Drug Mech Exam 1:3

By a numer of different molecular mechanisms (or transmembrane signaling pathways).

Each mechanism has evovled to transduce many different signals.

• -receptors within cells and on cell membranes
• -enzymes
• -signal transducer proteins
• -other molecules that generate, amplify, coordinate, and terminate postreceptor signaling by chemical second messengers

• 1. Intracellular Receptors for Lipid-Soluble Agents
• 2. Ligand-Regulated Transmembrane Enzymes (Receptor Tyrosine Kinases)
• 3. Cytokine Receptors
• 4. Ligand-Gated Ion Channels
• 5. G-Proteins and Second Messengers

A lipid-soluble ligand (drug) crosses the plasma membrane and acts on an intracellular receptor (an enzyme or regulator gene transcription)

• -nitric oxide
• -corticosteriods
• -sex hormones
• -thyroid hormones
• -vitamin D

Receptors are transmembrane protiens consisting of extracellular (ligand-binding domain) and a cytoplasmic intracellular (enzyme domain).

protein tyrosine kinase, serine kinase, guanylyl cyclase

a hydrophobic segment of the polypeptide that crosses the lipid bilayer of the plasma membrane.

Ligand binds to extracellular domain of receptor, activating (allosterically regulating) the enzymatic activity of the intracellular domain.

insulin, epidermal growth factor (EGF), platelet-derived growth factor (PDGF), transforming growth factor (TGF), and other trophic hormones.

Receptor Tyrosine Kinase

1. Ligand binds to receptor's extracellular domain results in conformational change

2. conformational change causes receptor molecules to bind to one another (including tyrosine domains)

3. Tyrosine domains become enzymatically active, and phosphorylate one another (as well as downstream signaling proteins)

Activated RTKs phosphorylate tyrosine residues on different signaling protiens and allow a single activated receptor to modulate several biochemical processes

EX: each growth factor ligands initiates complex events with specific targets (altered membrane permeability to changes in gene expression)

Specific inhibitors of growth factor-activated Receptor Tyrosine Kinases

(where over expression of growth factor receptors and excessive growth factor signaling occurs)

• 1. Ligand binds to extracellular domain
• 2. transmembrane receptor protein is already bound to a protein tyrosine kinase (JAK)
• 3. Protien tyrosine kinase is activated by receptor-ligand binding
• 4.activated tyrosine kinase phophorylates tyrosine residues on receptor
• 5. Receptor binds to another set of proteins (STATs) and phosphorylates them
• 6. Once phosphorylated, STATs (Signal Transducers and Activators of Transcription) dimerize, dissociate from receptor, and travel to nucleus to regulate gene expression

Cytokine receptors and RTKs are similar in mechanisms.

However, cytokine receptors, the protein tyrosine kinase activity is NOT intrinsic to the receptor molecule.

A SEPERATE protein tyrosine kinase JAK (janus-kinase) family binds noncovalently to receptor

-kinase enzyme not part of receptor

growth hormone, interferons, other regulators of growth and differentiation

A ligand-gated transmembrane ion channel is induced to open or close by binding of a ligand

• 1. ligand binds
• 2. receptor changes conformation
• 3. channel opens/closes

neurotransmitters acetylecholine, serotonin, B-aminobutyric acid (GABA), and excitatory amino acids (glycine, aspartic acid, and glutamic acid)

By mimicking or blocking the actions of endogenous ligands that regulate ion channels.

• 1. Ligand binds to the extracellular domain
• 2. Receptor stimulates GTP-binding signal transducer protein (G-Protein)
• 3. G-Protein activates Effector
• 4. Effector modulates production of intracellular second messengers

on the cytoplasmic face of the plasma membrane

enzymes or ion channels

Serpentine Receptors or 7-Transmembrane Receptors

All serpentine receptors transduce signals across membranes in same fashion

cAMP, Calcium ions, phosphoinositides

catecholaimes via B-adrenoceptor, Histamine via H2 receptor, vasopression via V2 receptor, glucagon, FSH, LH, thyrotropin, parathyroid hormone

Adynylyl Cyclase, a transmembrane protein converts ATP into cAMP via Gs.

Gs stimulates anynylyl cyclase after being stimulated by ligands that act via specific receptor

Binding and hydrolysis of GTP, amplyfies the transduced signal.

Amplification is attributed to the fact, active GTP-bound G protein remains active for a rel. long time (tens of seconds), because G-protein will have to undergo dephosphorylation to become inactive again.

Active G-protien (Gs) activates effector molecule, thus prolonging and transducing more signals

duration of activation of the G protein

• 1. G Protein is inactive when attached to GDP (Go)
• 2. GDP is phosphorylated to GTP
• 3. G Protein is active when attached to GTP (Gs)
• 4.GTP has to be dephosphorylated to GDP again for G Protein to become inactive

• cAMP (cyclic adenosine monophosphate)
• Calcium and Phosphoinositides
• cGMP (cyclic guanosine monophosphate)

• -hormonal responses (breakdown of carbohydrates and triglycerides)
• -conservation of water by kidneys
• -calcium homeostatsis
• -increased rate and contraction force of heart muscle
• -production of adrenal and sex steriods
• -relaxation of smooth muscle
• -numerous other endocrine and neural processes

The ligand

By stimulating cAMP-dependent protein kinases

The specificity of cAMP's effects is attributed to the distinct protein substrates (usually enzymes) of kinases that are expressed in different cells.

• 1. Dephosphorylation of protein substrates (phosphatases)
• 2. cAMP degradation to 5'-AMP (phosphodiesterases, PDE)

If more cAMP accumulates and is not degraded, the response created by the kinases will be prolonged, due to the presence of the cAMP.

ex: Viagra

False. They may oppose and/or compliment one another in different cells.

ex: cAMP-mediated smooth muscle relaxation vs. Calcium-phosphoinositide-mediated smooth muscle contraction)

Yes. Protein Kinases are important targets for discovery efforts in a number of disease.

Specific inhibitors of protein kinases have potential as drugs, esp. in cancer chemotherapy

Imatinib, is an inhibitor of a tyrosine kinase which is activated by growth factor signaling pathways, used to treat chronic myelogenous leukemia.