Exam 3 Notes Sessions 20-29*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. Lecture #20•Slide 1•General Principle and G-protein coupled receptors•Slide 2•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 specifically 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-proteinʼs 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 Gα subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to re-associate with Gβγ and starting a new cycle•Slide 6•There are 7 class of cell surface receptors known today, but weʼre focusing on G-protein coupled receptors•Slide 7•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'.•receptor binds onto the ligand which then triggers the release of the second messenger•relay of the information also amplifies 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•Galpha, s subunit activates membrane bound effector enzyme adenylyl cyclase (s=stimulatory)•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•Alpha2-Adrenergic receptor is coupled to the Galpha, i protein•Slide 11•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 cytosolic segments•N-terminus is on the exoplasmic face, and the C-terminus is on the cytosolic face of the plasma membrane•Slide 12 •Epinephrine binds to liver and muscle cells at specific receptors on the outside of surface cells membranes•The receptor then activates a series of enzymatic reactions inside the cells, resulting in the synthesis of large amounts of cAMP•Epinephrine canʼt 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) finally 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-specific 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•Slide