Receptors of Hormones & Neurotransmitters Seminar No. 8 - Chapter 22 - Q. What are general features of signal molecule? (see scheme on p. 130) Signal molecule (e.g. hormone) * carries information into cell * has extremely low concentration in blood (10^-9 -- 10^-15 mol/l) * specifically binds to receptor * signal molecule is quickly inactivated * agonist -- (external) molecule which acts the same way as physiological signal molecule * antagonist -- (external) molecule which blocks receptor TH no biological response Q. What is the amplification of signal? Amplification of signal = to make it more powerful 1 molecule of hormone ~ 100-1000 molecules of second messenger Two classes of hormones Concentration of hormone in blood generally does not correlate with its biological effects More factors are involved (transport systems, chemical modifications, activity of receptors etc.) Two principal types of receptors * membrane receptors * intracellular receptors The main types of membrane receptors Ligand gated ion channels * in synapses, activated by neurotransmitters, very quick response Receptors activating G-proteins * stimulate or inhibit adenylate cyclase /phospholipase C Receptors with guanylate cyclase activity * atrial natriuretic factors Receptors with tyrosine kinase activity * insulin Nicotinic acetylcholine receptor ^* transmembrane protein = channel for Na^+ and K^+ * heteropentamer (a[2]bgd) * a-subunits have two binding sites for acetylcholine (ACH) * nicotine is agonist of this receptor Four events on postsynaptic membrane (see the scheme and the graph on p. 131) * ACH binds to receptor TH channel opens TH influx of Na^+ and efflux of K^+ * partial depolarization of membrane (-60 (r) -40 mV) opens other type of voltage-dependent Na^+-channel TH further influx of Na^+ TH depolarization of postsyn. membrane ((r) +20 mV) * this depolarization opens K^+-channel (volt. dep.) TH efflux of K^+ TH membrane potential returns to normal value (-60 mV) = repolarization * Na^+,K^+-ATPase gets ion distribution to normal state (Na^+ TH OUT, K^+ TH IN) Q. Describe the formation of acetylcholine in the body. Acetylcholine GABA receptor * channel for chloride ion (Cl^-) * has the binding site for GABA TH channel opens TH Cl^- ions get into cell TH hyperpolarization ((r) -80 mV) TH decrease of excitability * benzodiazepines and barbiturates (synthetic substances) have similar effects like GABA, they are used as anxiolytics and/or sedatives * endozepines -- endogenous peptides have opposite effects, close the channel (are responsible for anxiety feelings) Q. Describe the synthesis of GABA. GABA formation GABA inactivation Diazepine Benzo[f]diazepine Benzodiazepines Barbiturates G-Protein linked receptors (scheme, p. 132) * extracellular part of receptor has a binding site for hormone * intracellular part has a binding site for G-protein * G-proteins are heterotrimers (abg) * in resting state, a-unit has GDP attached * after binding hormone TH (a-GDP)bg makes complex with receptor TH GDP is phosphorylated to GTP * activated G-trimer dissociates: (a-GTP)bg (r) a-GTP + bg * a-GTP interacts with effector (enzyme) TH activated/inhibited enzyme TH second messenger (-^ or -v) Main types of G-proteins * G[s] (stimulatory) * G[i] (inhibitory) * G[p] (phospholipid) * and other ... * see table on p. 132 !! Q. What reaction is catalyzed by adenylate cyclase? Adenylate cyclase reaction Adenylate cyclase (AC) * membrane bound receptor * catalyzes reaction: ATP (r) cAMP + PP (diphosphate) * G[s] protein stimulates AC TH conc. of cAMP -^ * G[i] protein inhibits AC TH conc. of cAMP -v Q. What is the function of cAMP? cAMP is the second messenger * cAMP activates protein kinase A TH phosphorylation of cell proteins: * Protein-OH + ATP (r) Protein-O-P + ADP ------------------------------------------------------------------------- * the second messenger