Chapter-21b: Hormones and Receptors

Transcription

1 1 hapter-21b: Hormones and Receptors Hormone classes Hormones are classified according to the distance over which they act. 1. Autocrine hormones --- act on the same cell that released them. Interleukin-2 on T-cells. 2. Paracrine hormones --- act only on cells close to the cell that released them. Prostaglandins and many polypeptide growth factors. 3. Endocrine hormones --- act on cells distant from the site of their release. Those are transported through bloodstream, etc. Endocrine hormones (insulin, epinephrine). Hormones act to: 1. maintain homeostasis (e.g., insulin vs. glucagon) 2. respond to stimuli (e.g., epinephrine) 3. control cyclic and developmental processes (e.g. sex hormones: steroids, peptides, and amino acid derivatives) 1

2 2 Responses are produced by second messengers, such as 3,5 -cyclic AMP (camp), inositol triphosphate (IP 3 ) and a 2+. H 2 A H 2 A P - camp H P - - H AMP H Four classes of receptors - Steroid receptors - Adrenoreceptors - Growth factor receptors - Acetylcholine receptors (Not discuss in this chapter) A. Steroid (hormone) receptors - are proteins in cytoplasm nucleus, but are not transmembrane proteins. - Steroid hormones are carried by serum binding protein in blood, and released near the target cells. The hormones diffuse into the target cells and bind the receptors in nucleus. - Hormones bound proteins bind to specific regulatory region of DNA and activated the DNA as transcription factors. B. Adrenoreceptors - are proteins on plasma membrane. - Polypeptides and amino acid derivatives bind to the plasma membrane receptors, and change concentration of a second messenger, camp, a 2+, etc. - Three examples are given: 1. β-adrenoreceptor system 2. α-adrenoreceptor system (Phosphoinositide system) 3. N system 2

3 3 Example-1: β-adrenoreceptor system - Human β-adrenoreceptor has 7 segments of membrane-spanning helices. - The N-terminal is on the extracellular surface where hormones bind, the -terminal is in the cytoplasmic side where G-proteins bind. R NH 2 H H 2 H R = H Norepinephrine (Noradrenalin) R = H 3 Epinephrine (Adrenalin) Receptor site H H G-protein binding site 3

4 4 G-proteins - are peripheral proteins, and are linked to membrane via a fatty acid on isoprenyl group. - receive the hormone (first messenger) message and transfer it to 2nd messenger. - G-protein has subunit structure, α, β, γ, or G α, G β, G γ. - GDP or GTP binds on G α. - The α-subunit (G α ) is separated from the βγ-subunits (G β and G γ ) when the bound GDP is replaced by GTP. The separated α-subunit (G α ) is the active form protein. - There are three kinds of G α proteins. G sα --- stimulating G α G iα --- inhibitory G α G qα --- phospholipid involved G α 4

5 5 Signal transduction process 1. Epinephrine binds to a specific receptor. 2. The occupied receptor causes replacement of the GDP bound to G s by GTP, activating G s. 3. G sα (α subunit) separates from G βγ subunits, and moves to adenylate cyclase (A) and activates it. 4. Activated A catalyzes the formation of 3,5 -cyclic AMP (camp) from ATP, i.e., [camp] is increased. 5. camp-dependent protein kinase (protein kinase A) is activated by camp. 6. Phosphorylation of cellular proteins by protein kinase A causes the cellular response to epinephrine. Epinephrine Norepinephrine Epinephrine Somatosteine pioids camp-dependent protein kinase (Protein kinase A) 5

6 6 Inhibitory mechanisms of signal transduction flow 1. GTP bound on activated G sα, G sα GTP is hydrolyzed to G sα GDP + P i. (step 3) 2. camp is degraded to AMP by phosphodiesterase. (step 5) 3. Phosphorylated proteins are dephosphorylated by phosphoprotein phosphatase. (step 6) 4. The binding of hormone to the inhibitory receptor, R i, triggers an almost identical chain of events except that the presence of G iα GTP complex inhibits A from synthesizing camp. Some inhibitors of the system (Note: These inhibitors increase the camp activity). 1. holera toxin - holera toxin (87 kd protein composed of A 1 B 5 ) inhibits the hydrolysis of G sα GTP G sα GDP + P i - AB 5 subunit structure of cholera toxin binds to receptor, and the A subunit takes into cell. - In the cell, A chain is divided to two chains, A 1 (195 AA) and A 2 (45 AA), by protease. Protease A A1 + A2 - A 1 catalyzes ADP ribosylation of an Arg residue in G sα. - This ADP-ribosylation prevents hydrolysis of GTP to GDP. Thus, G α GTP stays in active form. (Note: inactive form = G α GDP). - Therefore, camp is constantly produced. - - Epithelial cells of the small intestine secrete continuously digestive fluid (H 3 rich salt solution) in response to high [camp]. - onsequently, cholera patients have sever diarrhea and dehydration. - In this reaction, NAD + is a substrate rather than co-factor. 6

