Bài giảng Biochemistry 2/e - Chapter 34: The Reception and Transmission of Extracellular Information

Outline 34.1 Hormones and Signal Transduction Pathways 34.2 Signal-Transducing Receptors Transmit the Hormonal Message 34.3 Intracellular Second Messengers 34.4 GTP-Binding Proteins” The Hormonal Missing Link 34.5 The 7-TMS receptor

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Chapter 34The Reception and Transmission of Extracellular Informationto accompanyBiochemistry, 2/ebyReginald Garrett and Charles GrishamAll rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline34.1 Hormones and Signal Transduction Pathways34.2 Signal-Transducing Receptors Transmit the Hormonal Message34.3 Intracellular Second Messengers34.4 GTP-Binding Proteins” The Hormonal Missing Link34.5 The 7-TMS receptorOutline34.6 Specific Phospholipases34.7 Calcium as a Second Messenger34.8 Protein Kinase C34.9 The Single TMS-receptor34.10 Protein Modules34.11 Steroid HormonesSPECIAL FOCUS: Neurotransmission and Sensory Systems Classes of Hormones (There may be others, but we doubt it...) Steroid Hormones - derived from cholesterol- regulate metabolism, salt/water balances, inflammation, sexual function Amino Acid Derived Hormones - epinephrine, etc.- regulate smooth muscle , blood pressure, cardiac rate, lipolysis, glycogenolysis Peptide Hormones - regulate many processes in all tissues - including release of other hormones Signal-Transducing Receptors Transmit Hormone MessageNon-steroid hormones bind to plasma membrane and activate a signal-transduction pathway inside the cell Steroid hormones may either bind to the plasma membraneorenter the cell and travel to the nucleus Types of Receptors Three that we know of... 7-transmembrane segment receptors extracellular site for hormone (ligand) intracellular site for GTP-binding protein Single-transmembrane segment receptors extracellular site for hormone (ligand) intracellular catalytic domain - either a tyrosine kinase or guanylyl cyclase Oligomeric ion channels Second Messengers Many and there may be more!The hormone is the "first messenger" The second messenger - Ca2+, cAMP or other - is released when the hormone binds to its (extracellular) receptor The second messenger then activates (or inhibits) processes in the cytoplasm or nucleus Degradation and/or clearance of the second messenger is also (obviously) important cAMP and Glycogen Phosphorylase Earl Sutherland discovers the first second messenger In the early 1960s, Earl Sutherland showed that the stimulation of glycogen phosphorylase by epinephrine involved cyclic adenosine-3',5'-monophosphate He called cAMP a "second messenger" cAMP is synthesized by adenylyl cyclase and degraded by phosphodiesterase How are the hormone receptor and AC coupled? Purified AC and purified receptor, when recombined, are not coupled. Rodbell showed that GTP is required for hormonal activation of AC In 1977, Elliott Ross and Alfred Gilman at Univ. of Virginia discovered a GTP-binding protein which restored hormone stimulation to AC Hormone stimulates receptor, which activates GTP-binding protein, which activates AC G Proteins Many new developments in this area Two kinds: "heterotrimeric G proteins" and "small G proteins" X-ray diffraction structures for several of these are only recently available Structures shed new light on possible functions Heterotrimeric G Proteins A model for their activity Binding of hormone, etc., to receptor protein in the membrane triggers dissociation of GDP and binding of GTP to -subunit of G protein G-GTP complex dissociates from G and migrates to effector sites, activating or inhibiting But it is now clear that G also functions as a signalling device Signalling Roles for G() A partial list Potassium channel proteins Phospholipase A2 Yeast mating protein kinase Ste20 Adenylyl cyclase Phospholipase C Calcium channels Receptor kinases Stimulatory and Inhibitory G G proteins may either stimulate or inhibit an effector. In the case of adenylyl cyclase, the stimulatory G protein is known as Gs and the inhibitory G protein is known as Gi Gi may act either by the Gi subunit binding to AC or by the Gi complex complexing all the Gi and preventing it from binding to AC Read about the actions of