DRUG RECEPTOR INTERACTION

LECTURE 4PHARMACOLOGY

Drug Receptors and Pharmacodynamics

The action of a drug on the body, including receptor interactions, dose-response phenomena, and mechanisms of therapeutic and toxic action.

Receptors Definitions Classification

Ligands (Drugs) Ligand-Receptor interaction

Interaction and consequence Receptor-mediated mechanism of action

Drug Binding Sites (Receptors and Acceptors)

Binding Site Specific and Saturable

Receptor Binding Site + Effect

Acceptor Binding Site + no Effect

DRUG TARGETS many drugs inhibit enzymesEnzymes control a number of metabolic processes A very common mode of action of many drugs

in the patient (ACE inhibitors) in microbes (sulfas, penicillins) in cancer cells (5-FU, 6-MP)

some drugs bind to: proteins (in patient, or microbes) the genome (cyclophosphamide) microtubules (vincristine)

most drugs act (bind) on receptors in or on cells form tight bonds with the ligand exacting requirements (size, shape, stereo specificity) can be agonists (salbutamol), or antagonists

(propranolol) receptors have signal transduction methods

Signal transduction

1. enzyme linked(multiple actions)

2. ion channel linked(speedy)

3. G protein linked(amplifier) 4. nuclear (gene) linked

(long lasting)

Drug Receptor

A macromolecular component of a cell with which a drug interacts to produce a response

Usually a protein

Types of Protein Receptors

1. Regulatory – change the activity of cellular enzymes

2. Enzymes – may be inhibited or activated

3. Transport – e.g. Na+ /K+ ATPase

4. Structural – these form cell parts

B. Second Messengers• Small, nonprotein, water-soluble molecules

or ions• Readily spread throughout the cell by

diffusion• Two most widely used second messengers

are:

1. Cycle AMP

2. Calcium ions Ca2+

2. Calcium Ions (Ca2+) and Inositol Trisphosphate

• Calcium more widely used than cAMP• used in neurotransmitters, growth

factors, some hormones• Increases in Ca2+ causes many possible

responses:• Muscle cell contraction• Secretion of certain substance• Cell division

Two benefits of a signal-transduction pathway

1. Signal amplification

2. Signal specificity

A. Signal amplification• Proteins persist in active form long

enough to process numerous molecules of substrate

• Each catalytic step activates more products then in the proceeding steps

Receptors and Drug Action

Receptors: •Specific areas of cell membranes (proteins, glycoproteins)*•When bound to ligand, positive or negative biological response

Ligand

cellmembrane with receptor

Biological responce

Few ex. of free receptors in cytoplasmCell Nucleus

Cell membrane

Organelles

Cytoplasma

Extracellular fluid

Drugs that do not act on receptors:

Antacids: CaCO3 + HCl

Diuretics (osmotic)

Alkylating agents (cancer)

ClN

Cl N

Cl

NuN

Cl

Nu

Drugs that do act on receptors:

NO

O

O H

ca. 5Å

Acetylcholin (Neurotransmittor)

NO

O

H2N

Carbacholin

N

NMe

HO

O

Acetylcholin Agonists

Pilocarpine

Acetylcholin Antagonists

N

O

O

OH

Atropin

O

O

N

OH

Cyclopentolat

OO O

R

R'

HN

N

O

O

OO O

R

R'

HN

N

NNO

O NH2

ONN

NH2

O

h

Psoralenes

Agonist: Binds to (have affinity for) receptorBinding leads to biolog. response (Agonists have intrinsic activity / efficacy)

Antagonist:Affinity for receptorNo intrinsic activity

Types of receptors-Mechanism of Action

Super- Endogenous General structuresfamily ligands

1 Fast neurotransmitters Ligand gated ion channelsex. Acetylcholine

2 Slow neurotransmitters. ex. noradrenalin G-Protein coupled receptorsHormones

3 Insulin Enzyme coupled receptorsGrowth factors Catalytic receptors

4 Steroid hormones Cytoplasmic receptorsThyroid hormonesVitamin A, D

Ligand gated ion channels

Fastest intracellular response, ms

Binding of ligand - opening of channel - ion (K+, Na+) in or out of cell - response

LigandsFast neurotransmittersex. Acetylcholine (nicotinic receptors)

Nobel prize chemistry 2003, Roderick MacKinnon “for structural and mechanistic studies of ion channels”.

http://nobelprize.org/chemistry/laureates/2003/press.html

Membrane(Phospholipides)

Ion chanel

Ligand binding sites

G-Protein coupled receptorsG-protein: Guanine nucleotide binding protein

Extracellular fluid

Intracellular fluid

Ligand(Agonist)

