Powerpoint 4 Flashcards
Activation of the G-proteins leads to the generation of what 3 “second” messengers?
- cAMP
- DAG
- IP3
G-Protiin coupled receptors are the most numerous class of receptors with more than ___ members
140
What are 4 examples of non-endocrine signals that also act via G-Protiens?
- Glutamate
- Thrombin
- Odorants
- Photoreceptors
What are some examples of G-Protein coupled RECEPTORS?
TRH, GNRH, LH, FSH, TSH, oxytocin
How many times does the G-protein receptor cross the transmembrane domain? What is their shape?
7 times, alpha-helices
What do the 7 transmembrane alpha-helices form?
a pore
What do G-Proteins use as a switch? Which one activates it and which deactivates it?
GTP/GDP
GTP bound = G protein on
GDP bound = G protein off
When the G-Protein is ‘on’, what happens?
It moves along and activates adenylate cyclase and/or phospholipase C
What type of structure so G proteins have?
Trimeric
- Alpha subunit
- Beta subunit
- Gamma subunit
Which two G-Protein subunits bind together to form heterodimers?
Beta and Gamma subunits
What are the two different functional units of G-proteins?
- G-alpha
2. G-beta/gamma dimer
What happens to the G-protein functional subunits when GTP –> GDP?
G-alpha (carries the GDP/GTP) is displaced from G-beta/gamma dimer
- G(s)alpha:
- G(i)alpha:
- G(q)alpha:
- G(o)alpha:
- activates adenylate cyclase
- Inhibits adenylate cyclase
- activates phospholipase C
- activates ion channels
Is it that 1 hormone uses 1 G-protein, or can it use more than 1?
may use more than 1
Under what two conditions may G-protein use change?
- during development
2. depending on the concentration of the hormone or in different tissues
What happens in terms of G-protein function in some with the McCune Albright syndrome? What are 2 side effects?
Gain of G(2)alpha function: G-signaling pathway is on despite the absence of a hormone stimulant.
- Skin pigmentation (cafe au lait)
- breast development
- precocial puberty associated with ovarian cysts
What happens endocrinologicaly to someone who has Familial male precocious puberty?
High testosterone is produced by Lydia cells despite the absence of LH and FHS –> cells act autonomously
How many isoforms does Adenylate cyclase have? How do they differ?
> 10 - differ in their interactions with G-proteins
How many membrane spanning domains are adenylate Cyclades made up of? How many cytoplasmic domains do they have?
2 sets of 6 membrane spanning domains
2 cytoplasmic domains
What enzyme converts ATP to cAMP?
adenylyl cyclase
What amino acids of CREB does PKA phosphorylate?
serines and threonines, activating it
What does CREB stand for?
cAMP-response element binding protein
What is CREB?
a transcription factor which switches on genes by binding to the CRE sequence (enhancer sequence) of DNA to induce transcription of cAMP-inducible genes
How is the PKA signal terminated?
Phospho-diesterases hydrolyse cAMP to 5’AMP – this removes the cAMP from the regulatory subunits allowing them to bind back to PKA and inactivate it
- What is PIP2?
2. What are DAG and IP3?
- a minor phospholipid of the cell membrane
2. Second messengers
What are the primary actions of DAG and IP3?
DAG: activation of PKC
IP3: Release of Ca++ from ER
What is a result of increased Ca++?
activation of many enzymes including kinases (phosphorylation cascade)
How is the # of receptors regulated in cell membranes?
it constantly changes with the cell cycle, development, and cell differentiation - cells become more/less responsive to a hormone
A cell will have maximal response to its receptor if it that receptor occupies how much of the cell membrane?
3 %
A receptor may be up or down regulated in response to _______ stimulation.
hormonal
When are receptor complexes inactivated? In what 2 ways can it become inactivated?
After cell signalling has been initiated
- uncoupling
- endocytosis
What two types of hormones bind to a family of INTRACELLULAR receptors which are structurally related?
- steroid hormones
2. thyroid hormones
Where are intracellular hormone receptors located?
in the cytoplasm or the nucleus
What do intracellular hormone receptors function as?
Transcription factors
What is the response rate of intracellular hormone receptors? Why?
slow because transcription and translation of proteins is necessary
The family of steroid and thyroid receptors is made up of how many members?
> 150 (about 100 are orphan receptors as their ligand has not been identified)
Steroid and thyroid receptors are made up of how many peptides?
single peptide
What are the 3 domains of steroid and thyroid receptors?
- AF2: DNA hormone-specific binding domain with a region responsible for hormone dependent transcription activation at the C-terminus
- Highly conserved DNA-binding domain
- AF1: Transcription activating domain. Hypervariable, not dependent on the type of hormone. Can activate transcription continuously
Domains are interchangeable
What are the 2 main hormone receptor classes?
- cytosolic
2. nuclear
What is characteristic of DNA-binding domains?
2 loops that forms a “zinc finger”
What is the difference between Class 1 and Class 2 DNA-binding receptors?
Class 1:
- ligands = steroids
- form complexes with heat shock proteins
- form homodimers
- ex: hsp 70, hsp 90
Class 2:
- ligands = T3
- already bound to DNA
- can form heterodimers/homodimers or be monomeric
- activated by hormone binding
What 3 clinical effects are a result of a defect in the androgen (AR) nuclear receptor?
- partial or complete androgen insensitivity syndromes
- brast cancer
- prostate cancer
What clinical effect is a result of a defect in the glucocorticoid (GR) nuclear receptor?
generalized inherited glucocorticoid resistance
What clinical effect is a result of a defect in the Oestrogen (ER) nuclear receptor?
Usually lethal Oestrogen resistance
What clinical effect is a result of a defect in the Calcitriol (VDR) nuclear receptor?
calcitriol-resistance rickets
Hormones which act via nuclear receptors many be modified by _________.`
Target cells
What are two examples of modifications to nuclear receptor hormones that happen due target cells?
- conversion of thyroxin (T4) to the active T3 (removal of 5’-iodine by tissue specific deiodinase)
- Deactivation of cortisol to cortisone in aldosterone-responsive kidney cells (cortisol cross reacts with the aldosterone receptor)
What enzyme converts cortisol to cortisone?
Type 2 11-beta HDS
What receptors does cortisol bind to?
aldosterone receptor in addition to its own receptor
How does the convention to cortisone “protect” the cell?
by preventing inappropriate activation of the aldosterone receptor by cortisol
What enzyme in the hypothalamus converts the inactive testosterone (pro hormone) into an active form? What is this new active molecule called?
- aromatase
2. new molecule = estradiol
What enzyme in the prostate converts the inactive testosterone (prohormone) into an active form? What is this new active molecule called?
5-alpha-reductase
DHT
