Medical Biochemistry

1
Medical Biochemistry

• Membranes Membrane receptors G-proteins
• Lecture 73

2
Hormone Receptors

• All receptors have at least two functional
  domains
• Recognition domain binds hormone
• Second regions generates signal to some
  intracellular function

3
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

4
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)

5
Group II Hormones

• Largest group of hormones
• Have membrane receptors
• Use intracellular messengers
• cAMP
• cGMP
• calcium or phoshatidylinositols
• protein kinase cascade

6
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)

7
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

8
Bacterial toxins irreversibly activate adenylyl
cyclase

• Cholera toxin inactivates Gas GTPase activity,
  activates AC
• Pertussis toxin prevents Gai from being
  activated, activates AC

9
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
10
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)

11
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)

12
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

13
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)

14
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

15
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

16
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

17
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

18
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