ReseptorConform. change

GDP

GTP

G-protein

GDP

GTP

Target

GTP

GDP + P

Responce

HN

N N

N

O

OOPO

OOP

O

OO

OHHO

n=1; GDPn=2; GTP

n

H2N

Subtypes of G-proteins - Targets (Second messenger systems)

Ion channels: G12 Na+ / H+ exchange

Enzymes: Gi Inhib. Adenylyl cyclaseGs Stimul. Adenylyl cyclaseGq Stimul. Phospholipase C

One ligand can bind to more than one type of G-protein coupled receptors

second messenger pathways

N

N N

N

NH2

OOPO

OOP

O

OOP

OOO

OHHO

ATP

Adenylyl cyclase

P O P

N

N N

N

NH2

O

OH

c-ATP

OP OOO

Activate c-AMP dependent protein kinases (Phosphoryl. of proteins, i.e. enzymes)

Various responces(ex. metabolism, cell division)

Subtypes of G-proteins - Targets (Second messenger systems)

Ion channels: G12 Na+ / H+ exchange

Enzymes: Gi Inhib. Adenylyl cyclaseGs Stimul. Adenylyl cyclaseGq Stimul. Phospholipase C

second messenger pathways

Phopholipase C

OO

HOHO

OOH

P

P

PO

O

O

O

O

OR'

OR

PIP2Phosphatidylinositol besphophate

OO

HOHO

OOH

P

P

P

OH

O

O

O

O

OR'

OR

O

+

IP3Inositol-1,4,5-triphosphate

DAGDiacylglycerol

Activate protein kinases

Various processes in cell

Release Ca2+

Various processes in cell

Several steps Li (Treatmen manic depression)

G protein-linked receptorsStructure:•Single polypeptide chain threaded back and forth resulting in 7 transmembrane å helices•There’s a G protein attached to the cytoplasmic side of the membrane (functions as a switch).

2004-2005

Enzyme coupled receptors - Catalytic receptors

1) Binding of Ligand2) Dimerisation of reseptor

-OH -OH -OHTyr kinasedomain(Janus kinase)

Phosphoryl of Tyr

O P O P

STATSprotein

O P O P STATSprotein 1) Phosphoryl. of STAT

2) Release of STAT in cytoplasma3) STATto cell nucleus4) Intitation of transcription

STAT: Signal transducers and activators of transcription

Ligands: Peptide hormones

Ion channel receptors

Structure:• Protein pores in the

plasma membrane

Cytoplasmic receptors

(not bound to cell membranes)

DNA

DNA mRNA

Protein

ResponceHSP-90

Cell membrane

Cytoplasma

Lipophil. ligand thru cell membrane

HSP-90

Cell nucleus

(HSP-90: Heat shock protein)

Intracellular receptors

Not all signal receptors are located on the plasma membrane. Some are proteins located in the cytoplasm or nucleus of target cells.• The signal molecule must be able to pass through plasma membrane.

Examples:~Nitric oxide (NO)~Steroid (e.g., estradiol, progesterone, testosterone)

and thyroid hormones of animals).

Nuclear Receptors

Examples: Glucocorticoids: Inhibit transcription of COX-2; induce transcription

of Lipocortin Mineralcorticoids: Regulate expression of proteins involved in renal

function Retinoids (Vit A derivatives): Control embryonic development of

limbs and organs; affect epidermal differentiation => dermatological use (Acne)

PPARs (Peroxisome Proliferation-Activated Receptors): control metabolic processes: PPAR: Target of Fibrates (cholesterol lowering drugs: stimulate -

oxidation of fatty acids) PPAR: Target of Glitazones (anti-diabetic drugs: induce

expression of proteins involved in insulin signaling => improved glucose uptake)

Tyrosine-kinase receptorsStructure:

• Receptors exist as individual polypeptides• Each has an extracellular signal-binding site• An intracellular tail with a number of

tyrosines and a single å helix spanning the membrane

Receptor subtypesMost receptor classes - several sub-typesEach subtypes - differend A(nta)gonists

Sub types cholinerge receptors

Nicotinergereceptors

Muscarinergic receptors

O H

N

NHO H

O

HO

N

CNS

Effektor celleReseptor

Synapse

Acetylkolin

Noradrenalin

Det somatiske nervesystem Det autonome nervesystem

CNS CNS

Det sympatiske nervesystem

Det parasympatiske nervesystem

ganglion

4.3 Å

5.9 Å

M1: G-Protein coupled receptors Stimulate phospholipase A

M2: G-Protein coupled receptors Inhib. adenylyl cyclase

Nmuscle: Ligand gated ion channelsIncr. Na+/Ca2+

Nneuro: Ligand gated ion channelsIncr. Na+/Ca2+

NO

O

O H

ca. 5Å

Acetylcholin (Neurotransmittor)

