MOLECULAR DRUG TARGETS
LEARNING OUTCOMES
At the end of this session student shall be able to:
• List different types of druggable targets
• Describe forces involved in drug-receptor interactions
• Describe theories of drug receptor interaction
• Discuss the methods used to evaluate drug-receptor interaction
• Discuss the process of drug signal transduction
MOLECULAR DRUG TARGETS
• Molecular drug targets are cellular proteins that selectively bind the drug and initiate the drug’s effect
• A drug target has two characteristics:
– Recognition capacity
– Amplification capacity: ability to initiate a response
Drug targets include: Enzymes, Receptors, Ion channels and Transporters
There are about 8,000 targets in human proteins
Existing drugs interact only with 324 targets (266 human and 58 pathogens)
TERMINOLOGIES
Ligand: a molecule (drug) that binds to a target.
Pharmacophore: a fragment in the drug that enables it to bind to the receptor
Binding domain: a region on a target where a ligand binds
Signal transduction: the mechanism by which a message carried by the ligand is translated into a tissue response
TYPES OF LIGANDS Target Types of ligands Description
Enzyme
activator bind to enzymes and increase their enzymatic activity
competitive inhibitors Bind at the active site and inhibit enzyme activity
non-competitive inhibitors Bind at allosteric site and inhibit enzyme activity
Ion channel
openers bind to ion channels and allosterically open the ion channel
inhibitors (blockers) bind to ion channels and physically block the pore or cause an allosteric change that closes the pore
Transporter inhibitors bind to transporters and cause allosteric changes that
prevent proper functioning of the transporters
Receptor
agonist attach to binding site and give a similar response to that of the endogenous ligand
antagonist prevent the binding of the endogenous ligand and thus abolish the response
inverse agonists prevent the binding of agonists , elicit a response inverse to that of agonists
• Equilibrium between a drug, a receptor and a drug-receptor complex is:
• The biological activity of a drug depends on the stability of the D-R complex i.e. its affinity to the receptor
• The D-R complex can be stable if:
– the interaction between the drug and receptor is stronger than interaction between drug and receptor with solvent molecules.
– the enthalpy of interaction can compensate the entropic loss for both receptor and the drug.
• Thus, the Driving force for drug-receptor interaction is low energy state of drug-receptor complex
DRUG-RECEPTOR INTERACTIONS
The stability is measured as: 𝐾𝑑 =𝐷 [𝑅]
[𝐷−𝑅 𝑐𝑜𝑚𝑝𝑙𝑒𝑥]
the smaller the larger the more stable the cmplex
FORCES INVOLVED IN D-R COMPLEX
• Bonding between a drug and a receptor occurs if there is total decrease in free energy (i.e. G is negative)
G° = -RTlnKeq Keq = binding equilibrium constant
• Small change in energy have a major effect on drug-receptor equilibrium – E.g. decrease in G° of ~ 5.5 kcal/mol changes
binding equilibrium from 1% in drug-receptor complex to 99% in drug-receptor complex
FORCES INVOLVED IN D-R COMPLEX
Interactions in drug-receptor complex may involve covalent bond or non-covalent forces such as :
Hydrophobic interactions
Ion-dipole and dipole-dipole
Hydrogen bonding
Van-der-waals interactions
Charge-transfer
Ionic bonds
Covalent interactions
It is an irreversible link between the drug and the receptor
It is the strongest bond G from -40 to -110 kcal/mol
but is seldom found in drug action except with enzyme and DNA
Ionic interactions • Ionic bonding: are usually reversible and
their strength depends on the distance btw the two ions (G° ≈ -5 kcal/mol)
Arg, Lys and His can provide cations
Asp and Glu can provide anions
O
NH
O
O
NH
NH2
H2Npivagabine
Ion-dipole and dipole-dipole interactions
• dipole–dipole interaction: a dipole in a drug is attracted by a dipole in the receptor
N
N
N
N
O
H3 C
CH3
C
N
O O-
HO
ion-dipole
dipole-dipole
zaleplon
• ion–dipole interaction: a dipole in a drug is attracted by an ion in the receptor
G° ≈ -1 to -7 kcal/mol
Hydrogen bonds • Are a type of dipole-dipole interaction
between H on X-H and O, N or F
– (X is an electronegative atom)
G° ≈ -3 to -5 kcal/mol
intramolecular
intermolecular
O
O
O
H
H :OH
Charge transfer complexes Are forces btw an electron donor and an electron acceptor groups
– donors are usually π-electron-rich species
– Acceptors contain electron deficient π-orbitals
G° ≈ -1 to -7 kcal/mol
Chlorothalonil fungicide
+
CN
CN
Cl
Cl
Cl
Cl
OHacceptor donor
Hydrophobic interactions It occurs when non-polar sections of molecules are forced
together by a lack of water solubility
increased entropy of water decrease the free energy and stabilize the drug-receptor complex.
G° ≈ -0.7 kcal/mol (per CH2/CH2 interaction)
Hydrophobic Interaction
NH2
O
O
butamben
butamben - topical anesthetic
van der Waals forces
• Occurs due to the formation of transient dipoles within a structure
• G° ≈ -0.5 kcal/mol per CH2/CH2 interaction
charge transferor hydrogen bond
hydrophobic
dipole-dipoleor hydrogen bond
hydrophobic
ionic or ion-dipole
hydrogen bond
hydrophobic
:N
N
CH2CH2 N
H
CH2CH3
CH2CH3
H
OCH3CH2 CH2CH2O
+
dibucaine
hydrophobic
Dibucaine - local anesthetic
Drug receptor interaction - example
Means of measuring drug-receptor interactions
• Drug receptor interaction is measured by comparing a dose-response curve of endogenous ligand and the drug molecule. – If a molecule produce the same maximal response as the
ligand is called full agonist – If it show no response to the receptor but block the effect
of natural ligand depending on concentration of the ligand is a competitive antagonist
– If the effect of the antagonist is independent of the concentration of the ligand is called non-competitive antagonist.
