Lipids and Membranes Presented by Dr. Mohammad Saadeh The requirements for the Pharmaceutical Biochemistry I Philadelphia University Faculty of pharmacy
Membrane transport D. Endocytosis and Exocytosis Macromolecules that are too large to pass through biological membranes are taken into cells by a process called endocytosis. Receptor-mediated endocytosis being with binding of macromolecules to specific receptor protein in the plasma membrane of the cell. The membrane then invaginates, forming a vesicle that contains the bound molecules called phagosome (figure 9.36), then the vesicle can fuse with endosome (another type of vesicle) and then with a lysosome that is caused degraded. Alternatively, the ligand, the receptor, or both can be recycled from the endosome back to the plasma membrane, Exocytosis is similar to endocytosis, except the direction is reversed. The materials destined for secretion from the cell enclosed in the vesicles by the golgi apparatus. The vesicle then fuse with the plasma membrane.
Membrane transport D. Endocytosis and Exocytosis Figure 9.36: Endocytosis and Exocytosis
Drug-Receptor Theories Divided to A. Cell-surface (extracellular) receptors 1. G Protein as signal transducers such as 1. The adenylyl cyclase signaling pathway and 2. The Inositol-Phospholipid Signaling Pathway. 2. Tyrosine kinase receptor such as insulin tyrosine kinase receptor 3. Ligand-gated ion channels. B. Intracellular receptor; steroid hormones
Transduction of extracellular signals * The plasma membrane of all cells contain specific receptor that allow the cell to respond to external chemical stimuli that cannot cross membrane. * Chemotaxis: A chemical signal via cell surface receptor to the flagella in bacteria, causing the bacterium swim toward food or away from toxic chemicals. * Hormones: molecules that allow cells in one part of an organisms to communicate with cells in another part of the same organism. * Neurotransmitters: substances that transmit nerve messages at synapses. * Growth factor: proteins that regulate cell proliferation. * A general mechanism for signal transduction (figure 9.37) a ligand binds to its specific receptor on the surface of the target cell. This generates a signal passed through a membrane protein transducer to a membrane bound effector enzyme. The action of the effector enzyme generates an intracellular second messenger which may be nuclear effectors and cellular response.
Transduction of extracellular signals * A single ligand receptor can interact with a number of transducer which can activate several effector enzyme, production many second messenger to activate many kinase enzymes which catalyze phosphorylation of many target protein this series is called cascade.
Transduction of extracellular signals A. G Protein are signal transducers: such as 1. The adenylyl cyclase signaling pathway 2. The Inositol-Phospholipid Signaling Pathway
Transduction of extracellular signals B. The adenylyl cyclase signaling pathway: 1) Hormone binds to a specific receptor. 2) GDP exchange to GTP on Gs and dissociation of the β and γ (Figure 9.39). 3) (G S, α, and GTP), moves to adenylyl cyclase (effector enzyme) and activates it. 4) Adenylyl cyclase cleaves ATP to camp and PPi. Note: effector enzyme can respond to stimulatory G protein (G S ) or inhibitory G protein (G i ). 5) camp (second messenger) activate protein kinase A at the cytosol. 6) Phosphorylation of cellular proteins to get cellular responses to the hormone. (see figure 9.43).
Transduction of extracellular signals A. G Protein are signal transducers
Transduction of extracellular signals: B. The adenylyl cyclase signaling pathway
Transduction of extracellular signals B. The adenylyl cyclase signaling pathway: The effector enzyme is adenylyl cyclase. The second messenger is cyclic AMP (camp). Regulation of The adenylyl cyclase signaling pathway 1. Hormone that bind to inhibitory G protein (G i ) inhibit adenylyl cyclase activity (figure 9.43). 2. camp concentration in the cytosol increases. camp phosphodieterase catalyzes the hydrolysis of camp to 5ˋAMP. 3. GTPase convert GTP to GDP, results inhibit adenylyl cyclase activity (Figure 9.38).
Transduction of extracellular signals B. The adenylyl cyclase signaling pathway * camp concentration in the cytosol increases. camp phosphodieterase catalyzes the hydrolysis of camp to AMP. * the methylated purines caffeine and theophylline inhibit prolong and intensify camp phosphodieterase and hence the activating effects of the stimulatory hormones.
