RECEPTORS STRUCTURE AND FUNCTIONChapter 4

THE ROLE OF THE RECEPTOR• 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

• Different receptors specific for different chemical messengers

• Each cell has a range of receptors in the cell membrane making it responsive to different chemical messengers

THE ROLE OF THE RECEPTOR

Cell

Nerve

Messenger

Signal

Receptor

Nerve

NucleusCell

Response

THE ROLE OF THE RECEPTOR

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: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

CellMembrane

Cell

Receptor

Messenger

message

Induced fit

Cell

Receptor

Messenger

Message

Cell

Messenger

Receptor

THE BINDING SITE• A hydrophobic hollow or cleft on the receptor surface -

equivalent to the active site of an enzyme

• Accepts and binds a chemical messenger

• Contains amino acids which bind the messenger

• No reaction or catalysis takes place

ENZYME

Binding site

THE BINDING SITE

• 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

MessengeMessengerr

Induced fitInduced fit

MM

HOW DOES THE BINDING SITE CHANGE SHAPE?

Before – Intermolecular bonds not optimum length for maximum binding

strength After –

Intermolecular bond lengths optimised

Phe

SerO

H

Asp

CO2 Induced Fit

Phe

SerO

H

Asp

CO2

INDUCED FIT• 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

MM

MM

EERR RR

MM

EERR

Signal transductionSignal transduction

MAIN TYPES OF RECEPTORS

• ION CHANNEL RECEPTORS

• G-PROTEIN-COUPLED RECEPTORS

• KINASE-LINKED RECEPTORS

• INTRACELLULAR RECEPTORS

ION CHANNEL RECEPTORS• Receptor protein is part of an ion channel protein

complex

• Receptor binds a messenger leading to an induced fit

• Ion channel is opened or closed

• Ion channels are specific for specific ions (Na+, Ca2+, Cl-, K+)

• Ions flow across cell membrane down concentration gradient

• Polarises or depolarises nerve membranes

• Activates or deactivates enzyme catalysed reactions within cell

ION CHANNEL RECEPTORSHydrophilic

tunnel

Cellmembrane

Induced fit and opening

of ion channel

IONCHANNEL(open)

Cell

Cellmembrane

MESSENGER

Ionchannel

Ionchannel

Cellmembrane

RECEPTORBINDING

SITE

Cell

Lock Gate Ion

channelIon

channelCell

membraneCell

membrane

MESSENGER

ION CHANNEL RECEPTORS

Transmembrane Proteins

TM2 of each protein subunit ‘lines’ the central pore

ProteinsubunitsTM4

TM4TM4

TM3

TM3

TM3

TM3

TM3 TM2

TM2

TM2TM2

TM2

TM1

TM1

TM1

TM1

TM1

TM4 TM4

GATING

Five glycoprotein subunitstraversing cell membrane

MessengerReceptor

Inducedfit

‘Gating’(ion channel

opens)

Binding site

Cellmembrane

Cellmembrane

GATING

• Chemical messenger binds to receptor binding site

• Induced fit results in further conformational changes

• TM2 segments rotate to open central pore

Closed

Transverse view

TM2TM2

TM2

TM2

TM2

Cellmembrane

TM2 TM2

Open

Transverse viewTM2

TM2

TM2

TM2

TM2

GATING• Fast response measured in msec

• Ideal for transmission between nerves

• Binding of messenger leads directly to ion flows across cell membrane

• Ion flow = secondary effect (signal transduction)

• Ion concentration within cell alters

• Leads to variation in cell chemistry

• 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

G-proteinsplit

G-PROTEIN-COUPLED RECEPTORS

inducedfit

closed open

• G-protein subunit activates membrane bound enzyme• Binds to allosteric binding site• Induced fit results in opening of active site• Intracellular reaction catalysed

Intracellular reaction

active site(closed)

active site(open)

