Transcript Mechanism of hormone action

Mechanism of hormone action
Hormones
• Three types
– Proteins
• Glycoproteins
• Small pepstides
• Large proteins
– Lipids
• Cholesterol derivatives
• Eicosanoids
– Amino acid derivatives
• Hormones
– Innate by themselves
– Require mediation
• Receptors
– Binding sites for a hormone
• Very specific
Hormone receptors
• Two types
– Transmembrane
– Intracellular/nuclear
– Proteins regardless of the type
• Interaction between a hormone and a
receptor
– Initial step of hormone action
Transmembrane receptors
• Protein hormones
– Unable to pass through the plasma
membrane
• Size
• Charges
– Receptors must be located on the plasma
membrane
• Extracellular domain for interaction with hormone
• Intracellular signaling system
Types of transmembrane receptors
• Receptors with multiple transmembrane
domains
– Seven trans-membrane domain receptor
– No intrinsic enzymatic activity (C-terminus)
• Associated with intracellular proteins involved in signaling
– G-proteins
– Modification of extracellular domain (hormone binding
site, N-terminus)
• Glycosylation
– Crucial for hormone binding
• Trans-membrane domains (7)
– Alpha-helix
• Hydrophobic amino acids
• Loops
– Connect alpha helices
• May be linked by disulfide bridges (extracellular
loop 1 and 2)
• Intracellular/cytoplasmic domain
– Palmitoylation of some cysteine residues
• Attachment of fatty acids
• Fourth loop
– Site for phosphorylation
• General structure of seven transmembrane receptor
– Variations
• Amino acid sequences
– Variable length of N-terminus
– Affects binding of ligand/hormone
• Intracellular signaling
– Generated when a hormone interacts with
extracellular domain of the receptor
• Conformational change within the trans-membrane
helices
• Exchange of GDP to GTP on the alpha-subunit of
G-protein complex
– Activation of Ga subunit
• Dissociation of activated Ga from G-protein
complex (bg)
• Second messengers
– Cyclic nucleotides (cAMP and cGMP)
• cAMP
– Widely used secondary messenger
– Generated by adenyl cyclase
» Activated by activated Ga subunit of G-protein
complex
• Activation of cyclic nucleotide-dependent protein
kinases
– Protein kinase A (cAMP)
• Secondary messengers
– Amplification of hormonal signals
• Binding of hormone to the receptor
• Activation of adenyl cyclase by activated Ga
• Activation of protein kinase A by cAMP
– Rapid clearance and inactivation
• Phosphodiesterases
– Inhibited by methylxanthines (caffeine, theophylline, and
theobromine)
• Phosphoprotein phosphatases
• How do we know that cAMP is a secondary
messenger?
– Changes in production of cAMP after hormonal
treatment
– Correlation between amount of cAMP being produced
and cellular response to the hormone
– Inhibition of phosphodiesterase activity
• Presence of ligand but no effects
– Treatment with cAMP analogues/agonists
• Similar response to that of hormone
• Types of G-protein complex
– Ga subunit (20 different types)
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Gs (stimulatory Ga)
Gi (inhibitory Ga)
Go (associated with orphan receptors in neurons)
Gt (transducin found in retina, activates cGMPspecific phosphodiesterases)
– bg complex
• 4 or more
• Identification of specific G-protein complex
associated with particular receptor
– Structurally similar to each other
– Use of pertusis toxin (bacterial toxin)
• Uncoupling of G-protein complex from the receptor
– Gi is very susceptible
• G-protein complex coupled with secondary
messenger system other than cyclic
nucleotides
– Generated through phospholipid metabolism
• Inositol triphosphate (IP3)
• Diacylglycerol (DAG)
• Arachidonic acid
– Activation of phospholipase C (PLC) by
activated Ga
• IP3
– Water-soluble
• Stimulate release of Ca
• DAG
– Binds to protein kinase C
• Activated by elevated Ca
• Medical importance
– 65 % of prescription drugs target G-protein
coupled receptors
• Variety of ligands
Other protein hormone receptors
• Transmembrane receptors with intrinsic tyrosine
kinase activity
– Receptor tyrosine kinase
• Receptors for insulin and many growth factors
• Transmembrane receptors with associated
tyrosine kinases
– Cytokine receptors
• Receptors for growth hormone and prolactin
• No intrinsic kinase activity
• Interaction between receptor and hormone causes
recruitment and activation of tyrosine kinases associated with
receptor
Receptor tyrosine kinase
• Approximately 100 receptor tyrosine
kinases in human
– Highly conserved
• Domains
– Extracellular
• Hormone binding site
– Transmembrane
– Intracellular/cytoplasmic
• Tyrosine kinase activity
• 16 subfamilies
– Based on extracellular domain
– Variation on extracellular domain
• Interaction with variety of factors
– EGF, PDGF, and insulin
• Activation of receptor
– Dimerization
• Dimeric ligand (two
subunits)
– Each subunit binds to
a receptor
• Two binding sites within
a hormone
– One hormone
interacts with two
receptors
• Activation of receptor
– Pre-existence as a dimer
• Receptor is a dimer
• Activated through interaction with ligand
• Activation of receptor
– Conformational changes in the kinase domain
• Accessible to the substrate
– Autophosphorylation of tyrosine residues (3 in
insulin receptor)
• Activation loop
• Triggers conformational changes
– ATP binding
– Interaction with intracellular proteins
– Phosphorylation of other proteins