Transcript RECEPTORS STRUCTURE AND FUNCTION Chapter 4

RECEPTORS
STRUCTURE AND FUNCTION
Chapter 4
THE ROLE OF THE RECEPTOR
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Globular proteins
Located mostly in the cell membrane
Receive messages from chemical messengers coming from
other cells (CNS)
Transmit a message into the cell leading to a cellular effect
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Different receptors specific for different chemical
messengers
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Each cell has a range of receptors in the cell membrane
making it responsive to different chemical messengers
THE ROLE OF THE RECEPTOR
Nerve
Nerve
Signal
Messenger
Receptor
Response
Nucleus
Cell
Cell
THE ROLE OF THE RECEPTOR
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Neurotransmitters: Chemicals released from nerve
endings which travel across a nerve synapse to bind
with receptors on target cells, such as muscle cells or
another nerve. Usually short lived and responsible for
messages between individual cells
Hormones: Chemicals released from cells or glands
and which travel some distance to bind with receptors
on target cells throughout the body
Note: Chemical messengers ‘switch on’ receptors
without undergoing a reaction
THE ROLE OF THE RECEPTOR
•Receptors contain a binding site (hollow or cleft on the receptor
surface) that is recognised by the chemical messenger
•Binding of the messenger involves intermolecular bonds
•Binding results in an induced fit of the receptor protein
•Change in receptor shape results in a ‘domino’ effect
•Domino effect is known as signal transduction, leading to a chemical
signal being received inside the cell
•Chemical messenger does not enter the cell. It departs the receptor
unchanged and is not permanently bound
THE BINDING SITE
Messenger
Induced fit
Messenger
Messenger
Cell
Membrane Receptor
Cell
Receptor
Receptor
Cell
Cell
message
Message
THE BINDING SITE
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A hydrophobic hollow or cleft on the receptor surface equivalent to the active site of an enzyme
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Accepts and binds a chemical messenger
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Contains amino acids which bind the messenger
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No reaction or catalysis takes place
Binding site
ENZYME
THE BINDING SITE
Messenger
M
Induced fit
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Binding site is nearly the correct shape for the
messenger
Binding alters the shape of the receptor (induced
fit)
Altered receptor shape leads to further effects signal transduction
HOW DOES THE BINDING SITE CHANGE
SHAPE?
Phe
Phe
O
O
H
Ser
CO2
Asp
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CO2
Asp
Before –
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Induced
Fit
Ser
H
Intermolecular bonds not optimum length for maximum binding strength
After –
 Intermolecular bond lengths optimised
INDUCED FIT
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Binding interactions must be strong enough to hold
the messenger sufficiently long for signal
transduction to take place
Interactions must be weak enough to allow the
messenger to depart
Implies a fine balance
Designing molecules with stronger binding
interactions results in drugs that block the binding
site - antagonists
M
M
M
RE
R
RE
Signal transduction
MAIN TYPES OF RECEPTORS
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ION CHANNEL RECEPTORS
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G-PROTEIN-COUPLED RECEPTORS
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KINASE-LINKED RECEPTORS
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INTRACELLULAR RECEPTORS
ION CHANNEL RECEPTORS
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Receptor protein is part of an ion channel protein complex
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Receptor binds a messenger leading to an induced fit
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Ion channel is opened or closed
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Ion channels are specific for specific ions (Na+, Ca2+, Cl-, K+)
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Ions flow across cell membrane down concentration gradient
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Polarises or depolarises nerve membranes
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Activates or deactivates enzyme catalysed reactions within
cell
ION CHANNEL RECEPTORS
Hydrophilic
tunnel
Cell
membrane
MESSENGER
RECEPTOR
BINDING
SITE
Cell
membrane
Cell
Ion
channel
Lock
Gate
Ion
channel
ION
CHANNEL
(open)
Induced fit
and opening
of ion channel
Cell
membrane
Ion
Cell
membrane channel
Cell
MESSENGER
Ion
channel
Cell
membrane
ION CHANNEL RECEPTORS
Transmembrane Proteins
Protein
subunits
TM4
TM1
TM2
TM3
TM4
TM3
TM1
TM3 TM2
TM4
TM1
TM2
TM2
TM4
TM3
TM1
TM2 TM1
TM3 TM4
TM2 of each protein subunit ‘lines’ the central pore
GATING
Receptor
Cell
membrane
Binding site
Messenger
Induced
fit
‘Gating’
(ion channel
opens)
Five glycoprotein subunits
traversing cell membrane
Cell
membrane
GATING
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Chemical messenger binds to receptor binding
site
Induced fit results in further conformational
changes
TM2 segments rotate to open central pore
TM2
TM2
Cell
membrane
TM2
TM2
TM2
TM2
TM2
TM2
Transverse view
TM2
TM2
TM2
Closed
Open
TM2
Transverse view
GATING
