Download patrick_ch19_p3

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Document related concepts

G protein–coupled receptor wikipedia, lookup

NADH:ubiquinone oxidoreductase (H+-translocating) wikipedia, lookup

Drug design wikipedia, lookup

Signal transduction wikipedia, lookup

Endocannabinoid system wikipedia, lookup

Ligand binding assay wikipedia, lookup

Amino acid synthesis wikipedia, lookup

Biosynthesis wikipedia, lookup

Enzyme inhibitor wikipedia, lookup

Enzyme wikipedia, lookup

Clinical neurochemistry wikipedia, lookup

Metalloprotein wikipedia, lookup

Nerve agent wikipedia, lookup

Catalytic triad wikipedia, lookup

VX (nerve agent) wikipedia, lookup

Transcript
Patrick
An Introduction to Medicinal Chemistry 3/e
Chapter 19
CHOLINERGICS, ANTICHOLINERGICS
& ANTICHOLINESTERASES
Part 3: Cholinergics & anticholinesterases
©1
Contents
Part 3: Cholinergics & anticholinesterases
14. Acetylcholinesterase
14.1.
Role
14.2.
Hydrolysis reaction catalysed
14.3.
Effect of inhibition
14.4.
Structure of enzyme complex
14.5.
Active site - binding interactions
14.6.
Active site - Mechanism of catalysis (3 slides)
15. Anticholinesterases
15.1.
Physostigmine
15.2.
Mechanism of action (3 slides)
15.3.
Physostigmine analogues
15.4.
Organophosphates (9 slides)
15.5.
Anticholinesterases as ‘Smart Drugs’ (2 slides)
[30 slides]
©1
14. Acetylcholinesterase
14.1 Role
• Hydrolysis and deactivation of acetylcholine
• Prevents acetylcholine reactivating receptor
...
Nerve 1
Signal
...
Nerve 2
...
Acetylcholinesterase enzyme
©1
14. Acetylcholinesterase
14.2 Hydrolysis reaction catalysed
O
O
C
CH3
O
CH3
Ac e tylcholine
active
HO
C
NMe3
OH
+
NMe3
Choline
Ac e tic acid
inactive
©1
14. Acetylcholinesterase
Enzyme inhibitor
(Anticholinesterase)
14.3 Effect of inhibition
2o Message
Ach
•
•
•
•
Inhibitor blocks acetylcholinesterase
Ach is unable to bind
Ach returns to receptor and reactivates
it
Enzyme inhibitor has the same effect as
a cholinergic agonist
Nerve 2
©1
14. Acetylcholinesterase
14.4 Structure of enzyme complex
S
S
Enzyme
S
S
S
Enzyme
S
S
S
S
S
S
S
S
S
S
S
S
Enzyme
S
©1
14. Acetylcholinesterase
14.5 Active site - binding interactions
Ester binding region
Anionic binding region
Serine
OH
Aspartate
Histidine
N
hydrophobic
pockets
:
vdw
:O:
Me
N
Ionic
C
N
O
CH2
CH2
O
O
H-bond
H
CH3
O
vdwMe
Me
Tyrosine
•
•
•
Anionic binding region similar to cholinergic receptor site
Binding and induced fit strains Ach and weakens bonds
©1
Molecule positioned for reaction with His and Ser
14. Acetylcholinesterase
14.6 Active site - Mechanism of catalysis
O
O
:O :
CH2CH2NMe3
:N
:
:O
C
:
CH3
H
CH3
NH
C
R
O
:N
O
NH
H
Se rine
(Nucle ophile)
His tidine
(Ba se cata lys t)
:
: O:
His tidin e
(Ba s e)
C
O
:
CH3
:O :
R
CH3
O
C
OR
NH
H N
O
:N
NH
H
H istidine
Acid catalyst
Histidine
©1
14. Acetylcholinesterase
14.6 Active site - Mechanism of catalysis
