Binding Flashcards
Explain why binding, pharmacological response and enzyme kinetics all display saturability.
Kinetics that reach an asymptotic maximum are generally termed “saturable” . This is the result of a limited number of protein molecules available for interaction with ligands and substrates. When drug concentration is low, the rate of increase in binding or reaction velocity increases in a relatively linear manner. As the binding sites or active sites become more and more occupied, the receptors become “saturated” and the increase in binding or reaction velocity with every increase in drug concentration becomes smaller and smaller with a maximum amplitude dictated by the asymptote of the curve.
In occupancy theory is?
the response is proportional to the number of receptors occupied.
In rate theory
• In rate theory, the response is proportional to the rate of association of receptors. Another way to say this is that activation is proportional to the total numbers of encounters between ligand and receptor over a given unit of time. Activity is thus proportional to rates of dissociation and association, not occupancy
Identify fundamental assumptions commonly made when conducting binding experiments.
Assumptions:
• We can measure “bound receptor”
• All ligands are either free or bound to the receptor.
• Neither ligand nor receptor are altered in the interaction.
• The system is a equilibrium.
• The binding is reversible
What is Kd
Kd is typically referred to as the “dissociation constant” and is the inverse of the equilibrium constant for the binding reaction. It is a measure of affinity of the ligand for the receptor. Kd is equivalent to k-1/k1 , the rate of dissociation divided by the rate of association.
It is thus directly proportional to the rate of dissociation if the rate of formation is assumed to be constant (often a valid assumption with compounds of similar structure acting on the same receptor)
K1 and k-1 are
the forward and reverse rate constants respectively.
Bmax is the maximum amount of
binding and is equivalent to the total number of binding sites available to bind the ligand.
Since many receptors have multiple ligand binding sites, Bmax may not be a measure of the number of receptors.
Two primary pieces of data can be extracted from binding curves: Affinity (Kd) and maximum binding (Bmax)
True or false
True
Affinity measures how tightly the drug binds and is generally proportional to the ______________ of the drug from the receptor. It thus is a measure of how stable the ligand/receptor complex is.
Dissociation rate
A semilog plot allows several binding curves to be placed on the same x-axis even if affinities are differ by a large amount. Curves will line up indicating “relative” affinities of the drugs with those of highest affinity to the left and lowest affinity to the right:
Just read it
Read read read
high affinity drugs have _____ Kd values and low affinity drugs have ______ Kd values.
Low, high
Note that ligand binding data plot is similar to a semi-log dose response curve but the Y-axis describes _____________ rather than a physiological response.
Bound Receptors
As noted above, occupancy of a receptor is not always directly related to the magnitude of the physiological response, therefore binding and affinity are not the same as response and EC50
Read
Observing the Bmax will indicate how many binding sites are available in the preparation. If the preparation is identical and different Bmax values are reached for different drugs, this will indicate different numbers of binding sites available in the preparation for the two different drugs. Note that receptors often contain multiple binding sites on each receptor molecule
Read
Selectivity
Selectivity is a property of a ligand that describes its relative affinity for different receptor subtypes. A highly selective ligand will have high affinity for one receptor subtype with low affinity for others.
For example, a drug may activate only alph1 but not alpha2 adrenergic receptors. The relative affinity for different receptors is an important component of determining drug dose.
