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Lecture 13

Biochemistry- Lecture 13.docx

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BIOL 2021
Patricia Lakin- Thomas

Biochemistry- Lecture 13 G- protein regulated ion channels  binding directly or indirectly through cyclic nucleotide. Visual Cascade -Rod- photoreceptors  vertebrate -fastest known G-protein response -dim light vision  very sensitive -the diagram is the cell - the outer segment  contain the stuff that interact with the sunlight -the retina is an extention of the brain  send signals -photoreceptors = is a protein called Rhodopsin this is a G-protein photoreceptor -ligand is light -change in conformation - All trans-retinal  all the double bonds are trans - Retinal absorbs the light and changes the receptor conformation  this is due to the fact that there is an isomerasization of retinal - It goes from 11 cis to all trans in the double bonds - The activated receptor is now going to cause a change in the G-protein - The G-protein is called G c_+led Transducin  this activates cyclic GMP Phosphodiesterase The cGMP is made by guanylyl cyclase which is destroyed by cGMP phosphodiesterase. In the dark  there is high cGMp ion channels open And the c-GMP gated ion channels -the cell is depolarized  releasing transmitter at a high rate -it seems that there is a high rate of the transmitter -there is a depolarized cell  a high rate of transmitters In the light  the cGMP goes down -the cGMP gated ion channels close and as a result  the cell hyper polarizes and the we get a large membrane potential -the transmitter decreases -the signal to brain interprets this as the presence of LIGHT! -This is a good example of a signalling cascade  amplification  there is a small signal that turns to a very big response - this is a single photon of light  activates rhoposin  activates g- protein  activates the same amount of phosphodiester molecules -after this there is amplification after some of the molecules get destroyed - this is a good example of adaptation  this is turning the signals down -if you walk out of the room  outside into the bright sunlight  blinded  can’t see  in a short length of time  adapts -the visual system has to be able to adapt to different levels of sunlight -receptor desensitization  this is happening when the signal is still present -we want to turn down the volume of the signal  avoid blindness -deprotein coupled protein (GPCR) kinase phorphorylates activated receptor -the Arrestin binds to prevent the G-protein from activation -when going to the dark  this process is opposite -Enzyme-coupled cell surface receptors -Similar to GPCR: transmembrane protein and bind to ligand on the outside -different because on the cytosolic side  has an actual enzyme activity or directly binds an enzyme  there are no other intermediatary proteins -there is only one transmembrane domain - Receptor Tyrosine Kineases = RTK -phosphorylate on tyrosine on substrates and themselves  autophosphorylation -lignands are extracellular protein -cell survival, growth, division or protein on other cell Ephrins =contract- dependent signalling -activation by dimerization Activation by Dimerization: -ligand binding  2 receptors dimerize and there is transautophosphorylation -2 Results: 1) increases kinase activity 2) creates docking sites for other proteins  these attach to the phosphorylation sites **Question: Antibodies are Y-shaped molecules with 2 identical binding sites. What would happen if you exposed a cell to an antibody specific for the extracellular domain of a receptor tyrosine kinase? Would it be activated inactivated, or unaffected? **Answer: Probably will activate by bringing two kinases together to phosphorylate each other. -Form signalling complex  activated docked protein -these are activated because they are phosphorylated -sometimes they just get activated because they docked can tell if a particular molecule will be get docked  depends on the presence of a domain  SH2 domain = Src homology  phosphotyrosine binding -what kinds of proteins can be docked?  enzymes
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