Chapter 8

MM

MM

EERR

• Agonists are drugs designed to mimic the natural messenger• Agonists should bind and leave quickly - number of binding interactions is important• Antagonists are drugs designed to block the natural messenger• Antagonists tend to have stronger and/or more binding interactions, resulting in a different induced fit such that the receptor is not activated.

RR

MM

EERR

Signal transductionSignal transduction

Notes on Drug DesignNotes on Drug Design

Design of agonistsDesign of agonists

•Agonists mimic the natural messenger of a receptor • Agonists bind reversibly to the binding site and produce the same induced fit as the natural messenger - receptor is activated• Similar intermolecular bonds formed as with natural messenger• Agonists are often similar in structure to the natural messenger• The agonist must have the correct binding groups• The binding groups must be correctly positioned to interact with complementary binding regions• The drug must have the correct shape to fit the binding site

EE

AgonistAgonist

RR EE

AgonistAgonist

RR

Signal transductionSignal transduction

AgonistAgonist

RR

Induced fitInduced fit

van der Waalsbinding regionH-bond

binding regionIonic binding region

Binding groups

Neurotransmitter

OO2C

H

Binding site

Receptor

NH2Me

OHH

Design of a agonist and receptorDesign of a agonist and receptor

O

NH2Me

H

HO

O2

C

H

Binding site

Receptor

O

NH2Me

H

HO

O2

CH

Binding site

Receptor

INDUCED FIT

Induced fit allows stronger binding interactions

Design of an agonist and receptorDesign of an agonist and receptor

Hypothetical neurotransmitter

HONH2Me

H

Compare Binding groups:Compare Binding groups:• Identify important binding interactions in natural messenger• Agonists are designed to have functional groups capable of the same interactions• Usually require the same number of interactions

H-bondinggroup

van der Waals-bondinggroup

Ionic bindinggroup

H2NNH2Me

HNHMe

HO HONH2Me

HH

HMe

Possible agonists with similar binding groups

Design of an agonistDesign of an agonist

OO

2C

H

Binding site

Receptor

OO

2C

H

Binding site

Receptor

HCH2Me

H

Structure II has 2 of the 3 required binding groups - weak activity

HNH2Me

H

I

HCH2Me

H

II

HNH2Me

H

Structure I has one weak binding group - negligible activity

Design of an agonistDesign of an agonist

• Binding groups must be positioned such that they can interact with complementary binding regions at the same time• Example has three binding groups, but only two can bind simultaneously• Example will have poor activity

HN H

2M e

O H

H

OO 2C

H

Binding site

2 Interactions only

H

NH2Me

H

OH

No interaction

Design of an agonistDesign of an agonist

• One enantiomer of a chiral drug normally binds more effectively than the other• Different enantiomers likely to have different biological properties

OO

2C

H

Binding site

3 interactions

O

NH2Me

H

HO

O2

C

H

Binding site

2 interactions

OH

NH2Me

H

ONH2Me

H

H

OMeH2N

H

H

Mirror

Enantiomers Enantiomers of a chiral of a chiral moleculemolecule

Design of an agonistDesign of an agonist

O

NH 2

H

H

Me

CH 3

• Agonist must have correct size and shape to fit binding site• Groups preventing access are called steric shields or steric blocks

No Fit

O

O 2 C

H

Binding site

CH 3

Steric block

Me

Steric block

Design of an agonistDesign of an agonist

Design of antagonistsDesign of antagonists• Antagonists bind to the binding site but fail to produce the correct induced fit - receptor is not activated• Normal messenger is blocked from binding

O N

H

H

Me

H

H

OO

2C

H

Binding site

Perfect Fit(No change in shape)

Design of antagonistsDesign of antagonists

OHO

2C

Receptor binding site

Extra binding regions

O

O

Asp

-

HO

Design of antagonistsDesign of antagonistsAntagonists can form binding interactions with extra binding regions neighboring the binding site for the natural messenger

Extra hydrophobic binding region

Hydrophobic binding region

Ionic bindingregion

H-bondbinding region

Hypothetical neurotransmitter

NH2Me

HO

H

Hydrophobic region

O

O

Asp

-

HO

Design of antagonistsDesign of antagonistsInduced fit resulting from binding of the normal messenger

NH2Me

HO

H

Hydrophobic region

O

O

Asp

HO

-

NH2Me

HO

HInduced fit

Hydrophobic region

O

O

Asp

HO

Hydrophobic region

HO

Initial binding

-

Design of antagonistsDesign of antagonistsDifferent induced fit resulting from extra binding interaction

NHMe

HO

H

Hydrophobic region

O

O

Asp

HO

Different induced fit

-NHMe

HO

H

Irreversible antagonistsIrreversible antagonists

• Antagonist binds irreversibly to the binding site• Different induced fit means that the receptor is not activated • Covalent bond is formed between the drug and the receptor• Messenger is blocked from the binding site • Increasing messenger concentration does not reverse antagonism• Often used to label receptors

X

OH OH

X

O

Covalent Bond

Irreversible antagonism

1

Nu

Nu

Receptor

Propylbenzilylcholine mustard

Cl

Cl

Agonist binding site

Antagonistbinding site

Cl

Cl

HOO

O

NCl

Cl

Irreversible antagonistsIrreversible antagonists

Nu

Nu

Receptor

2Irreversible binding

Allosteric antagonistsAllosteric antagonists

• Antagonist binds reversibly to an allosteric binding site • Intermolecular bonds formed between antagonist and binding site• Induced fit alters the shape of the receptor• Binding site is distorted and is not recognised by the messenger• Increasing messenger concentration does not reverse antagonism

