BMB 462 Lecture 7 Outline of Last Lecture I Continuation of Primary Active Transport a P Type Continued b F Type and V Type ATPase c ABC Transporters II Secondary Active Transport a Lactose Permease b Na Glucose Symporter III Signal Transduction a General Features b General Process c Types of Receptors d Example Beta Adrenergic Receptor Outline of Current Lecture I Continued Analysis of the Beta Adrenergic Receptor a Review of Structure b Mechanism c Termination II Different Signal Transduction Cascades III G Protein Inhibitors IV Multivalent Proteins V Insulin Regulation of Gene Expression Current Lecture Concepts to remembers from previous courses lectures The structure of ATP adenosine sugar and 3 phosphate groups I Continued Analysis of the Beta Adrenergic Receptor a Review of Structure These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute i Beta Adrenergic Receptor has 7 transmembrane helices a hormone binding pocket facing the extracellular environment and is associated with the G protein GS 1 GS has 3 subunits alpha beta and gamma b Mechanism i A hormone binds to the hormone binding pocket causing a conformational change that makes the receptor protein shift a GDP leaves the associated GS protein allowing GTP to bind which activates the switch 1 Once the hormone binds and the conformation shift occurs G dissociates from the receptor protein 2 G is palmitoylated it is attached to a 16 0 fatty acid while G is prenylated it is attached to an isoprene G and G remain associated a Linking the G proteins to lipids cause the subunits to diffuse only in the plane of the membrane 3 The lipid linked proteins have a hydrophobic lipid portion in the membrane that keeps them associated with the membrane a When the hormone binds to the receptor you activate the G protein which has GTP bound to it and the subunit dissociates from the subunits i subunits stay associated to the membrane which means the 3 subunits can only move in a 2D plane they can t move out into the cytosol which increase the probability of beneficial interactions 1 The subunit is more likely to hit Adenylyl cyclase and interact with it if it s associated with the membrane than diffusing through the cytosol 4 Lipid rafts further cluster components i e signal transduction components so the GS protein doesn t have to diffuse around the entire cells to find them a i e Adenylyl cyclase is likely close to the GS proteinreceptor complex so the G protein can find it more quickly c Signal Termination if a signal is activated the cell needs ways to terminate it i G proteins these proteins are very important in many signal transduction pathways 1 They are a large family of small proteins that bind GTP and slowly hydrolyze it to GDP Pi they re also known as GTPases a The most famous example is Ras a protononcogene that converts to oncogene and causes cancer 2 G proteins serve 2 functions a Switch releasing GDP and binding GTP turns G proteins on makes them active They are turned off by a timing mechanism b Timer GTPase activity hydrolyzes GTP to release Pi GDP is now bound making the protein inactive 3 Most G proteins are pretty bad at releasing GDP to bind GTP they need help from GEFs Guanisine nucleotide Exchange Factors a receptor which aid in releasing GDP 4 Kcat of reactions second an enzyme can catalyze this value is usually 1000s 100 000s of reactions second a The Kcat for G proteins however is about 5 reactions minute they are really bad enzymes which is good because it gives them a period of time when they re active before they shut themselves off i Activity is moderated by GAPs GTPase Activator Proteins that speed up GTP hydrolysis this allows the cell to control how long the switch is on ii Phosphodiesterases 1 Phosphate is attached to the 3 and 5 Carbons of active cAMP in order to stop the signal you have to remove the Pi from cAMP 2 Phosphodiesterases hydrolyze the 2nd messenger cNMP i e cAMP or cGMP back to NMP a It uses water to dephosphorylate to NMP in order to stop the signaling molecule b There are some specific phosphodiesterases for cAMP and some for cGMP Many drugs work by inhibiting different components of signal transduction This results in changing what s going on in the cell which is useful therapeutically cGMP is really important in controlling smooth muscle contractions Increasing cGMP increases male arousal Viagra inhibits phosphodiesterase which increases the levels of cGMP it was originally going to be a medicine to regulate blood pressure Continuing types of signal termination iii Desensitization 1 In addition to just terminating the signal and shutting off the pathway you sometimes need to regulate pathway sensitivity a Desensitization is more refined than turning the complete pathway off b i e In eyes when you go into bright light the eyes desensitize and don t absorb as much light when you go into the dim room the eyes re sensitize so they can absorb more light c i e When there is a lot of adrenaline in the blood and still binding to receptors i GS GS are almost never bound But GS recruits the ARK protein a receptor kinase 1 ARK phosphorylates the receptor which gives it a lower affinity to epinephrine epinephrine is now bound for a shorter period of time which means less signaling 2 After phosphorylation ARR Arrestin is attracted to and binds to the phosphorylated C terminus it needs a Pi present to bind a This creates a receptor arrestin complex which gets endocytosed into the cell so that there are fewer receptors available on the cell surface to bind the signal b This results in a reduced sensitivity ii To restore sensitivity the receptor arrestin complex slowly dephosphorylates so ARR is no longer bound and the receptor can return to the plasma membrane II Different Signal Transduction Cascades Mix and Match components a Other G proteins there are potentially over 1000 types the cell creates this by mixing and matching components i GS activates adenylyl cyclase ii GI inhibits adenylyl cyclase and is associated with another complex iii GQ activates phospholipase C iv Transducin receptor cells in eyes activates phosphodiesterases to break down cGMP b Other 2nd Messengers i Activated adenylyl cyclase creates cAMP ii Active Phospholipase C creates IP3 and DAG Ca2 because IP3 opens the Ca2 ligand channel iii Depending on what the cell is activating and what components are available you get a
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