Lectures 1-6 Flashcards
What four classes of drug targets?
Receptors, ion channels, transporters and enzymes
Give a general definition for a receptor and state its two components
A molecule which binds to a soluble physiogical mediator to have a direct or indirect effect; comprises a ligand binding domain and a tranduction (effector) mechanism
What are the four general categories of receptors?
ligand-gated ion channels, GPCRs, Kinase linked receptors, nuclear receptors
Give a description of function and structure for ligand-gated ion channels
Cause hyperpolarisation or depolarisation as they let ions pass through after a ligand binds. Have a direct effect, with change occurring in miliseconds.
Structure= oligomeric assembly of subunits around a central pore
Give a description of function and structure of GPCRs
Endogenous mediator binds to effector and causes a 2nd messenger effect downstream. Effect is indirect via a G-protein and occurs in seconds.
Structure= monomeric or oligomeric assembly of subunits comprising 7 transmembrane helices with a g-protein coupling domain
Give a description of function and structure of kinase-linked receptors
Binding to the receptor causes activation of a kinase, leading to protein phosphorylation within hours (direct effect)
Structure= Single transmembrane helix linking extracellular receptor domain to intracellular kinase domain
Give a description of function and structure of nuclear receptors
Respond to mediator molecule getting in to the nucleus of the cell and binding to a receptor on the membrane- effects take hours as they are via DNA
Structure= monomeric structure with separate receptor and DNA binding domains
Give a detailed description of the structure of ligand gated ion channels
Protein has a C-terminal domain, an N-terminal domain and four transmembrane domains; 2nd transmembrane domain lines the channel pore
What is the specific structure of the glutamate activated ion channels?
3 transmembrane domains with 2TM domain a re-entrant loop that doesn’t go the whole way through the membrane but forms the pore.
How is the structure of the cys-loop family different in terms of the structure of subunits present? How does this compare to the structure of I glutamate subunits?
Subunits have a pentameric structure formed of 5 proteins, which can all be heteromeric. Two cysteines present in the N-terminal domain form a disulphide bridge
I glutamate subunits have a tetramer (4 protein) structure
Describe the general properties of ionotropic receptors and give examples of molecules that interact with these recetors
Generally, fast neurotransmitters act here. GABA can bind to the GABA-A receptor, acetylcholine can bind to the NAChR and glutamate can bind at NMPA/AMPA receptors.
4-5 protein subunits assembled around a central pore (can be heteromeric or homomeric) and in the absence of the NT, the pore is closed.
What are the different gaba subunits responsible for?
a1 = sedation, a2/3 = anxiety, a5= cognition
What is the purpose of using flumazenil as a control in diazepam studied?
Has high binding affinity to GABA receptors but no efficacy
Define the term agonist
A drug that binds to and activates a receptor
Define the term allosteric modulator
A drug that binds to a receptor at a site different to the active site. Induces a conformational change in the receptor, which alters the affinity for the receptor for the endogenous ligand
Define the term antagonist
A drug that attenuates the effect of an agonist. Can be competitive or non-competitive, reversible or irreversible
Define the term Bmax
Maximum amount of a drug/radioligand in pM that can bind to receptors in a membrane preparation
For what purpose is the Cheng-Pursoff eqn used?
Used to determine a Ki value from an IC50 value measured in a competition radioligand binding assay
Define the term desensitization
A reduction in response to an agonist while it is continuously present at a receptor
What is the EC50?
Molecular concentration of an agonist that produces 50% of the maximum response possible
What is the ED50?
Dose of a drug that produces 50% of the maximum response effect
Define the term ex vivo
Taking place outside a living organism
What is the IC50?
Concentration of an agonist or antagonist that produces 50% of its possible interaction
What is the ID50?
Dosage of a drug that produces 50% of the possible inhibition for that drug
What is Kd?
Dissociation constant for a radio-labelled drug. Concentration of the drug that occupies 50% of receptors
What class of drug targt is the GABA-A receptor?
ligand-gated ion channel
What is the purpose of the Michaelis-Menten Eqn?
Shows the rate of reaction of an enzymatic reaction. Vmax= max velocity of reaction, and km= Michaelis constant (1/2 max velocity)
What is the purpose of the Lineweaver Burke Plot?
Effectively the double reciprocal of the Michaelis-Menten eqn. Allows values to be calculated more easily. The point where the line crosses the y axis=1/vmax and where it crosses the x-axis = -1/km
How is Km related to the enzyme’s properties?
Is the concnetration of a substrate needed to achieve half the possible reaction rate of an enzyme- the lower the km, the tighter the binding
What is the receptor theory?
States that binding of a drug to a receptor is governed by affinity and activation of a receptor is governed by efficacy of the drug
What are constitutively active receptors?
Receptors that spontaneously convert from active to inactive conformation in the absence of an agonist
What is an inverse agonist?
Drugs that bind to a site distinct from the orthosteric ligand and can either increase affinity/ increase efficacy or have direct agonist effects
What is an allosteric modulator?
A drug that binds to a site distinct from the orthosteric ligand and can either increase affinity, increase efficacy or have a direct agonist effect
What are the advantages of allosteric modulators?
Work by potentiating the effects of an endogenous agonist; safer than continually applying an agonist (prevents desensitization); offer opportunity for sub-type selectivity
What is the law of mass action?
The rate of a chemical reaction is proportional to the product of the concentration of reactants
What are the assumptions of the law of mass action?
All receptors are readily accessible to ligands; binding does not alter ligand or receptor; binding is reversible
Define the terms: Kon, Koff and Ka/Kd
Kon = association rate constant (M-1,min-1)
Koff =dissociation rate constant (min-1)
Ka/Kd = equilibrium dissociation constant (M) (higher affinity of a ligand the lower the Kd)
What equation describes the law of mass action?
The Hill-Langmuir equation (proportion of receptors occupied is Na/Ntot
What is the Cheng-Prusoff Eqn?
Can convert an IC50 value to a value that is independent of radioligand used to get a value for affinity. Saturation curve described as a rectangular hyperbola
What can be used to get a symmetrical sigmoid curve from Cheng-Prusoff eqn?
Scatchard Plot
What is Bmax and Ka?
Bmax = total number of receptor binding sites available ka= affinity of ligand for receptor
What are the two features of an inhibition curve?
1- IC50 (concentration required to get half max inhibition)
2- Large Hill slope- want a hill slope of 1.0
What is a Schild Plot?
Calculated the pA2 (measure of the potency of a competitive antagonist)- shows concentration of an agonist needed to produce same effect as it would alone without the antagonist present
What is the IC50 value dependent on?
The affinity of the compound and the concentration of ligand used
What is the Kb value?
Dissociation equilibrium constant concentration for the antagonist