cAMP is quickly inactivated * cAMP is removed by hydrolysis, catalyzed by phosphodiesterase: * cAMP + H[2]O (r) AMP General scheme of phosphorylation Q. Which aminoacids can be phosphorylated? A. * AA with a hydroxyl group in a side chain * serine * threonine * tyrosine Q. What reaction is catalyzed by protein phosphatase? A. Protein-O-P + H[2]O (r) Protein-O-H + P[i ] Phosphatidyl inositol system (p. 133) * G[p] protein activates phospholipase C (PL-C) * PL-C catalyzes the hydrolysis of phosphatidyl inositol bis phosphate (PIP[2]): * PIP[2] + H[2]O (r) IP[3] + DG * both products (IP[3], DG) are second messengers The structure of PIP[2 ]Q. What is the source of inositol in human body? The origine of inositol DG and IP[3] as second messengers * DG activates protein kinase C TH phosphorylation of intracellular proteins * IP[3] opens calcium channel in ER TH Ca^2+ concentration in cytoplasm increases TH Ca^2+ ions are associated with special protein calmodulin (CM) TH Ca^2+-CM complex activates certain types of kinases TH biological response Insulin receptor * has four subunits (a[2]b[2]) * extracellular a-units bind insulin * intracellular b-units have tyrosine kinase activity TH phosphorylation of tyrosine phenolic hydroxyl of intracellular proteins including insulin receptor itself (autophosphorylation) TH cascade of further events TH biological response Intracellular receptors: - cytoplasmatic - nucleic for steroids, iodothyronines, calcitriol, retinoids Intracellular receptors Steroid and thyroid hormones * insoluble in water TH in ECF are transported in complex with transport proteins * hormone themselves diffuse easily across cell membrane * they are bound to cytoplasmatic or nuclear receptors * in nucleus, the hormone-receptor complex binds to HRE (hormone response element) in regulation sequence of DNA * this leads to induction of mRNA synthesis = transcription of gene Events on synapses Cholinergic synapses * neurotransmitter: acetylcholine * two types of receptors * nicotinic rec. (ion channel) -- e.g. neuromuscular junction * muscarinic rec. (G-prot.) -- e.g. smooth muscles Cholinergic receptors Q. How is acetylcholine released from presynaptic terminal? A. * influx of Ca^2+ triggers the fusion of presynaptic vesicles (contaning acetylcholine) with cell membrane and exocytosis of acetylcholine * acetylcholine is liberated into synapse Q. What reaction is catalyzed by acetylcholinesterase? A. Q. What is nicotine? Nicotine is the main alkaloid of tobacco (Nicotiana tabacum) Q. Why nicotine triggers the release of adrenaline? Main effects of nicotine * nicotine binds to acetylcholine nicotinic receptors in brain and other tissues including cells of adrenal medula * stimulates the secretion of adrenaline -- because it binds to receptors in adrenal medula (p. 135 !) - silent stress other effects: * increases the secretion of saliva and gastric juice * increase intestinal peristalsis * vasoconstriction Q. What is muscarine? Muscarine is an alkaloid in some mushrooms Inhibitors of acetylcholinesterase * Reversible -- carbamates (N-substituted esters of carbamic acid), e.g. fysostigmine, neostigmine * they are used to improve muscle tone in people with myasthenia gravis and routinely in anesthesia at the end of an operation to reverse the effects of non-depolarising muscle relaxants. It can also be used for urinary retention resulting from general anaesthetia * Irreversible -- organophosphates, very toxic compounds Carbamates -- General formulas Organophosphates Adrenergic synapses * neurotransmitter: noradrenaline [* ] four types of receptors: a[1], a[2], b[1], b[2 ]* all of them are G-protein linked receptors * occur in various cells and tissues Adrenergic receptors Q. Describe the synthesis of noradrenaline. The formation of DOPA and dopamine Noradrenaline and adrenaline The next seminar April 24, 2006 Chapter 21 - I. part