Lecture 20 Flashcards
3 ways of cellular communication
- synaptic signaling (neurotrasnmitter)
- endocrine signaling (hormone)
- direct contact (paracrine or autocrine) e.g. thromboxane
2 types of receptor
- Intercellular receptor
- on cell surface, which produce secondary messengers and thus signal transduction cascade - Intracellular receptor
- bind to cytosolic protein receptor or DNA sequence
2 ways that hormone can be released
- Endocrine - directly into bloodstream
2. Exocrine - first secreted into duct and then into bloodstream
How is hormone controlled? (2)
- Coordinated counter-regulatory of opposing pathways (Gs and Gi)
- Antagonistic action of other hormone
eg. insulin and glucagon
What is Bmax?
max hormone binding (analogue to Vmax)
What is Kd?
dissociation constant of receptor-hormone complex (same as half the receptors which have formed receptor-hormone complex) = 1/2Bmax (analogue to Km)
How is cascade reaction switched off? (2)
- hormone will dissociate from membrane receptor and removed from circulation by liver.
- Then cAMP is degraded by phosphodiesterases producing AMP.
How can cAMP degradation be inhibited?
Phosphodiesterase inhibitors such as methylxanthine can be used which potentiates the effect of cAMP
4 Principle groups of receptors
- Type 1 : Ligand-gated ion channels
- Type 2: G-protein-coupled receptors (GPCR)
- Type 3: Enzyme-linked receptor (tyrosine kinase)
- Type 4 : Steroid hormone receptors
Example of Type 1 receptor
nicotinic cholinergic neuroreceptors
Mechanism of type 1 receptor
Binding of neurotransmitter opens the receptor, allowing a rapid influx of ions into cytoplasm. When terminating signal, the neurotransmitter at synaptic cleft is cleaved or degraded by enzyme (eg acetylcholinesterase)
Structure of Type2 receptor (3)
GPCR contain 7 transmembrane alpha helices (heptahelical receptor)
Extracellular domain interact with ligand (hormone or neurotransmitter)
Intracellular domain interact with G proteins.
Mechanism of type2receptor
- Ligand binds to receptor which cause GTP to bind to G protein alpha subunit, causing conformational change.
- As a result, alpha subunit of G protein dissociates from beta and gamma subunits, then activates adenylyl cyclase, which convert ATP to cAMP.
Glucagon interaction with GPCR
- glucagon binds to GPCR which cause GTP to bind to G protein alpha subunit, causing conformational change.
- Alpha subunit of G protein dissociates and activates adenylyl cyclase which turns ATP to cAMP.
- cAMP then activates protein kinase A which stimulates glycogenolysis and inhibits hepatic glycolysis (inhibit pyruvate kinase by phosphorylating it)
2 ways of termination of glucagon
- Hydrolysis of GTP
2. Hydrolysis of cAMP to AMP by phosphodiesterase
2 ways G-protein can activate other protein
- Gs activates adenylyl cyclase which convert ATP to cAMP
- Gq activates phospholipase C which react with PIP2 (localized inner membrane leaflet of membrane), producing IP3 (produced to cytoplasm) and DAG (produced in membrane)
Which type of G protein is linked with beta 1 receptor
Gs linked with B1 Receptor
Which type of G protein is linked with alpha 1 receptor?
Gq linked with alpha 1 receptor
Which type of G protein is linked with alpha 2 receptor?
Gi (inhibitory) linked with alpha 2 receptor
What does Gq protein activate?
Phospholiapse C > PIP2 >IP3+DAG
What does Gs protein activate?
activate adenyly cyclase
What does Gi protein activate?
inhibit adenylyl cyclase by replacing GTP with GDP
Epinephrine has higher affinity for Beta or alpha receptor?
Beta1 receptor
Norepinephrine primarily acts through which receptor?
alpha receptor
Effect of IP3
Increase calcium influx causing smooth muscle contraction
Effect of DAG
activate PKC
Effect of activation of Gi (3)
- inhibit adenylyl cyclase leading decrease cAMP
- Increased opening of K+ channel, leading to membrane hyperpolarization
- increased closing of neuronal calcium channel, leading to decreased NE release from nerve terminal
What is difference between beta 1 and beta 2 receptor?
beta 2 is more specific to epinephrine while beta 1 has equal binding to epinephrine and norepinephrine
Effect of binding of epinephrine to beta-adrenergic receptors on skeletal myocytes (2)
- stimulate glycogenolysis
- inhibit glycogenesis
Factors that affect arterial BP
Cardiac output (heart rate, stroke volume) peripheral resistance
What is Type 3 receptor made up of?
single transmembrane protein Kinase
How does Type 3 receptor work?
- hormone binds to receptor which activates protein kinase that phosphorylates intracellular proteins.
Mechanism of insulin receptor
- insulin binds to receptor which activates tyrosine kinase activity in intracellular domain of Beta subunit.
- Then, tyrosine residue of beta subunit are autophosphorylated.
- Then, tyrosine kinase phosphorylates tyrosines in other proteins such as insulin receptor substrates (IRS).
- Phosphorylated IRS promote activation of protein kinases and phosphatases, leading to the metabolic effects of insulin.
What is insulin receptor made up of?
two alpha and two beta subunits
alpha subunits held by disulfide bond induce conformational change that stimulates tyrosine kinase acitvity of beta subunit.
Effects of insulin (3)
- rapid increase in glucose transport into cytoplasm of adipose and muscle tissue
- stimulate transcription of specific genes for glycolytic enzyme such as hepatic glucokinase & phosphofructokinase
- activate lipogenesis in adipocytes and inactivate lipolysis
What is difference between lipid-soluble hormones (type 4 receptor) to other types of signals?
- target intracellular receptors which bind to regulatory regions of DNA that induce or repress specific genes.
- have longer half life
Mechanism of cortisol receptors
- Cortisol binds to cortisol receptors in cytosol which leads to dissociation of Hsp from receptor (Usually bound to receptor to make it inactive)
- Activated receptor then homodimerize and translocate to nucleus where they bind to DNA at hormone response element (HRE) in the promoter region, controlling gene expression.
- Thus it increase transcription of gluconeogenic enzyme.
Why does steroid hormones have similar structures with minor changes bring about different cellular function?
Because they bind to different receptors which causes different cellular function
e.g. testosterone and progesterone (male/female)
