Mechanism of hormone action

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Mechanism of hormone action
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Hormones

• Three types
• Proteins
• Glycoproteins
• Small pepstides
• Large proteins
• Lipids
• Cholesterol derivatives
• Eicosanoids
• Amino acid derivatives

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• Hormones
• Innate by themselves
• Require mediation
• Receptors
• Binding sites for a hormone
• Very specific

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Hormone receptors

• Two types
• Transmembrane
• Intracellular/nuclear
• Proteins regardless of the type
• Interaction between a hormone and a receptor
• Initial step of hormone action

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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

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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

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• Trans-membrane domains (7)
• Alpha-helix
• Hydrophobic amino acids
• Loops
• Connect alpha helices
• May be linked by disulfide bridges (extracellular
  loop 1 and 2)

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• Intracellular/cytoplasmic domain
• Palmitoylation of some cysteine residues
• Attachment of fatty acids
• Fourth loop
• Site for phosphorylation

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• General structure of seven trans-membrane
  receptor
• Variations
• Amino acid sequences
• Variable length of N-terminus
• Affects binding of ligand/hormone

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• 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)

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• 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)

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• 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

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• 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

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• Types of G-protein complex
• Ga subunit (20 different types)
• Gs (stimulatory Ga)
• Gi (inhibitory Ga)
• Go (associated with orphan receptors in neurons)
• Gt (transducin found in retina, activates
  cGMP-specific phosphodiesterases)
• bg complex
• 4 or more

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• 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

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• 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

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• IP3
• Water-soluble
• Stimulate release of Ca
• DAG
• Binds to protein kinase C
• Activated by elevated Ca

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• Medical importance
• 65 of prescription drugs target G-protein
  coupled receptors
• Variety of ligands

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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

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Receptor tyrosine kinase

• Approximately 100 receptor tyrosine kinases in
  human
• Highly conserved
• Domains
• Extracellular
• Hormone binding site
• Transmembrane
• Intracellular/cytoplasmic
• Tyrosine kinase activity

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• 16 subfamilies
• Based on extracellular domain
• Variation on extracellular domain
• Interaction with variety of factors
• EGF, PDGF, and insulin

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• 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

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• Activation of receptor
• Pre-existence as a dimer
• Receptor is a dimer
• Activated through interaction with ligand

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• 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