Each of the following molecules contains at least one

Lecture 13 Cell Biology  Types of intercellular Signaling o Autocrine – cell produces signal and its receptors get the message o Paracrine – cell produces signal and other close by cells’ receptors get the messages o Endocrine – hormones are secreted into the blood stream and travelled over long distances to their target cells.  Two major types of signaling pathways o G-protein linked receptors o Receptor protein – tyrosine kinases (RTK’s)  G Proteins and their Receptors o Membrane receptor has 7 transmembrane helices o Monomeric G protein – Ras o Heterotrimeric G protein (Name break down - different polypeptide chains, 3 of them)  Alpha subunit binds GDP and GTP  Beta and gamma subunits o G proteins bind GDP to GTP o G protein coupled receptors – GPCR – the largest family of genes in humans, more than 1/3 of prescription drugs act as ligands for GPCR’s  G Protein Activation o Ligant binds to GPCR, G protein-coupled receptor o GDP on the alpha subunit is replaced by GTP on the alpha subunit.  GDP is stripped off by GTP  Cell does not use phosphylation because delta G is positive and requires a large amount of energy. o Alpha – GTP subunit dissociates from beta gamma subunits o Alpha – GTP activates adenyl cyclase (adenylyl is the same enzyme often written this way in biochemistry  This is the EFFECTOR  Receptor Mediated Activation of Effector by Heterotrimeric G Proteins o The main goal is to synthesize the second messenger which is cyclic AMP o There are 8 steps which are gone through to achieve the goal. 1. When the ligant binds to the receptor, the g protein can bind to the receptor 2. GDP is stripped off and GTP is bound to the alpha subunit. 3. The alpha subunit and GTP now becomes bound to the effector 4. The effector is activated ATP  cAMP Adenylyl cyclase – the effector, the second messenger is made. 5. G protein with GTP on it will hydrolyze GTP back to GDP. (IF the effector is left on the cAMP continues to be produced and can cause cancer. It is important to note that you CANT leave the effector turning out cAMP, the effector must be turned OFF.) 6. GDP binds back to the alpha subunit and the effector is inactivated. 7. GRK – g protein coupled receptor kinase – kinases use ATP to phosphorylate the GPCR (GDP). 8. Once the GPCR is phosphylated it attracts Arrestin.  Re-forming heterotrimer G protein o Alpha GTP ---GTPase activy -- alpha GDP o Alpha – GDP binds to beta gamma subunits. o Ready for next ligand binding  Arrestin Mediated Internalization of GPCRs o GPCR is phosphorylated and arrestin protein. o Binding of the arrestin protein signals that it should be endocytosed in one of the clathrin coated pits.  G Linked Processes o Stimulus and ligand binding o GPRC is the receptor o Effector  Adenylyl cyclase  Phospholipase C – has lipids as signaling molecules o Physiological response  cAMP as 2 nd messenger o G protein activated the effector which is adenylyl cyclase o Synthesis of the 2 ndmessenger cAMP o cAMP activates PKA which is phosphokinase A  Formation of cAMP from ATP is catalyzed by adenylyl cyclase o Has 12 helicies. Has a cytoplasmic portion that contains the enzymes active site  Basic Process – Glycogen Breakdown o Ligand binds to GPCR (receptor) o G protein is activated o Adenylyl cyclase (effector) is activated o Generate cAMP which is 2 ndmessenger o Enzymatic cascade leafing to activated glycogen phosphorylase o Activated glycogen phosphorylase cleaves glycogen to give glucose-1-P  Enzyme that cleaves glycogen that gives us glucose- 1-Phosphate  Lipid Derived Second Messengers o Sites where phospholipase (PL) A1, A2, C, and D cut. o Phosphatidyl-inositol derivatives. o Head group – inositol ( a sugar) o Phospholipase a1 and 2 cleave off the fatty acid glycerol. o DAG – Diacyl glycerol