BC 7&8 Signal Transduction I&II Flashcards
Four Cell Signaling Mechanisms
Autocrine
Paracrine
Endocrine (Estrogen, progesterone, testosterone, insulin, glucagon, cortisol, epinephrine
Neuronal
Diabetes
Endocrine: signaling ligand receptor response
Autoimmune: B Cells destroyed, no insulin for signaling
II Less and less effective to insulin response. (Insulin resistance). B Cells stop producing insulin (B cell exhaustion)
Neuronal
Specialized Paracrine
- through the neuron via an electrical potential
- signaling target cells via neurotransmitter (1 cell)
ligand dependent signaling
Hydrophobic signaling molecule transported via carrier protein. Diffuse across membrane, bind to ligand within the cell.
Binding and Activation of cell surface receptors. Alters structure on cytoplasmic side delivering signal inside cell.
steroid hormones
intracellular ligands.
Hydrophobic uncharged, diffuse across
not soluble in blood, require carrier.
testosterone, estradiol, cortisol, progesterone, aldosterone.
ALL DERIVED FROM CHOLESTEROL, maintain same four ring structure.
cortisol, binds to receptor in cytoplasm, travels across nuclear membrane altering gene expression,.
Nitric Oxide
Intracellular Ligand
Nervous, immune, and circulatory systems.
synthesized from argenine by NO synthase. from endo cells: eNOS
soluble in both aqueous and lipid medias, diffuses across membrane. VERY short half life…only close cells affected.
Vasodilation (NO)
- release of ACH from nerve terminus
- ach binds GPCR on surface of endothelial cell of blood vessel wall
- NO synthase activated. Arg->NO & Citrulline
- NO diffuses out to smooth muscle cell
- NO reacts with Fe in the active site of the enzyme guanalyl cyclase. produces cGMP
- cGMP activates path for relaxation of smooth muscle
Nitroglycerin
Converted to NO when administered in body. Does NOT involve NO synthase
Used for angina (chest pain, too little O2 in heart)
Transmembrane Receptors
Initiate phosphorylation Cascades
1. GPCR (g-protein coupled receptor) (seven pass)
- Receptor tyrosine Kinase ( Two separate monomers come together when bound to ligand)
Pre dimerized by disulfide bonds to form INSULIN structure, otherwise, bound just by interactions and ligand
Ligands: either gene or modified molecule
Agonists: activate
Antagonists: interfere with activation by normal ligand (can be synthesized: benedryl/histamine)
Phosphorylation
Transfer of P is COVALENT modification. Requires the action of an enzyme to either form a new phosphate bond (kinase) or break it (phosphatase)
Serine, Threonin, and Tyrosine have OH for phosphorylation. (pattern of AA on either side of these depend if it will react here)
Kinase
When kinase phosphorylation cascade takes place, they do not pass on P, they grab a new one and attach it to the next and stay active, exploding effect.
Targets:
other kinases
phosphatase enzymes: increases DEphophorylation
metabolic enzymes: activate or inactivate
FAST
transcription factors: change gene expression: SLOW
GPCR protein
Mediate hormones, NT’s, local mediators
7 membrane spanning alpha helices
40% of drugs target these…in every major organ
Phopholipase C:
alpha 1
Adenylate Cyclase:
alpha 2
B1
B2
B3
heterotrimeric G proteins
ligand binds to extracellular portion of GPCR and activates heterotrimeric GTP binding protein. Tethered to inner leaflet by fatty acyl tail.
3 Subunits
alpha- gtp binding/hydrolysis (timer)
- Ga s- activates adenylate cyclase +cAMP+PKA
- Ga i - inhibits adenylate cyclase-cAMP -pKA
- Ga q - activates phospholipase +Ca2 +PKC
beta
gamma
basal level of adenylate cyclase all the time either inhibited or promoted
GPCR Activation
Ga s
- Inactive g proteins coupled a b g
- Activation of receptor, alpha releases GDP and binds GTP, alpha dissociates and b g stay together.
- Ga s activates adenylate cyclase
- timer goes off, hydrolyzes GTP, now inactive
- alpha recombines with b g dimer
same for Ga I but inhibits
B and Alpha2 Andrenergic Receptors
Adenylate Cyclase Activity
B Ligand: epinephrine G protein: Ga s Effect: AC stimulated cAMP: High
Alpha 2 ligand: epinephrine G protein: Ga I Effect: AC inhibited cAMP: low
G Protein Defects
Cholera and Pertussis
ADP Ribosylase: adds adp ribose to argenine reside of Ga subunits
Both INCREASE adenylate cyclase
Cholera: toxin from bacteria results in Ga s that cannot hydrolyze GTP. High cAMP
cAMP activates chloride exporter at apical membrane, abnormal electrolyte export. 1L/hr
Pertussis: Ga I cannot function to decrease adenylate cyclase activity.
cAMP
most effect of cAMP, activation of serine/threonine kinase, Protien Kinase A (PKA)
Inactive PKA: tetramer, two catalytic and two regulatory
Active: cAMP binds to regulatory, dissociation from catalytic, activating them
Active PKA phosphorylates and regulates enzymatic activity in cytoplasm
can PhLate trans factor CREB for gene expression. regulates glucose metabolism
TURN DOWN:
cAMP phophodiesterase: removes phophodiester ring, back to normal non functioning AMP
alpha 1 adrenergic receptors
activate phospholipase C
Gq proteins: consists of a b g subunits.
Ga q activates phospholipase C.
Phospholipase C cleaves phosphotidyl inositol (PIP2). produces two intermediate signaling molecules
- diacylglycerol (DAG)
- Inositol triphosphate (IP3)
IP3 hydrophilic, travels in cytoplasm
-IP3 receptor in ER. Opens calcium channels, flows into cytoplasm. Ca binds to PKC and recruits it to the membrane. PKC binds to DAG and Ca2+ and is fully activated. Phophorylates variety of enzymes causing cascade
Ca2+ ATPase can kick Ca back into ER.
Ca2+ activates calmodulin. Binds and allows calmodulin to HUG other enzymes changing confirmation and activity. CAM-Kinase II
or causes CAMkinase II to be AUTOphosphorylated. Keeping it active without further Ca2+
Receptor Tyrosine Kinase
RTK
over 50: EGF, Vascular endothelial GF (VEGF), insulin
inactive as monomers, dimerize for ligand binding. DiSulfide bonds hold dimer together
leads to activation of a Ras
Ras GTPase
Activation of a RTK also begins signaling pathway for activation of GTPase called Ras
lipid tail, membrane bound
GEF’s & GAP’s
Guanine Exchange Factor
-stimulates exchange of GDP for GTP (to activate Ras)(Gets an Energized Form)
GTPase Activation Factor
-stimulates hydrolysis of GTP to inactivate ras (Gives up a Phosphate)
both influence the rate of GTP binding but do not actually do the work!!!
Signalling with RTK
- GF binds to RTK forms dimer
- residues in cytoplasm autophophorylated by eachother
- new binding site in cytoplasm forms
- GRB2 contains SH2 domain to bind to phosphotyrosine residues (GRB solely structural)
- GRB2 recruits SOS protein ( a GEF) and stimulates Ras to exchange GDP for GTP (Activates Ras)
- Ras recruits cytoplasmic protein kinase RAF, RAF becomes activated and phosphorylated
- activated RAFK leaves, starts other cascades
- phophotases will eventually turn off - Fas hydrolyzes GTP and returns to inactive state
insulin operates with RTK as well