cholera toxin and pertussis toxinThe ras Gene and p21ras An oncogene and its product a gene first found in rat sarcoma virus Normal cellular ras protein activates cellular processes when GTP is bound and is inactive when GTP has been hydrolyzed to GDP Mutant (oncogenic) forms of ras have severely impaired GTPase activity, so remain active for long periods, stimulatingexcessive growth and metabolic activity - causing tumors to form 7-TMS Receptors Receptors that interact with G proteins Seven putative alpha-helical transmembrane segments Extracellular domain interacts with hormone Intracellular domain interacts with G proteins Adrenergic receptors are typicalNote desensitization by phosphorylation, either by ARK or by protein kinase APhospholipases Release Second Messengers Inositol phospholipids yield IP3 and DAG PLC is activated by 7-TMS receptors and G proteins PLC is activated by receptor tyrosine kinases (via phosphorylation) Note PI metabolic pathways and the role of lithiumOther Lipids as Messengers Recent findings - lots more to come More recently than for PI, other phospholipids have been found to produce second messengers! PC can produce C20s, DAG and/or PA Sphingomyelin and glycosphingolipids also produce signals Ceramide (from SM) is a trigger of apoptosis - programmed cell death Ca2+ as a Second Messenger Several sources of Ca2+ in cells! [Ca2+] in cells is normally very low: < 1M Calcium can enter cell from outside or from ER and calciosomes CICR - Calcium-Induced Calcium Release - is very, very similar to what happens at the foot structure in muscle cells! IP3 (made by action of phospholipase C) is the trigger See Figures 34.17-34.19Calcium Oscillations! M. Berridge's model of Ca2+ signals Ca2+ was once thought to merely rise in cells to signal and drop when the signal was over Berridge's work demonstrates that Ca2+ levels oscillate in cells! The purpose may be to protect cell components that are sensitive to high calcium, or perhaps to create waves of Ca2+ in the cell Ca2+-Binding Proteins Mediators of Ca2+effects in cells Many cellular proteins modulate Ca2+ effects 3 main types: protein kinase Cs, Ca2+-modulated proteins and annexins Kretsinger characterized the structure of parvalbumin, prototype of Ca2+-modulated proteins "EF hand" proteins bind BAA helicesTransduction of two second messenger signals PKC is activated by DAG and Ca2+Most PKC isozymes have several domains, including ATP-binding domain, substrate-binding domain, Ca-binding domain and a phorbol ester-binding domain Phorbol esters are apparent analogues of DAG Cellular phosphatases dephosphorylate target proteins Read about okadaic acidSingle TMS Receptors Three main classesExtracellular domain to interact with hormone Single transmembrane segment Intracellular domain with enzyme activity Activity is usually tyrosine kinase or guanylyl cyclase Each of these has a "nonreceptor" counterpart src gene kinase - pp60v-src was first known Two posttranslational modificationsReceptor Tyrosine Kinases Membrane-associated allosteric enzymes How do single-TMS receptors transmit the signal from outside to inside?? Oligomeric association is the key! Extracellular ligand binding The Polypeptide Hormones Common features of synthesis All secreted polypeptide hormones are synthesized with a signal sequence (which directs them to secretory granules, then out) Usually synthesized as inactive preprohormones ("pre-pro" implies at least two precessing steps) Proteolytic processing produces the prohormone and the hormoneProteolytic Processing A mostly common pathway Proteolytic cleavage of the hydrophobic N-terminal signal peptide sequence Proteolytic cleavage at a site defined by pairs of basic amino acid residues Proteolytic cleavage at sites designated by single Arg residues Post-translational modification: C-terminal amidation, N-terminal acetylation, phosphorylation, glycosylation Gastrin as an Example Heptadecapeptide secreted by the antral mucosa of stomach Gastrin stimulates acid secretion in stomach Product of preprogastrin - 101-104 residues Signal peptide cleavage leaves progastrin - 80-83 residues Cleavage at Lys and Arg (basic) residues and C-terminal amidation leaves gastrin N-terminal residue of gastrin is