– If it produced less than the maximal response is called partial agonist
– If it showed opposite response is an inverse agonist
% M
usc
le C
on
trac
tio
n
Dose-response curve of Ach
W showed same response as Ach is agonist
Y showed less than maximal response is a partial agonist
X blocks the response independent on conc. of Ach is a non-competitive antagonist
X showed no response X blocked the response depending on conc. of Ach is a competitive antagonist
Z showed less maximal response opposite to Ach is a partial inverse agonist
Z showed full response but opposite to Ach is an inverse agonist
Stages in drug-receptor interactions • Drug receptor interaction involves two stages
1. Affinity: capacity of drug to bind to the receptor 2. Efficacy (): ability of drug to initiate a biological effect
• Affinity and efficacy are uncoupled: a compound can have great affinity but poor efficacy (and vice versa).
• Agonist and antagonist depends on the biological system.
– A compound can be an agonist for one receptor and an antagonist or inverse agonist for another receptor
Equal efficacies Different affinities Equal affinities Different efficacies
Drug-Receptor Theories
• Occupancy theory
• Rate theory
• Induced-Fit theory
• Activation-Aggregation theory
• Multistate model
Occupancy Theory
• In 1926
• Intensity of pharmacological effect is directly proportional to number of receptors occupied
• Does not rationalize why two drugs that can occupy the same receptor can act differently. (as agonists, antagonist or inverse agonists)
Rate Theory (1961)
In 1961
• Activation of receptors is proportional to the total number of encounters of a drug with its receptor per unit time.
• Does not rationalize why different types of compounds exhibit the characteristics they do.
Induced Fit Theory
In 1958
•
• Agonist induces conformational change – response
• Antagonist does not induce conformational change - no response
• Partial agonist induces partial conformational change - partial response
Activation-Aggregation Theory
In 1965-1967
• Receptor is always in a state of dynamic equilibrium between activated form (Ro) and inactive form (To).
𝑅𝑜 ⇌ 𝑇𝑜
• Agonists shift equilibrium to Ro
• Antagonists shift equilibrium to To
• Partial agonists bind to both Ro and To
• Binding sites in Ro and To may be different, accounting for structural differences in agonists vs. antagonists
biological response
no biological response
Two-state (Multi-state) Receptor Model
• R and R* are in equilibrium (equilibrium constant L), which defines the basal activity of the receptor.
In the multi-state model there is more than one R state to account for variable agonist and inverse agonist behavior for the same receptor type.
• Partial agonists bind preferentially to R*
• Partial inverse agonists bind preferentially to R
• Antagonists have equal affinities for both R and R* (no effect on basal activity)
• Full agonists bind only to R*
• Full inverse agonists bind only to R
G-PROTEIN-COUPLED RECEPTORS (GPCRs)
GPCRs has seven transmembrane –helices in a single chain of 350–1,200 residues,
The amino-terminal contains N-linked glycosylation sites.
Interaction with G-protein is through the third loop, and the C-terminal
Are integral plasma membrane proteins that transduce signals from extracellular ligands to signals in intracellular G-proteins (GTP binding proteins)
Types of GPCRs Receptors
About 860 genes of the human genome encode GPCRs.
More than 50% of GPCRs are activated by sensory stimuli (8 by light, 22 by taste compounds and 388 to 460 by odorant stimuli).
The endogenous ligands of GPCRs are small neurotransmitters, neuropeptides, peptide hormones, inflammatory mediators, lipids and ions
Types of GPCRs Receptors Endogenous ligand Example
Biogenic amines Acetylcholine, Adrenaline, Dopamine,
Histamine, Serotonin
Peptides/Proteins Adrenocorticotrophin (ACTH,
Adrenomedullin, Amylin, Angiotensin II,
Bradykinin, chemokines, Gastric
inhibitory peptide, Gastrin, Neuropeptide
Y/W/FF, Opioids
Amino acids Glutamate, GABA
Lipids Leukotriene, Lysophosphatidylcholine,
Platelet-activating factor, Prostacyclin,
Prostaglandin, Thromboxane A
Nucleotides/Nucleosides Adenosine, ADP, ATP, UDP, UTP
Proteases Thrombin, Trypsin
Ions Calcium
•G-protein consist is complex of three units GαGβγ.
•At rest Gα bound to GDP
•When a receptor is activated: 1. GDP is converted to GTP. 2. complex dissociates to active Gα-GTP and Gβγ 3. Hydrolysis of GTP leads to reassociation
G-protein signaling pathways
Note
• Signaling may take up to tens of seconds to be completed.
• in a few cases, such as vision using rhodopsin and transducin, the responses take only tens of milliseconds.
G-protein Pathway
Gαs activates adenylyl cyclases, increase cAMP, stimulates PKA
Gαi inhibit most adenylyl cyclases
Gαq activate PI3K increase IP3 and DAG that release Ca+2 and activate PKC, respectively
Gα12/13 enhance Rho kinase
Gαtransducin Release cGMP actiavte vision and taste systems
Gβγ inhibit opening of Cav channels stimulate PLC β and (PI3K).
G-protein signaling pathways
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