Enzyme Enzyme

G-PROTEIN-COUPLED RECEPTORS

Transmembranehelix

C-Terminal chain

G-Proteinbinding region

Variableintracellular loop

Extracellularloops

Intracellular loops

N-Terminal chain

HO2C

NH2

VII VI V IV III II IMembrane

G-PROTEIN-COUPLED RECEPTORS

LIGAND BINDING SITE - varies depending on receptor type

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

Ligand

B DCA

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 2-adrenergic receptors now solved - better templates

Bacteriorhodopsin & Rhodopsin Family

Common ancestor

EndothelinsOpsins, Rhodopsins

Tachykinins

Monoamines

alpha beta

H2 1

muscarinic

H12 4 15 3 2A 2B 2C D1A D1B D5D4 D3 D2 3 2 1

Bradykinin, Angiotensin. Interleukin-8

Muscarinic Histamine -Adrenergic Dopaminergic -Adrenergic

Receptortypes

Receptorsub-types

• Receptor types and subtypes not equally distributed amongst tissues.• Target selectivity leads to tissue selectivity

Heart muscle 1 adrenergic receptorsFat cells 3 adrenergic receptors Bronchial muscle 1& 2 adrenergic receptorsGI-tract 1 2 & 2 adrenergic receptors

RECEPTOR TYPES AND SUBTYPES

Tyrosine kinase - linked receptors

• Bifunctional receptor / enzyme

• Activated by hormones

• Overexpression can result in cancer

• 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

closed

messenger

inducedfit

active site open

intracellular reaction

closed

messenger

Tyrosine kinase-linked receptors

N H 2

C O 2 H

Cell membrane

Catalytic binding region Catalytic binding region (closed in resting state)(closed in resting state)

Ligand binding regionLigand binding regionExtracellularExtracellularNN-terminal-terminalchainchain

IntracellularIntracellularCC-terminal-terminalchainchain

Hydrophilic Hydrophilic transmembranetransmembraneregion (region (-helix)-helix)

Tyrosine kinase-linked receptors

Reaction catalysed by tyrosine kinase

N C

O

Protein Protein

OH

Tyrosineresidue

TyrosinekinaseMg++

ATP ADP

N C

O

Protein Protein

O

Phosphorylatedtyrosineresidue

P

Epidermal growth factor receptor (EGF- R)

Inactive EGF-R monomers

Cellmembrane

Binding site for EGFEGF - protein hormone - bivalent ligand

Active site of tyrosine kinase

Induced fitopens tyrosine kinase active sites

Ligand binding and dimerisation

OH

OHOH

HO

Phosphorylation

ATP ADP

OP

OPOPPO

EGFEGF

• 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

Epidermal growth factor receptor (EGF- R)

Insulin receptor (tetrameric complex)

Insulin

Cellmembrane

Insulin binding siteKinase active site

OP

Phosphorylation

ATP ADPOP

OPPO

Kinase active siteopened by induced fit

OHOHOH

HO

Growth hormone receptorTetrameric complex constructed in presence of growth hormone

Growth hormone binding site

Kinase active site

Kinase active siteopened by induced fit

GH

OHOH

OHHO

kinases

GH receptors(no kinase activity)

GH binding&

dimerisation

OPOPOP

PO

ATP ADP

Activation and phosphorylation

OH

Binding of kinases

OHOHHO

Intracellular receptors•Chemical messengers must cross cell membrane

• Chemical messengers must be hydrophobic

• Example-steroids and steroid receptors Zinc

Zinc fingers contain Cys residues (SH)Allow S-Zn interactions

CO2H

H2N

DNA binding region(‘zinc fingers’)

Steroidbinding region

CellCellmembranemembrane

Intracellular receptor Mechanism

4. Binds co-activator protein

1. Messenger crosses membrane2. Binds to receptor3. Receptor dimerisation

5. Complex binds to DNA6. Transcription switched on or off

7. Protein synthesis activated or inhibited

MessengerMessenger

ReceptorReceptor

Receptor-ligand Receptor-ligand complexcomplex

DimerisationDimerisation

Co-activatorCo-activatorproteinprotein

DNADNA