pyroglutamate C-terminal amidation involves destruction of Gly Protein-Tyrosine Phosphatases The enzymes that dephosphorylate Tyr Some PTPases are integral membrane proteins But there are also lots of soluble PTPases Cytoplasmic PTPases have N-term. catalytic domains and C-terminal regulatory domains Membrane PTPases all have cytoplasmic catalytic domain, single transmembrane segment and an extracellular recognition site Guanylyl Cyclases Soluble or Membrane-Bound Membrane-bound GCs are the other group of single-transmembrane-segment receptors (besides RTKs) Peptide hormones activate the membrane-forms Note speract and resact, from mammalian ova Activation may involve oligomerization of receptors, as for RTKs Soluble Guanylyl Cyclases Receptors for Nitric Oxide NO is a reactive, free-radical that acts either as a neurotransmitter or as a second messenger NO relaxes vascular smooth muscle (and is thus involved in stimulation of penile erection) NO also stimulates macrophages to kill tumor cells and bacteria NO binds to heme of GC, stimulating GC activity 50-fold Read about NO synthesis and also see box on Alfred Nobel Protein Modules in Signal TransductionSignal transduction in cell occurs via protein-protein and protein-lipid interactions based on protein modulesMost signaling proteins consist of two or more modules This permits assembly of functional signaling complexesLocalization of Signaling ProteinsAdaptor proteins provide docking sites for signaling modules at the membraneTypical case: IRS-1 (Insulin Receptor Substrate-1)N-terminal PH domainPTB domain18 potential tyrosine phosphorylation sitesPH and PTB direct IRS-1 to receptor tyrosine kinase - signaling events follow!Signaling Pathways from Membrane to the NucleusThe complete path from membrane to nucleus is understood for a few casesSee Figure 34.38Signaling pathways are redundantSignaling pathways converge and divergeThis is possible with several signaling modules on a signaling proteinModule Interactions Rule!The interplay of multiple modules on many signaling proteins permits a dazzling array of signaling interactionsSee Figure 34.40We can barely conceive of the probable extent of this complexityFor example, it is estimated that there are more than 1000 protein kinases in the typical animal cell - all signals!Steroid Hormones Glucocorticoids, mineralocorticoids, vitamin D and the sex hormones May either act at nucleus or at plasma membrane Steroids are hydrophobic and cannot diffuse freely to nucleus Receptor proteins carry steroids to the nucleus Steroid receptor proteins are all apparently members of a gene superfamily and have evolved from a common ancestral precursor Steroid Receptor Proteins Hydrophobic domain near C-terminus that interacts with steroid itself Central, hydrophilic domain that binds to DNA Central DNA-binding domains are homologous to one another, with 9 conserved Cys residues Three pairs of Cys residues are in Cys-X-X-Cys sequences - as in Zinc-finger domainsSteroid-receptor complex may bind to DNA or to transcription factors Thyroid hormone receptor proteins are similar Extracellular Effects of steroid hormones Two lines of evidence: action of steroids on calcium channels and other membrane proteins and the speed of certain steroid hormone effects Example: testosterone rapidly stimulates transport of glucose, calcium and amino acids into rat kidney cells Several demonstrations now of tight binding of steroid probes to GABA receptor and other proteins Cells of Nervous Systems Neurons and Neuroglia (Glial Cells) Neurons contain processes, including an axon and dendrites Axon is covered with myelin sheath and cellular sheath, except at nodes of Ranvier The axon ends in synaptic termini, aka synaptic knobs or synaptic bulbs Three kinds of neurons: sensory neurons, motor neurons and interneurons Ion Gradients The source of electrical potentials in neurons Nerve impulses consist of electrical signals that are transient changes in the electrical potential differences (voltages) across neuron membrane Know resting concentrationsLearn to use the equation for actual potential difference in the box on page S-43 Difference between Nernst potential and actual potential represents a thermodynamic push The