Cell Signaling 2020 PDF

Cell signalling Stuart Cruickshank  Events in signal transduction Events often occur in a relay chain, sometimes called a cascade. First messenger – extracellular molecule (signal), binds to a receptor. Binding activates receptor protein, which then activates relay protein. Re...

Cell signalling Stuart Cruickshank  Events in signal transduction Events often occur in a relay chain, sometimes called a cascade. First messenger – extracellular molecule (signal), binds to a receptor. Binding activates receptor protein, which then activates relay protein. Relay protein stimulates another membrane protein which acts as an effector (effects changes in cell). Effector protein – enzyme that produces a secondary messenger (cytoplasmic molecule that triggers metabolic and/or structural responses within cell).  Second messengers Intracellular signaling molecule whose concentration changes in response to binding of extracellular hormone (the first messenger) to cell- surface receptor Include: cAMP (3´,5´-cyclic AMP) cGMP (3´,5´-cyclic GMP) DAG (1,2-diacylglycerol) IP3 (inositol 1,4,5-trisphosphate) Ca2+  Signal Transduction Signal transduction pathway – chain of molecular interactions leading to various responses within the cell. Signal transduction can lead to changes anywhere in the cell (nucleus, cytoplasm, membrane) Signal transduction pathways can regulate enzyme activity but also synthesis of proteins, cell movement etc. Signal transduction involves amplification, allows for acute sensitivity of the cell to very small amounts of signal. Highly efficient Signal transduction pathways are specific. Different kinds of cells have different kinds of receptors, protein kinases, etc. - each kind of cell can have its own particular response to the same hormone based on how it puts together its signal transduction pathway.  Amplification Amplification by transduction involving a series of enzymes (usually protein kinases) Enzymes may catalyze a chemical reaction without being used up in the process; one activated protein kinase may phosphorylate many more than one individual target protein (or instead may activate more than one of the same kind of target protein) Process allows an exponential increase in the number of activated proteins (e.g., one protein activates two, which together activate four, which together activated eight, etc.), reception of few ligands at the cell surface can lead to dramatic changes in enzyme activity within the cell.  Why does cell use a signaling cascade in signal transduction? Signal amplification Regulation and interaction of signaling pathways Protein kinase A kinase is an enzyme that phosphorylates another protein (or, in the case of a tyrosine-kinase receptor, also themselves). ATP supplies the phosphate group. A tyrosine kinase is therefore an enzyme that phosphorylates tyrosine amino acids found on target proteins. Protein phosphatase A protein phosphatase catalyzes the reverse reaction of the one catalyzed by a protein kinase, i.e., the hydrolytic removal of a phosphate added to a protein. Protein phosphatases allow reversibility to the protein-kinase-mediated phosphorylation of a protein, thus contributing to the dynamic nature of a cell.  serine (Ser ) threonine (Thr) H H H 3 N+ - H 3 N+ - C C OO C C OO C H2 C H OH OH C H3 Many enzymes are regulated by covalent attachment of phosphate, in ester linkage, to the side-chain hydroxyl group of a particular amino acid residue (serine, threonine, or tyrosine).  Protein Kinase O - P ro te in O H + ATP P ro te in O P O + ADP - O Pi H2 O Protein Phosphatase  A protein kinase transfers the terminal phosphate of ATP to a hydroxyl group on a protein.  A protein phosphatase catalyzes removal of the Pi by hydrolysis.  Protein Kinase O - P ro te in O H + ATP P ro te in O P O + ADP - O Pi H2 O Protein Phosphatase Protein kinases and phosphatases are themselves regulated by complex signal cascades. For example:  Some protein kinases are activated by Ca2+-calmodulin.  Protein Kinase A is activated by cyclic-AMP (cAMP).  Cell signalling The G-Protein Coupled Receptor Stuart Cruickshank 1) GPCRs = the largest class of cell surface receptors. 2) Primary mechanism by which cells sense and respond to their external environment. 3) Activated by a wide range of extracellular signals; small biogenic amines, large protein hormones, neuropeptides, chemokines, 4) Fundamental receptors for the sensory perception of light, taste and smell. G-protein coupled receptors (GPCRs) is one of the most important families of drug targets. Of the top 200 best selling prescription drugs, more than 20% interact with GPCRs (sales ~ £11 billion).  GPCR = G Protein Coupled Receptor = Serpentine Receptor = 7-TM Receptor Invariable: 7-TM Variable: Ligand binding (Terminal region + extracellular loops) Cytoplasmic loop regions allow various downstream events Single ligand could bind multiple receptors: Adrenaline: 9 GPCR Acetylcholine: >5 GPCR Serotonin: >15 GPCR The G-Protein. What is it? The heterotrimeric G Proteins The structure of an inactive G Protein: GDP bound Gα complexed with GβGγ Gα has GTPase activity Gβ interacts with GTPase domain of Gα and keep it GDP bound conformation. Gα and Gγ are membrane anchored Gα undergoes conformation change but not the Gβγ proteins  G-Protein-coupled receptors One intracellular loop interacts with G-protein G-Protein is membrane protein 3 subunits (αβγ) α-subunit possesses GTPase activity On binding α-subunit dissociates- free to activate membrane enzyme or ion channel Activation terminated on hydrolysis of GTP molecule Examples include muscarinic, adrenoceptors  G-Protein activation results in two potential signaling components. Ground state Ligand Binding G protein activation Gα specific Gβγ specific targets targets