002 hydrophilic hormones and enzyme cascade Flashcards
give some examples of hydrophilic hormones
- proteins e.g. insulin
- peptides
- amino acid derivatives e.g. adrenaline
what is unique about hydrophilic hormones?
- can dissolve into plasma in blood easily as hydrophilic without needing to transport via proteins
(unless they are already bound to some protein e.g. albumin) - however they cant cross plasma membrane as they are hydrophilic so use GPCR
what do endocrine hormones do?
- act on cells far from the site of release
- secreted into the blood
- only target cells that express the receptor
- e.g. insulin, adrenaline
what do paracrine hormones do?
- act on nearby cells only
- diffuse into interstitial fluid and are rapidly inactivated by local enzymes
- e.g. histamine
what do juxtacrine hormones do?
- the hormone is either bound to the membrane (requiring physical contact between cells)
- or the hormones is secreted into extracellular matrix
what do autocrine hormones do?
- act on the cell that released the hormone
- e.g. T cells, interleukin-2
what are the 4 types of hormone receptors?
- ligand binding ion channels
- receptor enzymes
- enzyme-recruiting receptors
- G-protein coupled receptors
describe ligand-gated ion channels and give an example
- the signal is transduced to the cell via a change in membrane potential/ when the ion channel opens
- e.g. ACh receptors
describe receptor enzyme hormone receptors and give an example
- enzymatic activity of receptor is activated by hormone binding
- e.g. insulin
describe enzyme recruiting receptors and give an example
- hormone binding induces the recruitment and activation of protein kinases
- e.g. cytokine receptors
describe G-protein coupled receptors and give an example
- hormone binding activates GTP-binding proteins/second messengers
- e.g. adrenaline receptors
describe the pathway of general signal transduction of hormones
- hormone is released
- hormone binds to receptor on plasma membrane of cell, which induces a conformational change in recptor’s cytosolic region that alters its function
- concentration of a 2nd messenger increases through enzymatic action
- effectors are stimulated or inhibited by second messenger (pumps, enzymes, tfs)
- signalling pathway is shut down, effectors return to original state and messengers removed/become ineffective
describe the overall process/effects of a hormone binding to a GPCR
- hormone-bound receptor causes the exchange of GDP for GTP, activating G alpha subunit to do different actions
- G alpha subunit then eventually hydrolysis the GTP back into GDP, becoming inactive again
what are the 4 types of G alpha (GPCR) subunits?
- Gs, Gi, Gq, Gt
what is the function of Gs alpha subunits?
- activate adenylyl cyclase –> increase cAMP
what is the function of Gi alpha subunits?
- inhibits adenylyl cyclase –> decreases cAMP
what is the function of Gq alpha subunits?
- activates phospholipase C –> increases DAG, IP3, Ca
what is the function of Gt alpha subunits?
- activates retinal cyclic GMP phosphodiesterase
what are the different types of GPCR?
- G alpha = s,i,q,t
- G beta (5 isoforms)
- G gamma (6 isoforms)
what do G beta and gamma GPCRs do?
- may alter the specificity of receptor G protein binding, cooperate in transduction or shut pathways down
what enzymes does protein kinase A phosphorylate?
- hormone-sensitive lipase
- acetyl CoA carboxylase
- glycogen synthase
- transcription CREB
= immediate metabolic effects and long term effects via gene transcription
what effects can PKA have in the liver?
increase glycogenolysis and gluconeogenesis
what effects can PKA have in adipose tissue?
- increase lipolysis
what effects can PKA have in ovarian follicles?
increases synthesis of oestrogen and progesterone
how is the adenylyl cyclase/cAMP/PKA pathway amplified?
- adrenaline receptor complex can catalyse GDP:GTP on multiple G proteins
- each activated G alpha subunit can only bind to 1 adenylyl cyclase
- each adenylyl cyclase can catalyse formation of many cAMP
- 4 cAMP activate 2 PKA
- each PKA can phosphorylate many things
describe the structure and function of receptor enzymes (hormone binding)
- have an extracellular ligand-binding domain and an enzyme active site on the intracellular section, connected by a single transmembrane segment
- many of the enzymes are tyrosine kinases e.g. the insulin receptor
- the ligand binding either activates the enzyme activity or brings it in proximity to its target
describe the structure and function of receptor tyrosine kinases
- function as dimers with an extracellular hormone-binding domain and an intracellular protein tyrosine kinase domain
- when hormone binds, RTK monomers cross-phosphorylate each other
- this makes it an attachment site for proteins with SH2 or PTB domains
e.g. insulin
describe the steps of the MAPK pathway (epidermal growth factor receptor)
- epidermal growth factor (EGF) binds to each EGFR monomer inducing a structural change = dimerize
- proximity of cytosolic domains allows cross-phosphorylation, so PTK is active
- tyrosine-phosphates act as docking sites for Grb-2 which is attached to Sos
- Sos catalyses exchange of GDP for GTP on membrane bound Ras, activating it
- GTP-Ras binds and activates Raf, a membrane-bound protein kinase
- a series of protein kinases are phosphorylated and activated e.g. MAPK
- P-MAPK phosphorylates transcription factors, altering their activity
what is the outcome of the MAPK pathway (EGF)?
- EGF (epidermal growth factor) binds to its receptor causes cross-phosphorylation of its monomers causing cascade of phosphorylation
- eventually phosphorylates MAPK which then phosphorylates transcription factors
what is Sos?
a guanine nucleotide exchange factor (GEF)
- catalyses the exchange of GDP for GTP on Ras, but only when it has been recruited to the membrane via Grb-2 (in MAPK/EGF pathway)
what is Ras?