H2O
ROH
:
:O :
CH3
O
C
OR
:N
O
NH
CH3
C
N
O
H
NH
Histidine
Histidine
_
:
: O:
O
CH3
C
H
O
::
O
NH
:N
CH3
C
O
OH
H
NH
:N
H
H is tidine
Histidine
Basic catalyst
©1
14. Acetylcholinesterase
14.6 Active site - Mechanism of catalysis
_
:
:O :
:O :
CH3
C
CH3
OH
O
H
NH
C
OH
NH
:O :
:N
H
N
Histidine
Basic catalyst
Histidine
(Acid catalyst)
_
:
:O :
CH3
C
O
OH
NH
:N
O
CH3
C
OH
OH
:N
NH
H
His tidine
©1
14. Acetylcholinesterase
•
Serine and water are poor nucleophiles
•
Mechanism is aided by histidine acting as a basic catalyst
•
Choline and serine are poor leaving groups
•
Leaving groups are aided by histidine acting as an acid catalyst
•
Very efficient - 100 x 106 faster than uncatalysed hydrolysis
•
Acetylcholine hydrolysed within 100 msecs of reaching active
site
•
An aspartate residue is also involved in the mechanism
©1
14. Acetylcholinesterase
The catalytic triad
•
An aspartate residue interacts with the imidazole ring of
histidine to orientate and activate it
:
:
:O
H
:N
:
H
:O:
N
O
Serine
(Nucleophile)
Histidine
(Base)
Aspartate
©1
15. Anticholinesterases
•
Inhibitors of acetylcholinesterase enzyme
•
Block hydrolysis of acetylcholine
•
Acetylcholine is able to reactivate cholinergic receptor
•
Same effect as a cholinergic agonist
©1
15. Anticholinesterases
15.1 Physostigmine
O
C O
Me
N
H
Urethane
or
Carbamate
•
•
•
•
•
Me
N
N
Me
Me
Pyrrolidine N
Natural product from the African calabar bean
Carbamate is essential (equivalent to ester of Ach)
Aromatic ring is important
Pyrrolidine N is important (ionised at blood pH)
Pyrrolidine N is equivalent to the quaternary nitrogen of Ach
©1
: :
15.2 Mechanism of action
O
MeNH
H
H
O
:N
NH
MeNH
O Ar
C O
:N
Ar
NH
:O :
:
C
O
Physostigmine
O
:O :
Ar
N
NH
MeNH
C
:O :
:N
O Ar
NH
H
:
: :
C O
:
MeNH
O
H
©1
15.2 Mechanism of action
-ArOH
O
MeNH
C
O Ar
O
:N
NH
C
MeNH
H
O
:
:O :
:N
Stable carbamoyl
intermediate
O
Hydrolysis
very slow
H
:N
NH
Rate of hydrolysis slower by 40 x 106
©1
NH
15.2 Mechanism of action
O
O
H
H
C
N
Me
O
C
N
O
Me
Carbonyl group
'deactivated'
©1
15.3 Physostigmine analogues
Me
O
C
Me
O
CH
NMe2
N
H
•
•
•
•
O
Me
C
O
NMe3
N
Me
(ionised at blood pH)
Miotine
Neostigmine
Simplified analogue
Susceptible to hydrolysis
Crosses BBB as free base
CNS side effects
•
•
•
•
•
Fully ionised
Cannot cross BBB
No CNS side effects
More stable to
hydrolysis
Extra N-methyl group
increases stability
©1
Hydrolysis mechanisms
Possible mechanism 1
N
:
Me
 -O
+C
: O:
Me
N
OAr
H
Water
H
O
C
Me
N
OH
C
OAr
OH
MeNH2
+
CO 2
H
©1
Hydrolysis mechanisms
Possible mechanism 2
O
Me
-H
C
N
H2O
Me
N
OAr
C
O
MeNH2
+
CO 2
Me
C
O
N
OH
H
H
Too reactive
Compare:
O
Me
-Me
No hydrolysis
C
N
OAr
Me
©1
15.4 Organophosphates
a) Nerve gases
i
PrO
O
i
PrO
P
i
PrO
P
F
Dyflos
(Diisopropyl fluorophosphonate)
•
•
•
•
O
Me
F
Sarin
Agents developed in World War 2
Agents irreversibly inhibit acetylcholinesterase
