26 Asma Karameh BAHAA NAJJAR Ebaa' Alzayadneh
Generally speaking, all cells have been programmed during development to response to specific set of extracellular signals produced by other cells.these signals act in various combinations to regulate the behavior of cells... Those are 4 forms of intracellular signaling: Important to mention that cell signaling requires complementary set of of receptor proteins in each cell that bind and respond to those molecules in characteristic ways. And those are 3 major classes (extracellular receptors): Enzyme linked proteins Covalently or non-covalently G-protein coupled receptors Ionotropic receptors Dear friends this lecture has many repeated information however our aim is to ensure these information by slight addition so don t be confused of physiology but enjoy this experience of seeking deeply ^-^ 1 P a g e
Firstly G-protein coupled receptor (GPCR) form the largest family of cell-surface receptors and found in all Eukaryotes They mediate the response to an enormous diversity of signal molecules including hormones, neurotransmitters and local transmitters The same signal can activate many different receptor family members We have just one structure of those receptors although the chemical and functional diversity of the signal molecules. Notice: These domains facing the extracellular side of the plasma membrane responsible of recognizing & binding ligands. Ligand binding promotes oligomerization which affects activity of the receptor. Examples: adrenergic receptors,opioid receptors,adh receptors, serotonin And Rhodopsin, olfactory receptors. Binding of a ligand to these receptors causes conformational change that enables them to activate GTP-binding proteins, and usually synthesis second messenger. es It sense light, via a bound chromospheres, retinal. 2 P a g e
These proteins are attached to the cytoplasmic face of the plasma membrane They serve as relay molecules,functionally coupling the receptors to ion channels and enzymes in this membrane There are various types of G-proteins,each specific for a particular set of receptors and for a particular set of downstream target proteins in the plasma membrane In the unstimulated state "resting state, alpha subunit has GDP bound. Subtypes of G-PROTEINS INCLUDE,Gi, G q, Gs..etc. G-proteins sisassemble to relay signals from G-PROTEIN LINKED RECEPTORS When stimulated by an activated receptor, alpha releases its GDP allowing GTP to bind in its place. This exchange causes the trimmer to dissociate into two activated subunits : Alpha subunit, and Beta -Gamma complex This change causes Alpha subunit to adopt a new shape that allows it to interact with its target protein The beta gamma complex doesn t change its conformation,but the surface previously masked by the alpha subunit is now available to intract with a second set of target proteins (enzymes or ion channels) Reassosiation of three subunits together occurs by alpha subunit which is a GTPase hydrolyses its bound GTP into GDP, reversing the activation process. The time during which those subunits are active and apart is usually short and depends on how quickly the alpha subunit hydrolyses its bound GTP. 3 P a g e
C-AMP Is synthesized from ATP by an enzyme called ADENYLYL CYCLASE C-AMP Is continuously destroyed by c-amp phosphidiesterase that hydrolyses c-ampinto 5-AMP. MANY extracellular signals work on increasing c-amp CONTANT BY increasing the activity of adenylyl cyclase rather than decreasing the activity of c-amp Notice: GS involved in increasing c-amp by direct activation of adenylyl cyclase. Although it is usually alpha subunit that regulate the cyclase, the bet a-gamma complex does so a well. Depending on the particular beta-gamma complex and the isoform of the cyclase Hormones That Use the Adenylyl Cyclase camp Second Messenger System: FLAT CHAMP F: FSH C:CRH L:LH H:HCG A: ACTH A:ADH T:TSH M:MSH P:PTH C-AMP can directly activate certain types of ion channels of highly specialized cells C-AMP exerts its effect mainly by activating c-amp-dependent-protein- 4 P a g e
R2C2 Mechanism of regulating cells activities by PKA: 1) When c-amp binds to R a conformational change causes R to release S. 2) The catalytic subunits can then catalyze the transfer of terminal phosphate group from ATP to OH specific threonines or sirens in a particular 5-aminoacidsequence of selected target cells thereby regulating their activities Consider that PKC ARE DIFFERENT KINASES ACTIVATED BY Ca ++ As we almost reached the end of this part ^-^: It s the last step in signaling in which signal must stop propagation in many ways: 5 P a g e
1) 1-G-alpha itself contains GTP-ase which hydrolyses GTP INTO GDP and pi so the presence of GDP on G-ALPHa causes it to re-bind to the inhibitory BETA-GAMA complex. (Automatically release of Phosphate group and they can bind to certain isoforms of adenylate cyclase and deactivate it.) 2) Phosphidiesterase :this enzyme becomes active after initiation of a signal,it is activated by some kinases which cause phosphorylation of it (c-amp itself mediates its distribution(degradation)) 3) Down regulation of the receptor, assuming hormone didn t exposed to metabolism and still bounded to the receptor, this step must be ended. By Phosphorylation of the receptor itself,it becomes as tagged which helps to be recognized by protein BETA-Arrestin which promotes removal of the receptor from the membrane by clathrin-mediated endocytosis (internalization of the receptor so it's no longer available for the ligand 4) Termination by phosphatases: they remove the phosphate group that was added to some proteins thus a complete removal of the signal and the things resulted from the signal. So we need another signal for a continuation of the response. Important to consider that signals may cause activation of I and activation of s however the net result will be either to increase c-amp or neutralization of c-amp to normal level,so effect is based on the net result. Bg considered as inhibitor for G alpha, and sometimes an inhibitor for ADENYLYL CYCLASE ITSELF. Here are examples of molecules may be found in the signaling pathways,all of them are GTP binding proteins,their role is to provide GTP for a rapid exchange of GDP to GTP.. But involved to different special kinds of cells. Some of them are active when they are GDP bound but those are few. Other kinds of proteins called helper proteins, in which most GTP binding proteins depend on them in the exchange process: Gaps: GTPase Activating Proteins, promote GTP hydrolysis. 6 P a g e
GEFs: Guanine Nucleotide Exchange Factors, promote GDP/GTP exchange (GDP GTP) Another reference: molecular biology of cell P.P. QUESTION: Thank you all すべてのいただきありがとうございます ^-^ 1- You made up a drug that bind to G protein coupled receptor (GPCR) and increase c- AMP; what method should we use in this drug: ANS: causes irreversible binding of GDP to αs subunits 2- Glucagon binds to G-protein coupled receptor adenylyl cyclase will be activated and camp is formed: ANS: second messenger 3- The enzyme which transfer phosphate group from ATP to proteins Ans: PKA 4- Consider this pathway: epinephrine G protein linked receptor G protein adenylyl cyclase camp. Identify the second messenger: Ans: C-AMP 5- When a receptor binds to its G protein, which of the following happens next: Ans: GDP is replaced by GTP 6- Under which of the following situations would receptor down-regulation most likely occur? (Down-regulation means that the receptors decrease) Ans: The concentration of a hormone is too high. 7 P a g e