- a small G protein
- monomeric (unlike heterotrimeric G proteins, slower than heterotrimeric)
- Ras-GTP binds to and activates Raf
what is the clinical relevance of Epithelial growth factor receptors and cancer?
- EGFR is overexpressed in some epithelial cancers –> uncontrolled growth
- a small amount of receptor can dimerize in the absence of ligand (EGFR)
- this is enough to initiate the cascade and phosphorylate transcription factors, causing the cell to inappropriately grow and divide
- cetuximab (therapeutic antibody) targets and blocks the extracellular domain of the receptor preventing it from dimerises = halting cancer
- used in colorectal cancer
describe the PI-3K/insulin receptor signalling pathway
- insulin binds to dimeric receptor, causing PTK to cross-phosphorylase
- first round of cross-P fully activated kinase activity and then cross-Ps again
- these phosphorylated tyrosine residues act as docking sites for IRS-1, which is then phosphorylated
- P-IRS-1 can now bind PI-3K at membrane which now phosphorylates PIP2 into PIP3
- PIP3 allows PDK1 and PKB to associate with the membrane
- PKB is phosphorylated and dissociates from membrane and phosphorylates target proteins
what is the use/outcome of the PI-3K/insulin receptor pathway?
- responsible for GLUT4 translocation to allow glucose transport for blood glucose regulation
what is the function of IRS-1(insulin receptor substrate-1)?
- insulin receptor substrate-1is phosphorylated on several tyrosine residues
- IRS-1 is already associated with the membrane due to its PH domain which can bind to PIP2
- IRS-1 is a docking protein, it can bind to may proteins, including Grb-2 (activating MAPK pathway)
- therefore insulin can simultaneously stimulate numerous pathways with different effects
what is the function of transforming growth factor (TGF-beta)?
- TGF- beta family is a large family of proteins involved in regulating development
- in most cells they prevent proliferation by inducing synthesis of proteins that inhibit the cell cycle
- also plays a role in tissue organisation, promoting expression of extracellular matrix proteins and adhesion molecules
describe the transforming growth factor (TGF-B) receptor signalling pathway
- TGF-B binds to TBR-II
- then bind to TBR-I and phosphorylates its glycine serine rich domain (GS), activating serine/threonine kinase
- TBR-I can then phosphorylate a class of transcription factors called R-Smads
- upon phosphorylation 2 R-Smads and a Co-Smad form a heterotrimer, and nuclear localization signals are also exposed
- in nucleus, heterotrimer interacts with transcription factors
what is the clinical relevance of the TGF-B receptor and cancer?
- the TGF-B receptor pathway often inhibits growth in cells
- loss of either TBRI or TBRI function due to inactivating mutations is found in many human tumours
- these tumours are resistant to growth inhibition by TGF-B
- mutations in the Smad proteins also prevent TGF-B signalling, most pancreatic cancers contains a deletion in a Co-Smad
describe the structure and function of cytokine hormone receptors
- cytokines are a family of small signalling molecules with a characteristic arrangement of 4 alpha helices, controlling the growth and differentiation of a number of cells
- cytokine receptors recruit an enzyme (no intrinsic enzyme activity)
- the receptors all have a tyrosine kinase called JAK bound to their cytosolic domains which phosphorylate transcription members of the Signal Transduction and Activation of Transcription (STAT) family
- cytokine receptors can activate other pathways (e.g. MAPK)
- however JAK/STAT pathway is only activated by cytokines
what is erythropoietin?
- a cytokine released by the kidney in response to low blood O2
what is the function of erythropoietin receptor signaling pathway?
- stimulate the transcription of genes in erythroid progenitors that prevent them from undergoing apoptosis and stimulate them to differentiate into erythrocytes
(to increase blood O2 capacity)
what is the clinical/sport relevance of erythropoietin?
- the use of supplemental erythropoietin to increase the level of erythrocytes in blood in banned in international athletic competitions
- it is dangerous as surplus erythrocytes can clot small blood vessels causing stroke
describe the steps of the erythropoietin receptor signalling pathway
- dimeric JAK2 kinase with low activity is bound to cytosolic domain of EpoR
- Epo simultaneously binds 2 EpoRs, bringing JAK kinases close enough for to phosphorylate each other
- this lowers Km of kinase for its substrate, activating it
- JAK kinases phosphorylate receptors allow STAT5 to bind and also be phosphorylated
- phosphorylated STAT5s dissociate from receptor, dimerize, exposing nuclear localisation sequence
- STAT5 dimer enters nucleus and its DNA-binding domain binds to specific DNA regulatory sequences to control expression of target genes
describe the switching off JAK/STAT pathway
- SHP1 is a phosphotyrosine phosphatase, that binds phosphorylated receptor and dephosphorylates JAK kinase, inhibiting pathway when cytokines are no longer binding to receptor
- mutation in erythropoietin receptor can lead to unable to bind to SHP1 phosphatase –> resulting in increased intracellular signalling, so more RBCs than normal
what kind of receptor are the adrenaline signaling pathway?
- GPCR
- specifically Gsa
what kind of receptor are the tyrosine kinases?
- intracellular enzyme receptors
what kind of receptor pathway is the epidermal growth factor receptor in?
- intracellular enzyme receptors
- tyrosine kinases
what kind of receptor pathway is the insulin receptor in?
- intracellular enzyme receptor
- tyrosine kinases
what kind of receptor pathway is the transforming growth factor B receptors in?
- intracellular enzyme receptor
what kind of receptor pathway is the erythropoietin receptor in?
- enzyme recruiting receptors
- cytokine receptors