Permanent activation of cholinergic receptors by Ach
Results in death
©1
15.4 Organophosphates
b) Mechanism of action
-H
Serine
Serine
H
O
O
i
PrO
i
PrO
O
P
i
PrO
•
•
F
P
O
i
PrO
STABLE
Irreversible phosphorylation
P-O bond very stable
©1
15.4 Organophosphates
c) Medicinal organophosphate
O
Me 3N
CH2
CH2
S
P
OEt
Ecothiopate
OEt
•
•
•
•
Used to treat glaucoma
Topical application
Quaternary N is added to improve binding interactions
Results in better selectivity and lower, safer doses
©1
15.4 Organophosphates
d) Organophosphates as insecticides
EtO
MeO
S
CO2Et
P
P
EtO
S
O
NO2
MeO
S
CH
CH2 CO2Et
Parathion
•
•
•
Malathion
Relatively harmless to mammals
Agents act as prodrugs in insects
Metabolised by insects to produce a toxic metabolite
©1
15.4 Organophosphates
d) Organophosphates as insecticides
MAMMALS
EtO
INSECTS
S
EtO
P
EtO
O
P
O
NO2
PARATHION
(Inactive Prodrug)
Insect
Oxidative
desulphurisation
EtO
O
NO2
Active drug
Mammalian
Metabolism
EtO
S
Phosphorylates enzyme
P
EtO
OH
INACTIVE
& excreted
DEATH
©1
15.4 Organophosphates
e) Design of Organophosphate Antidotes
Strategy
• Strong nucleophile required to cleave strong P-O bond
• Find suitable nucleophile capable of cleaving phosphate esters
• Water is too weak as a nucleophile
• Hydoxylamine is a stronger nucleophile
O
O
NH 2OH + RO P OR
Hydroxylamine
•
•
OR
O P OR
H2 N
+
ROH
OR
Hydroxylamine is too toxic for clinical use
Increase selectivity by increasing binding interactions with active
site
©1
15.4 Organophosphates
e) Design of Organophosphate Antidotes
Pralidoxime
N
CH3
•
•
•
•
CH N
OH
Quaternary N is added to bind to the anionic region
Side chain is designed to place the hydroxylamine moiety in
the correct position relative to phosphorylated serine
Pralidoxime 1 million times more effective than hydroxylamine
Cannot act in CNS due to charge - cannot cross bbb
©1
15.4 Organophosphates
e) Design of Organophosphate Antidotes
O
N
CH N
N
H
O
O
Me
CH N
Me
O
P
OR
OR
OR
P
OR
CO 2
H
Active Site (Blocked)
CO 2
O
OH
SER
SER
Active Site (Free)
©1
15.4 Organophosphates
e) Design of Organophosphate Antidotes
H
H
ProPAM
NOH
N
CH3
•
•
•
Prodrug for pralidoxime
Passes through BBB as free base
Oxidised in CNS to pralidoxime
©1
15.5 Anticholinesterases as ‘Smart Drugs’
•
Act in CNS
•
Must cross blood brain barrier
•
Used to treat memory loss in Alzheimers disease
•
Alzheimers causes deterioration of cholinergic receptors in
brain
•
Smart drugs inhibit Ach hydrolysis to increase activity at
remaining receptors
©1
NMe2
15.5 Anticholinesterases as ‘Smart Drugs’ Me
Et
CH
O
N
C
Me
O
Cl
O
MeO
NH2
CH2
Rivastigmine (Exelon)
(analogue of physostigmine)
N
H
MeO
Donepezil
H
N
N
Tacrine (Cognex)
Toxic side effects
N
O
P
MeO
MeO
HO
Anabaseine
(ants and marine worms)
CCl3
OH
Metrifonate
(organophosphate)
O
MeO
S
N
N
N
Galanthamine
(daffodil and snowdrop bulbs
Me
O
Me
N
Me
Xanomeline
©1