Competition
Competition occurs when two (or more) ligands are present that bind to the same site on a protein. The protein may be a target (such as a receptor), or a soluble plasma protein, or a protein involved in clearance of the ligands (such as a CYP enzyme or a renal transporter). As a result of competition, binding of all ligands involved to the protein of interest will be reduced. The consequences would then depend on the role the protein plays, and the degree to which binding of each ligand is reduced.
For example, competition for binding to plasma proteins could increase free (unbound) concentrations of drugs that are normally highly bound to plasma proteins, possibly increasing clearance of those drugs, increasing volume of distribution for those drugs, and increasing the therapeutic effects (and side effects) of those drugs. Competition for binding to a drug metabolising enzyme between two drugs might decrease clearance of one or both drugs, and might increase oral bioavailability of one or both drugs. Competition for binding to a target protein, such as that occurring between an endogenous agonist and an administered antagonist, would reduce binding of (and therefore the cellular response to) the endogenous agonist. This is how antagonists usually work.
The binding curve for one compound (binding versus concentration) is shifted to the right when a competing ligand is present. As a result of this shift, the KD for the compound is apparently increased, but the Bmax is not changed. This curve shift is observed regardless of whether binding data are plotted on a linear concentration axis (when the curve would be a hyperbola), or on a logarithmic concentration axis (when the curve would be sigmoidal).
The following example demonstrates how to determine the proportion of a population of target proteins occupied by each of two or more competing ligands.
Fractional occupancy of three ligands
In order to determine the proportion of a population of target proteins occupied by each of two or more ligands competing for a common binding site, it is necessary to know the concentration of each ligand present, as well as the KD for the binding interaction between each ligand and the target protein. In this example, we will consider the contributions of 3 ligands present together with their common target protein.
- Ligand A: KD 10 µM, concentration 5 µM
- Ligand B: KD 2 nM, concentration 6 nM
- Ligand C: KD 1 mM, concentration 2.5 mM
First, we normalise the data by expressing the concentration of each ligand present as a multiple of its own KD value for the target protein.
- Ligand A (KD 10 µM, concentration 5 µM) is present at 0.5 × KD
- Ligand B (KD 2 nM, concentration 6 nM) is present at 3 × KD
- Ligand C (KD 1 mM, concentration 2.5 mM) is present at 2.5 × KD
Then, we obtain the sum of the ligand concentrations, expressed as a multiple of their KD values. In this example, we would add 0.5 × KD, 3 × KD and 2.5 × KD, to obtain 6 × KD.
We then use the Hill-Langmuir equation to determine the total fractional occupancy of the population of target proteins. If competing ligands are present at 6 (× KD) then to obtain b/Bmax, the proportion of targets occupied, we can simply express the value for the KD as 1 (× KD):
b/Bmax = [L]/(KD+[L]), b/Bmax = [6]/(1+[6]), b/Bmax = 6/7 or 85.7%
Note that the same answer would be obtained by choosing any arbitrary value for KD, and using a value 6 × greater for the concentration of ligand present.
We have determined that 85.7% of the receptors are occupied by a ligand of some kind. It is straightforward to determine the relative contributions of each of the three competing ligands to this total occupancy.
- Ligand A is present at 0.5 × KD and would occupy (0.5/6) of 85.7%, or 7.1% of the targets
- Ligand B is present at 3 × KD and would occupy (3/6) of 85.7%, or 42.9% of the targets
- Ligand C is present at 2.5 × KD and would occupy (2.5/6) of 85.7%, or 35.7% of the targets
