Exam 3 Notes Sessions 20 29 Lecture 20 Slide 1 Slide 2 these follow through the slides all the slides are numbered if there is no information for a slide it just means it is pretty self explanatory or there was no information on that topic in the book General Principle and G protein coupled receptors Video of an amoeba chasing a pen point the amoeba was using cell surface receptors to detect cAMP immediately G protein coupled receptors are the most numerous class of receptors their ligand binding site is exposed outside the surface of the cell their effector site extends into the cytosol receptor activation by ligand binding triggers activation of the coupled trimeric G protein which interacts with downstream signal transduction proteins remember a ligand is just any substance e g hormone drug functional group etc that binds speci cally and reversibly to another chemical entity to form a larger complex All GPCR signaling pathways share the following elements 1 a receptor that contains 7 membrane spanning domains 2 a coupled trimeric G proteins which functions as a switch by cycling between active and inactive forms 3 a membrane bound effector protein 4 Feedback regulation and desensitization of the signaling pathway A second messenger also occurs in many GPCR pathways Slide 3 GPCR pathways usually have short term effects in the cell by quickly modifying existing proteins either enzymes or ion channels Slide 4 Endocrine signaling effects distant target cells endocrine system is a system of glands each of which secretes a type of hormone directly into the bloodstream to regulate the body circulatory system distributes hormone Paracrine signaling is a form of cell signaling in which the target cell is near para near the signal releasing cell nerve cells Autocrine signaling is a form of signaling in which a cell secretes a hormone or chemical messenger called the autocrine agent that binds to autocrine receptors on the same cell leading to changes in the cell tumor cells Slide 5 G protein coupled receptors sense molecules outside the cell and activate inside signal transduction pathways and ultimately cellular responses We will mainly discuss the cAMP signal pathway In general when a ligand binds to the GPCR it causes a conformational change in the GPCR which allows it to act as a guanine nucleotide exchange factor GEF The GPCR can then activate an associated G protein by exchanging its bound GDP for GTP The G protein s alpha subunit together bound with GTP can then dissociate from the beta and gamma subunits to further affect intracellular signaling or target cells directly depending on the alpha subunit type The G subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity allowing it to re associate with G and starting a new cycle Slide 6 protein coupled receptors Slide 7 There are 7 class of cell surface receptors known today but we re focusing on G G proteins function as molecular switches When they bind guanosine triphosphate GTP they are on and when they bind guanosine diphosphate GDP they are off messenger receptor binds onto the ligand which then triggers the release of the second relay of the information also ampli es the information Most of the signal molecules are hydrophilic the phospholipid bilayer provides a barrier for hydrophilic molecules most of the signal molecule thus remains in the extracellular space Slide 8 receptor G protein Activation of Effector Second messenger Overview of G protein coupled Second Messenger System the ligand binds to a site on the extracellular portion of the receptor this activates a G protein associated with the cytoplasmic C terminal This initiates the production of a second messenger The most common of these are cyclic AMP cAMP which is produced by adenylyl cyclase from ATP inositol 1 4 5 triphosphate IP3 DAG and cGMP remember Adenylyl Cyclase catalyzes ATP cAMP PPi The second messenger in turn initiates a series of intracellular events In the case of cAMP these enzymatic changes activate the transcription factor CREB cAMP response binding element activated CREB turns on gene transcription and the cell begins to produce the appropriate gene products in response to the signal it had received at its surface Slide 9 diagram on slide first you have the receptor at the resting state inactive binding of ligand triggers the conformational change of the receptor then you have the effector molecule which is inactive until it binds onto the receptor all of these are membrane associated proteins second messenger is not shown on this slide Slide 10 s stimulatory Galpha s subunit activates membrane bound effector enzyme adenylyl cyclase Once activated this enzyme catalyzes synthesis of the second messenger cAMP the second messenger receptor is Beta Adrenergic epinephrine In contrast the Galpha i protein subunit inhibits andenylyl cyclase leading to a decrease in cAMP Slide 11 Alpha2 Adrenergic receptor is coupled to the Galpha i protein Figure General Structure of G protein coupled receptors All G protein coupled receptors have the same orientation in the membrane and contain 7 transmembrane alpha helical segments 4 extracellular segments and 4 N terminus is on the exoplasmic face and the C terminus is on the cytosolic face cytosolic segments of the plasma membrane Slide 12 surface cells membranes Epinephrine binds to liver and muscle cells at speci c receptors on the outside of The receptor then activates a series of enzymatic reactions inside the cells resulting in the synthesis of large amounts of cAMP Epinephrine can t cross the cell membrane so its signal is transmitted inside the cell via the second messenger cAMP cAMP then switches on a cascade of enzymes mostly kinases nally resulting in the activation of glycogen phosphorylase and enzyme that breaks down glycogen into its glucose units and then releases the glucose into the blood stream Slide 13 the epinephrine cAMP causes different responses in different cells it induces cell speci c responses In cardiac muscle epinephrine induces an increase in the contraction rate In skeletal muscle it induces an increase in the conversion of glycogen to glucose protein 5 Slide 15 remember coats Slide 14 most important concept is the mechanism ligand binds onto cell surface receptor which is located in the plasma membrane integral protein with several transmembrane domains G protein alpha beta gamma subunits are associated with the membrane The Galpha and G beta gamma