Bacterial toxin and G Proteins Cholera toxin 1. Inter to the cytosol and activated the adenylyl cyclase by inactive GTPase so prevent forming GDP from GTP and prevent forming the inactive G protein (Gs, GDP, αβγ subunits) ; so 2. the camp level stay high 3.Dehydration result Pertussis toxin 1. Inhibit Gi protein due to the GDP can not replace with GTP; so 2. camp level stay high 3. Whooping cough
C. The Inositol-Phospholipid Signaling Pathway: see video 1. Hormone binds to to transmembrane receptor. 2. GDP exchange to GTP on Gq and dissociation of the β and γ. 3. (Gq, α, and GTP), moves to Phospholipase C (PLC) which is the effector enzyme and activates it. 4. PLC cleaves phosphatidylinositol 4,5 bis-phosphate (PIP 2 ) to inositol triphosphate (IP 3 ) and Diacylglycerol (DAG). (IP 3 and DAG are second messenger) 5. IP 3 binds to a specific intracellular receptor calcium channel on the endoplasmic reticulum, releasing sequestered Ca +2. (Ca +2 is a second messenger). 6. Diacylglycerol (DAG) and Ca +2 activate protein kinase C at the surface of the plasma membrane. 7. Phosphorylation of cellular proteins produces some of the cellular responses to the hormone. The Inositol-Phospholipid Signaling Pathway IP3, DAG and Ca+2 are second messengers Phospholipase C (PLC) is an effector enzyme.
C. The Inositol-Phospholipid Signaling Pathway: see video IP3, DAG and Ca +2 are second messengers Phospholipase C (PLC) is an effector enzyme. Note: The calcium signal is short time Ca +2 is pumped back into lumen of endoplasmic reticulum when channel is closed. DAG activates protein kinase C by increasing its affinity for calcium ions. protein kinase C catalyzes phosphorylation of protein (serinethreonine kinase family).
C. The Inositol-Phospholipid Signaling Pathway Several ways turned off the Inositol-phospholipid pathway 1. When GTP is hydrolyzed, Gq returns to its inactive. 2. The activation of IP3 is transient (short). IP3 hydrolyzed to other inositol phosphate. DAG converted to phosphatidate. (Both inositol and posphatidate are recycled back to phosphatidylinositol) 3. Phosphatidylinositol and extracellular signals lead to hydrolases sphingolipids to 1. sphingosine, 2. sphingosine1-phosphate and 3. ceramide. sphingosine inhibit protein kinase. Ceramide activate protein kinase and protein phosphatase. sphingosine1-phosphate activate phospholipase D
C. The Inositol-Phospholipid Signaling Pathway
D. Receptor Tyrosine kinases :Receptor tyrosine kinase exists of monomer free flooding in the cell membrane each receptor has an extracellular domain and intracellular domain. Activation of receptor tyrosine kinase: 1. Ligands bind to receptor on extracellular domain induce receptor dimerization and become activated. 2. After tyrosine kinase dimerises it autophosphorylation, the tyrosine kinase of one monomer add phosphate group to other monomer. 3. The phosphorylated dimer can catalyze phosphorylation of cytosolic proteins.
D. Receptor Tyrosine kinases example: The insulin receptor is an α 2 β 2 tetramer. Figure 9.47
D. Receptor Tyrosine kinases Activation of insulin tyrosine kinase receptor: 1. Insulin bind to receptor tyrosine kinase to activated, 2. leading to autophosphorylation of insulin-receptor substrates (IRSs) interact with phosphatidylinositide 3-kinase (PI kinase) at the plasma membrane. 3. PI kinase act as effector enzyme, catalyzes phosphorylated PIP2 to PIP3. 4. PIP3 act as a second messenger and activate the protein kinases to phosphorylate the cellular proteins (cellular response). Note: The effector enzyme is phosphatidylinositide 3-kinase (PI kinase). The second messenger is PIP3.
E. Ligand-and Voltage-Gated Channels: 1. When acetylcholine bind to the α- subunits, a conformational change causes the opening of the ion in the nicotinic acetylcholine receptor (nachr) also called ionotropc receptor, 2. which allows Na+ to follow downs its concentration gradients into cells, producing a localized excitatory postsynaptic potential a depolarization. 3. Involved in fast synaptic transmission.
F. Intracellular receptor; steroid hormones Lipophilic or hydrophobic ligands (steroid hormones) enter the cells by diffusion and bind to and activate a receptor in cytoplasm or nucleus. The activator receptor complex regulates transcription and protein synthesis. Example of intracellular receptors: 1. Steroid hormones receptor 2. IP 3 binds to a specific intracellular receptor calcium channel on the endoplasmic reticulum, releasing sequestered Ca +2.