Transcript Medical Biochemistry Membranes: Membrane receptors; G-proteins Lecture 73 Hormone Receptors • All receptors have at least two functional domains – Recognition domain binds hormone – Second.

Medical Biochemistry
Membranes: Membrane receptors;
G-proteins
Lecture 73
Hormone Receptors
• All receptors have at least two functional domains
– Recognition domain binds hormone
– Second regions generates signal to some intracellular
function
Two general groups of hormones
• Signal transduction occurs in two general ways
– Polypeptide hormones, catecholamines bind receptors
in plasma membrane, generates signal that regulates
intracellular function (often changing enzyme activity)
– Steroid, thyroid hormones bind intracellular receptors,
complex provides the signal
Group I Hormones
• Have intracellular receptors
• Affect gene expression
– activates or inactivates
specific gene expression
• At least two control sites
– PE - controls transcription
initiation
– HRE - modulate initiation rate
(functions like enhancer)
Group II Hormones
• Largest group of hormones
• Have membrane receptors
• Use intracellular messengers
–
–
–
–
cAMP
cGMP
calcium or phoshatidylinositols
protein kinase cascade
cAMP as Second Messenger
• Intracellular [cAMP] increased or
decreased by various hormones
• Effect varies by tissue
• cAMP derived from ATP by
adenylyl cyclase
• can terminate signal by cAMP
hydrolysis by phosphodiesterase
– inhibited by caffeine (mimics or
prolongs action of hormones)
Adenylyl cyclase system
• Receptors that couple to effectors
through G protein typically have
7 membrane-spanning domains
• Adenylyl cyclase (AC) regulated
by Gs (stimulatory) and Gi
(inhibitory) complexes
Bacterial toxins irreversibly activate
adenylyl cyclase
• Cholera toxin inactivates
Gas GTPase activity,
activates AC
• Pertussis toxin prevents Gai
from being activated,
activates AC
Superfamily of GTPases
• Classified into four subfamilies
• Some ai stimulate K+ channels, inhibit Ca2+
channels, some as have opposite effects
• Gq family members activate phospholipase C
Gs
Gi
Gq
G12
cAMP-dependent protein kinase
• cAMP binds to inactive, heterotetrameric protein kinase
• cAMP-regulatory subunits dissociate from catalytic subunits that can
phosphorylate and activate protein substrates (e.g., cAMP-response
element binding protein - CREB)
• gives rise to diverse biological responses (e.g., induction of glycogen
breakdown in muscle cells by epinephrine)
• Animation: Extracellular_signaling.mov
• hormonal control of phosphoprotein phosphatases (dephosphorylation)
cGMP as Second Messenger
• Guanylyl cyclase forms cGMP from GTP
• Atriopeptins (e.g., atrial natriuretic factor - ANF) in
cardiac atrial tissues cause natriuresis, diuresis,
vasodilation, and inhibition of aldosterone secretion
• Nitric oxide (NO) binds soluble
guanylyl cyclase, increase cGMP,
activates cGMP-dependent protein
kinase, phosphorylates smooth muscle
proteins  vasodilation
– inhibitors of cGMP phosphodiesterase
enhance and prolong responses (Viagra)
Hormones act through calcium
• Ionized calcium regulates muscle contraction,
stimulus-secretion coupling, blood clotting
cascade, enzyme activity, and membrane
excitability
• Three ways of changing cytosolic [Ca2+]
– hormones that enhance membrane permeability to Ca2+
(e.g., acetylcholine) using Na+-Ca2+ exchange
– Ca2+-2H+ ATPase-dependent pump that extrudes Ca2+ in
exchange for H+
– Ca2+ can be mobilized from mitochondrial and ER
pools
Calmodulin
• Calcium-dependent regulatory protein
– Four Ca2+ binding sites, binding leads to
conformational change
– Ca2+-calmodulin can activate or inactivate enzymes
(analogous to binding of cAMP to protein kinase)
Phosphotidylinositol metabolism
• Binding of hormones (e.g., acetylcholine, antidiuretic
hormone) to receptors coupled to Gq leads to
activation of phospholipase C (PLC)
• Catalyzes hydrolysis of PIP2  IP3 + diacylglycerol
(DAG)
• IP3 binds intracellular receptor, releases Ca2+ from
sarcoplasmic reticulum and mitochondria
Phosphotidylinositol metabolism
• DAG (plus free calcium) activates protein kinase C
(PKC)
• both activated PKC and Ca2+-calmodulin dependent
protein kinase can phosphorylate and activate specific
substrates
Receptor tyrosine kinase (RTK) cascade
• Several receptors involved in growth control and
differentiation have intrinsic tyrosine
kinase activity (e.g., insulin, EGF)
• Binding ligand to receptor leads to
receptor phosphorylation and activation
of a cascade of protein kinases
• phosphorylation of transcription factors
activates (inactivates) gene transcription
Non-receptor tyrosine kinase
• Hormone-receptor interaction
(e.g., growth hormone, cytokines)
activates cytoplasmic tyrosine
kinase (e.g., JAK1)
• Tyrosine kinases phosphorylate
proteins that dock with other
proteins via SH2 domains (bind to phosphotyrosines)
• STAT binds phosphorylated receptor, becomes
phosphorylated, dimerizes, translocates to nucleus,
binds specific DNA elements, regulates transcription
Signaling crosstalk and convergence
• The same cellular responses (e.g., glycogen
breakdown) may be induced by multiple signaling
pathways
• Many RTKs and GCPRs activate multiple signaling
pathways, and different second messengers sometimes
mediate the same cellular response
• Interaction of different signaling pathways permits
fine-tuning of cellular activities