Full responce

Spare receptors

Full agonist

Weak responce

Partial agonist

Full responce

Spare receptors - Partial agonist

Absens of full agonist

Responce = Agonist

Presence of full agonist

Responce = Antagonist

Desensitizing

Sensitizing

Receptor and normal amount of ligand = agonist

Overstimulated receptor

Receptor and normal amount of ligand = Antagonist

Overstimulated receptor

Binding of ligand to receptor• Covalent bond• Ionic bond• Hydrogen bond• Hydrophobic interaction

Covalent bondstrong; 50-150 kcal/mol,Normally irreversible bonding

ex. Acetylcholine esterase inhibitors

Effector celleSynapse

reseptor

AcChNerve potensial

AcCh

AcCh (exess)

AcCh-esterase

Ch

ChCh-acetyl-transferase

AcCh

Nerve cell

N O

O

Acetylkolin (AcCh)

Acetylkolin-esteras e

Kolin AcyltransferaseN OH

Kolin (Ch)

Reversible inhibitors

OH O OOH O

OH

NO

O

NHO

k3(AcCh)AcCh

Inhibitor

NeostigminePyridostigmine

Myasthenia gravis (weak muscles, reduced sensitivity to Acetylcholine)O

N O N

O

N O N

OHO O

NMe2OH O

Me2N OH

RO

O

Me2N

RHO

k3(inhibitor)

More difficult to cleave

Reversible inhibitor (drugs): k3 (inhib) < k3(AcCh)

Irreversible Inhibitors Not drugs, nerve gasses, insecticides etc.

LPO

OR

R2

OH O POH

L

OR

R1O

HOPO

OR

R1

AgingO P

OH

R1O

Pralidoxim

N

N

OH

Me

OH

Pralidoximmotgift

N

N

O

Me

PO OR

R1

Gen structur mustard gasses

PO

O

N

N

Tabun

FPO

O

Sarin

LPO

R1R2

L: Leaving groupR1: alkoksyR2: alkyl, alkoksy, amino

O

O

O

O

S PS

O

O

Malation

only insects

O

O

O

O

S PO

O

O

Malaoxon

Act as mustard gassesnot tox.

Ionic bond5-10 kcal/mol,Reversible bonding

NO

O

O H

Acetylcholin

Hydrogen bond2-5 kcal/mol,Reversible bonding

NO

O

O H

Hydrophobic interaction0.5-1 kcal/mol,Reversible bonding

OHO

NH

OH

Anion

cavity

H-bind acceptor

Lipophilic area

The occupancy theory: The more receptors sites occupied by ligand, the stronger response

The rate theory: The more ligand-receptor interact / unit time, the stronger response

The induced-fit theory:

ReceptorConformation ANo ligand bound

+ Agonist

Agonist

Responce

+ Antagonist

Conformation B

AntagonistResponce

ConformationAorInactive conformation C

The macromolecular pertubation theory:(induced fit + rate theory)

The activation -aggregation theory:

Conformation B(Active)

Responce

Always dynamic equilibrium.

Responce+ Agonist

Agonist

Responce+ Antagonist

Antagonist

Responce A+ Inverse agonist

Inv. Agonist

Responce B(opposite of responce A)

Dose-Response Relationships

A + B A-B [A] - equil. towards AB

L + R L-R [L] - equil. towards LR ??

L

Biological responce

R

L-R R locked in membrane (do not move freely)L dissolved in extracellular fluid

Reaction on solid - liquid interface

Ligands (Receptor binding molecules)Drugs or endogenous compounds binding

to receptors are described as Ligands.

Ligands are classified into 2 groupsAgonist: molecule that binds to receptor and

produces similar response to that of the endogenous ligandPartial agonist agonist that produce partial

effectAntagonist: molecule that binds to a receptor,

but does not cause a responseCompetitive reversible or weak bindingNon-competitive non-reversible or strong

binding

Ligands (Agonist and Antagonist)

Affinity and Efficacy Affinity: the

attraction of the drug for the receptor. high affinity: low

concentrations bind low affinity: high

concentrations bind no affinity: does

not bind Efficacy: the

intrinsic activity Max. effect

efficacy = 1 Min. effect

efficacy = 0

ELIMINATION OF DRUGS

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