ACTIVE SITE (open)

ENZYMEReceptor

AllostericAllostericbinding sitebinding site

Binding siteBinding site

(open)ENZYMEReceptor

Inducedfit

Binding siteBinding siteunrecognisableunrecognisable

Antagonist

Antagonists by the umbrella effectAntagonists by the umbrella effect

• Antagonist binds reversibly to a neighbouring binding site • Intermolecular bonds formed between antagonist and binding site• Antagonist overlaps the messenger binding site• Messenger is blocked from the binding site

Antagonist

Binding sitefor antagonist

Binding sitefor messenger

messenger

Receptor Receptor

Partial agonistsPartial agonists

Agents which act as agonists but produce a weaker effect

Partialagonist Slight shift

Partial openingof an ion channel

Receptor

OO 2C

H

1

NHMeO

H

H H

Receptor

O

O 2C

2

NHMeO

H

H

Possible explanationsPossible explanations• Agent binds but does not produce the ideal induced fit for maximum effect• Agent binds to binding site in two different modes, one where the agent acts as an agonist and one where it acts as an antagonist• Agent binds as an agonist to one receptor subtype but as an antagonist to another receptor subtype

Inverse agonistsInverse agonists

Properties shared with antagonistsProperties shared with antagonists• Bind to receptor binding sites with a different induced fit from the normal messenger • Receptor is not activated• Normal messenger is blocked from binding to the binding site

Properties not shared with antagonistsProperties not shared with antagonists• Block any inherent activity related to the receptor (e.g. GABA receptor) • Inherent activity = level of activity present in the absence of a chemical messenger • Receptors are in an equilibrium between constitutionally active and inactive forms

Explanation of how drugs affect receptor equilibriaExplanation of how drugs affect receptor equilibria

B) Addition of agonist

C)Addition of antagonist

D) Addition of inverse agonist

E) Addition of partial agonist

Inactive conformations Active conformation

Agonist binding site

DesensitizationDesensitization

• Receptors become desensititized on long term exposure to agonists• Prolonged binding of agonist leads to phosphorylation of receptor • Phosphorylated receptor changes shape and is inactivated• Dephosphorylation occurs once agonist departs

Receptor

O O2C

1

H Ion channel(closed)

AgonistNH3

Receptor

O

H

AgonistNH3

O2C

DesensitizationDesensitization

• Receptors become desensititized on long term exposure to agonists• Prolonged binding of agonist leads to phosphorylation of receptor • Phosphorylated receptor changes shape and is inactivated• Dephosphorylation occurs once agonist departs

•Induced fit alters protein shapeInduced fit alters protein shape•Opens ion channelOpens ion channel

Receptor

O

H

AgonistNH3

O2C

DesensitizationDesensitization

• Receptors become desensititized on long term exposure to agonists• Prolonged binding of agonist leads to phosphorylation of receptor • Phosphorylated receptor changes shape and is inactivated• Dephosphorylation occurs once agonist departs

Receptor

O

H

AgonistP

O2C

NH3

DesensitizationDesensitization

• Receptors become desensititized on long term exposure to agonists• Prolonged binding of agonist leads to phosphorylation of receptor • Phosphorylated receptor changes shape and is inactivated• Dephosphorylation occurs once agonist departs

• Phosphorylation alters shape• Ion channel closes• Desensitization

SensitizationSensitization• Receptors become sensititized on long term exposure to antagonists• Cell synthesises more receptors to compensate for blocked receptors• Cells become more sensitive to natural messenger• Can result in tolerance and dependence• Increased doses of antagonist are required to achieve same effect (tolerance)• Cells are supersensitive to normal neurotransmitter • Causes withdrawal symptoms when antagonist withdrawn • Leads to dependence

SensitizationSensitization

Antagonist

Neurotransmitter

Normal response

Receptorsynthesis

No response

Response

StopantagonistExcess response No response

Increaseantagonist

Tolerance

Receptorsynthesis

Sensitization

Dependence

No response

No response

O

Me OH

H

H H

H

H

H2O

His 524

Glu353

Arg394

Hydrophic skeleton

Oestradiol

• Phenol and alcohol of estradiol are important binding groups• Binding site is spacious and hydrophobic• Phenol group of estradiol is positioned in narrow slot• Orientates rest of molecule• Acts as agonist

Design of an antagonist for the estrogen receptorDesign of an antagonist for the estrogen receptor

Action of the oestrogen receptorAction of the oestrogen receptor

Oestradiol

H12

Oestrogenreceptor

Binding site

AF-2 regions

Dimerisation &exposure of AF-2 regions

CoactivatorCoactivator

Nucleartranscription

factor

CoactivatorCoactivator

DNA

Transcription

OH

S

O

O

Raloxifene

Asp351

His 524

O

Glu353

Arg394

N

H

H

Side chain

Raloxifene is an antagonist (anticancer agent)Phenol groups mimic phenol and alcohol of estradiolInteraction with Asp-351 is important for antagonist activitySide chain prevents receptor helix H12 folding over as lidAF-2 binding region not revealedCo-activator cannot bind

Design of an antagonist for the estrogen receptorDesign of an antagonist for the estrogen receptor

Anticancer agent

CH2CH3

O

Me2N

Tamoxifen as an antagonist Tamoxifen as an antagonist for the estrogen receptorfor the estrogen receptor