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Fast response measured in msec
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Ideal for transmission between nerves
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Binding of messenger leads directly to ion flows
across cell membrane
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Ion flow = secondary effect (signal transduction)
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Ion concentration within cell alters
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Leads to variation in cell chemistry
G-PROTEIN-COUPLED RECEPTORS
• Receptor binds a messenger leading to an induced fit
• Opens a binding site for a signal protein (G-protein)
• G-protein binds, is destabilised then split
messenger
induced
fit
closed
open
G-protein
split
G-PROTEIN-COUPLED RECEPTORS
• G-protein subunit activates membrane bound enzyme
• Binds to allosteric binding site
• Induced fit results in opening of active site
• Intracellular reaction catalysed
Enzyme
active site
(closed)
Enzyme
active site
(open)
Intracellular
reaction
G-PROTEIN-COUPLED RECEPTORS
Extracellular
loops
NH2
N-Terminal chain
Membrane VII
VI
V
IV
III
II
I
G-Protein
binding region
HO2C
C -Terminal chain
Variable
intracellular loop
Intracellular loops
Transmembrane
helix
LIGAND BINDING SITE varies depending on receptor type
Ligand
A
B
C
D
A) Monoamines: pocket in TM helices
B) Peptide hormones: top of TM helices + extracellular loops
+ N-terminal chain
C) Hormones: extracellular loops + N-terminal chain
D) Glutamate: N-terminal chain
Bacteriorhodopsin & Rhodopsin Family
• Rhodopsin = visual receptor
• Many common receptors belong to this same family
• Implications for drug selectivity depending on similarity (evolution)
• Membrane bound receptors difficult to crystallise
• X-Ray structure of bacteriorhodopsin solved - bacterial protein similar to
rhodopsin
• Bacteriorhodopsin structure used as ‘template’ for other receptors
• Construct model receptors based on template and amino acid sequence
• Leads to model binding sites for drug design
• Crystal structures for rhodopsin and b2-adrenergic receptors now solved better templates
Bacteriorhodopsin & Rhodopsin Family
Common ance stor
Monoamines
muscarinic
beta
alpha
Opsins, Rhodopsins
Bradyk inin,
Endothelins Angiote nsin.Tachykinins
Inte rleuk in-8
2
4
5
Mu scarin ic
Rec eptor
types
3
1
H1 H2
Histamine
1 2A 2B 2C
D4 D3 D2
-Adre ne rgic
D1A D1B D5
Dopamine rgic
3
2
1
b-Adre ne rgic
Rec eptor
sub-types
RECEPTOR TYPES AND SUBTYPES
• Receptor types and subtypes not equally distributed amongst tissues.
• Target selectivity leads to tissue selectivity
Heart muscle
Fat cells
Bronchial muscle
GI-tract
b1 adrenergic receptors
b3 adrenergic receptors
1& b2 adrenergic receptors
1 2 & b2 adrenergic receptors
Tyrosine kinase - linked receptors
• Bifunctional receptor / enzyme
• Activated by hormones
• Overexpression can result in cancer
Tyrosine kinase-linked receptors
• Protein serves dual role - receptor plus enzyme
• Receptor binds messenger leading to an induced fit
• Protein changes shape and opens active site
• Reaction catalysed within cell
• Overexpression related to several cancers
messenger
messenger
induced
fit
closed
active site
open
intracellular reaction
closed
Tyrosine kinase-linked receptors
Extracellular
N-terminal
chain
Ligand binding region
NH2
Hydrophilic
transmembrane
region (-helix)
Cell membrane
Catalytic binding region
(closed in resting state)
Intracellular
C-terminal
chain
C O2 H
Reaction catalysed by tyrosine kinase
O
N
Protein
C
Protein
OH
Tyrosine
residue
Tyrosine
kinase
Mg++
ATP
ADP
O
N
Protein
C
Protein
O
Phosphorylated
tyrosine
residue
P
Epidermal growth factor receptor (EGF- R)
EGF
Cell
membrane
Ligand binding
and dimerisation
Phosphorylation
OH
HO
OH
Inactive EGF-R
monomers
OH
ATP
Induced fit
opens tyrosine kinase
active sites
Binding site for EGF
EGF - protein hormone - bivalent ligand
Active site of tyrosine kinase
OP
PO
ADP
OP
OP
Epidermal growth factor receptor (EGF- R)
• Active site on one half of dimer catalyses phosphorylation of Tyr residues on
other half
• Dimerisation of receptor is crucial
• Phosphorylated regions act as binding sites for further proteins and enzymes
• Results in activation of signalling proteins and enzymes
• Message carried into cell
Insulin receptor (tetrameric complex)
Insulin
Phosphorylation
Cell
membrane
HO
OH
OH
OH
ATP ADP
Kinase active site
opened by induced fit
Insulin binding site
Kinase active site
PO
OP
OP
OP
Growth hormone receptor
Tetrameric complex constructed in presence of growth hormone
GH
GH binding
&
dimerisation
Binding
of kinases
GH receptors
(no kinase activity)
Activation and
phosphorylation
ATP ADP
HO
kinases
OH
HO
OH
OH
Growth hormone binding site
Kinase active site
OH
OH
OH
Kinase active site
opened by induced fit
PO
OP
OP
OP
Intracellular receptors
•Chemical messengers must cross cell membrane
CO2H
• Chemical messengers must be hydrophobic
• Example-steroids and
steroid receptors
Steroid
binding region
Zinc
DNA binding region
(‘zinc fingers’)
H2N
Zinc fingers contain Cys residues (SH)
Allow S-Zn interactions
Intracellular receptor Mechanism
Co-activator
protein
Receptor
DNA
Messenger
Receptor-ligand
complex
Dimerisation
Cell
membrane
1. Messenger crosses membrane
5. Complex binds to DNA
2. Binds to receptor
6. Transcription switched on or off
3. Receptor dimerisation
7. Protein synthesis activated or inhibited
4. Binds co-activator protein