Action Potential A somewhat misleading name - it refers to the series of changes in potential that constitute a nerve impulse Small depolarization (from -60 to -40 mV) opens voltage-gated ion channels - Na flows in Potential rises to +30 mV, Na channels close, K channels open. K streams out, lowering potential Action potentials flow along the axon to the synapse Number and frequency important, not intensity Voltage-Gated Na, K Channels Clustered in Nodes of Ranvier See Figure 34.52 for Na, K channel effectors Arrangement of Na channel in membrane is like DHP receptor in muscle See Figure 34.55 for diagram of how the channel is formed in membrane These channels are voltage-sensitive - voltage changes cause conformational changes and gating Communication at the Synapse A crucial feature of neurotransmission Ratio of synapses to neurons in human forebrain is 40,000 to 1! Chemical synapses are different from electrical Neurotransmitters facilitate cell-cell communication at the synapse Note families of neurotransmitters in Table 34.6The Cholinergic Synapse A model for many others Synaptic vesicles in synaptic knobs contain acetylcholine (10,000 molecules per vesicle) Arriving action potential depolarizes membrane, opening Ca channels and causing vesicles to fuse with plasma membrane Acetylcholine spills into cleft, migrates to adjacent cells and binds to receptors Toxin effects: botulism toxin inhibits Ac-choline release, black widow's latrotoxin protein overstimulates Two Classes of Ac-Ch Receptor Nicotinic and muscarinic As always, toxic agents have helped to identify and purify hard-to-find biomolecules Nicotinic Ac-Ch receptors are voltage-gated ion channels Muscarinic Ac-Ch receptors are transmembrane proteins that interact with G proteins Acetylcholinesterase degrades Ac-Ch in cleft Transport proteins and V-type H+-ATPases return Ac-Ch to vesicles - called reuptake Other Neurotransmitters Excitatory and inhibitory Glutamate is good example: nerve impulse triggers Ca-dependent exocytosis of glutamate Glutamate is either returned to neuron, or carried into glial cells, converted to Gln and taken back to the neuron from which it was released See 4 types of glutamate receptors in Fig. 34.68 NMDA receptor is best understood for now Note phencyclidine (angel dust) storyGABA and Glycine Inhibitory Neurotransmitters Inhibitory neurotransmitters diminish the actions of activating neurotransmitters See Figure 34.70 for glutamate degradation Excitatory glutamate is broken down to inhibitory GABA, which is broken down to non-signals GABA & glycine receptors are chloride channels Glycine receptor is site of action of strychnine Catecholamine Neurotransmitters Epinephrine, norepinephrine, dopamine and L-dopa are all neurotransmitters Synthesized from tyrosine - see Fig. 34.72 Excessive production of dopamine (DA) or hypersensitivity of DA receptors produces psychotic symptoms and schizophrenia Lowered production and/or loss of DA neurons are factors in Parkinsonism Neurological Disorders Depression, Parkinsonism, etc. Defects in catecholamine processing are responsible for many neurological disorders Norepinephrine and dopamine systems are keys Breakdown of NE and DA by catechol-O-methyl transferase and monoamine oxidase Reuptake by specific transport proteins MAO inhibitors are antidepressants Tricyclics - antidepressants that block reuptake Cocaine blocks reuptake, prolongs effects of DA Peptide Neurotransmitters Lots more to learn here! Likely to be many peptide NTs Concentrations are low; purification is hard Roles are complex Endorphins and enkephalins are natural opioids Endothelins affect smooth muscle contraction, vasoconstriction, mitogenesis, tissue changes Vasoactive intestinal peptide stimulates AC (to make cAMP) via G proteins, and its effects are synergistic with those of other neurotransmitters Sensory Transduction Similarities between sight, taste, smell Specialized sensory cells translate stimulus into electrical signals in adjacent neurons Vision is the paradigm system Absorption of light quanta by rhodopsin isomerizes retinal (11-cis to all-trans) Light is absorbed by rhodopsin in the outer segments of rod and cone cells
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