits GAP domain G-alpha, prevents deactivation of signal, leads to massive diarrhea and death due to dehydration |
| What inactivates the G-alpha domain in cAMP signaling? | Ras domain of G-alpha binds GTP, GAP domain induces hydrolysis of GTP to GDP, inactivates signal |
| G-alpha domain is similar to ... | Ras; AA sequence is similar; also, GAP is similar; structure of G-alpha similar to Ras and GAP when they are bound to each other |
Created by:
JaneO