Question

Which ligand binds the tightest? Ligand A, with a Kd of 10^-9M Ligand B, with a Kd of 10^-3M Ligand C, with a percent occupancy of 30% at one micromolar

Ligand D, with a percent occupancy of 80% at 10 nm

Answer 1

Ligand A with a Kd of 10^-9M binds the tightest, indicating the highest affinity for its receptor as compared to the other ligands listed.

Step-by-step explanation:

The tightest binding ligand among the given options is indicated by the smallest dissociation constant (Kd). This value speaks to the affinity of a ligand for a receptor, with a smaller Kd reflecting a higher affinity. Ligand A has a Kd of 10-9M, suggesting a very tight binding. Ligand B has a significantly higher Kd at 10-3M, indicating weaker binding. Ligand C's percent occupancy at a certain concentration does not provide an immediate Kd value but suggests a moderate affinity. Ligand D, with 80% occupancy at 10 nm, also suggests a high affinity, yet without the Kd we cannot compare it directly with Ligand A. However, given that Ligand A's Kd is very low, it appears to bind tighter than the rest, assuming standard conditions and similar systems for comparison.

Answer 2

To answer this item, we must take note that the ligand that binds the tightest is the one with the lowest dissociation constant, Kd. Kd's for both A and B are already given so, we only need to solve Kds for C and D.
Kd of C
0.3 = (1x10⁻⁶)/(1x10⁻⁶ + Kd) ; Kd = 2.3x10⁻⁶
Kd of D
0.8 = (1x10⁻⁹)/(1x10⁻⁹ + Kd) ; Kd = 2.5x10⁻10
Since Ligand D has the least value of dissociation constant then, it can be concluded that it binds the tightest.