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 trans-membrane 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 Gsubunit

• Dissociation of activated G from G-protein complex (

• Second messengers– Cyclic nucleotides (cAMP and cGMP)

• cAMP– Widely used secondary messenger– Generated by adenyl cyclase

» Activated by activated Gsubunit 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 G • 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– Gsubunit (20 different types)

• Gs (stimulatory G• Gi (inhibitory G• Go (associated with orphan receptors in neurons)• Gt (transducin found in retina, activates cGMP-

specific phosphodiesterases)

– 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 G

• 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