7 7 2. Pertussis toxin - ADP ribosylated G iα prevents exchange of GTP from GDP. Thus, G iα GDP stays in inactive form. - Thus, G iα cannot inhibit A activity, consequently, [camp] is increased. 3. affeine - affeine & theophylline inhibit the (camp AMP) reaction catalyzed by phosphodiesterase. - Thus, [camp] stays the same level, and constantly stimulate the camp-dependent protein kinase activity. 7

8 8 Example-2: α-adrenoreceptor system (Phosphoinositide pathway) - G-proteins also affect a 2+, phosphatidylinositol-4,5-bisphosphate (PIP 2 ), and sn-1,2- diacylglycerol (DG). Signal transduction processes 1. Hormone binds to a specific receptor. 2. The occupied receptor causes GDP-GTP exchange on G qα. 3. G qα, with bound GTP, moves to phospholipase (PL) and activate it. 4. Active PL cleaves phosphatidylinositol-4,5-bisphosphate to inositol-triphosphate (IP 3 ) and diacylglycerol (DG)* (see next page). 5. IP 3 binds to a specific receptor on the endoplasmic reticulum, releasing sequestered a 2+ to cytosol. 6. a 2+ stimulates a-calmodulin protein kinases, and/or a 2+, DG and PS (phosphatidylserine) activate protein kinase at the surface of the plasma membrane. 7. Phosphorylation of cellular proteins by a 2+ -am kinase and protein kinase produces the cellular response to the hormone. [a 2+ ] = 10-7 M a 2+ -almodulin Protein Kinase [a 2+ ] = 10-4 M 8

9 9 - IP 3 is degraded to IP 2 by inositol triphosphatase, and PIP 2 is regenerated. R 2 H 2 H 2 H P R 1-2- H P 3 H H P 3 2- Phosphatidyl-iositol-4,5-bisphosphate PL R 2 H 2 H 2 H + H 2- P 3 2- H P 3 H H P 3 2- R 1 Diacylglycerol (DG) Inositol - triphosphate (IP 3 ) Example-3: a 2+ activates Protein Kinase G via N as a biological messenger 1. N synthase is activated by a 2+ -calmodulin which is increased by external hormonal signal. 2. Nitric oxide (N) is produced from the Arg breakdown reaction catalyzed by N synthase. 3. The produced N activates guanylate cyclase. 4. Guanylate cyclase catalyzes the cgmp formation reaction from GTP. 5. cgmp activates cgmp-dependent protein kinase (protein kinase G). 6. Protein kinase G activates target proteins by phosphorylation, consequently, the smooth muscle cells are relaxed. a 2+ External hormonal signal NADPH Arg H 2 N 2 NADP + H N 2 a 2+ -am itrulline H 2 N 2 Nitroglycerin N synthase N Guanylate cyclase in smooth muscle cells cgmp cgmp - dependent protein kinase (Protein kinase G) Smooth muscle relaxation Phosphorylation of target proteins - Note: It is well-known that nitroglycerin works on angina pectoris (is a disease caused by insufficient blood flow to the heart muscle, leading to severe chest pain). Nitroglycerin produces N very quickly, and the N consequently relaxes the heart smooth muscle by stimulating the production of cgmp. 9

10 10. Receptors represented by growth factor receptors - are growth factor receptors - are transmembrane proteins consisted of α-subunit (receptor site) and β-subunit (transmembrane and tyrosine-specific protein kinase). - Binding growth factor such as insulin on the receptor triggers autophosphorylation of the Tyr residues in the -terminal domain of β-subunit. Protein H + ATP Protein P ADP - This phosphorylation allows the tyrosine kinase domain to catalyze phosphorylation of other target proteins. - Phosphorylated target proteins activate various enzymes. Tyrosine Kinase 10

11 11 Several different receptor tyrosine kinases 11