Question

Protein X has a Kd of 0.25 micromolar; protein Y has a Kd of 0.5 micromolar, and protein Z has a Kd of 0.75 micromolar for ligand A. Which one of the following is true?

Group of answer choices

1-Protein X has the highest affinity for ligand A.

2-Protein Z has the highest affinity for ligand A.

3-Protein X has the lowest affinity for ligand A.

4-Protein Y affinity for ligand A is higher than that of protein X.

Answer 1

Answer: Option (1) is the correct answer. Protein X has the highest affinity for ligand A.

Formula for affinity estimation is as such:

Ka = 1/Kd,

It means, higher Kd, indicates lower affinity or smaller Kd indicates higher affinity.

Kd of protein X is = 0.25 µM (2.5 x 10^-7M)

Kd of protein Y is = 0.5 µM (5.0 x 10^-7M)

Kd of protein Z is = 0.75 µM (7.5 x 10^-7M)

Protein X has the lowest Kd value, it indicates that it has highest affinity for ligand A. Protein Z has the highest Kd value, it means it has lowest affinity for ligand A.

Detail explanation :

Kd is called an equilibrium dissociation constant. The equilibrium concentrations of reactants and products could also be characterized by an equilibrium association constant (Ka) which is simply the reciprocal of Kd (as shown in above formula).

Binding affinity is the strength of the binding interaction between a single biomolecule (e.g. protein or DNA) to its ligand/binding partner. Binding affinity is typically measured and reported by the equilibrium dissociation constant (KD), which is used to evaluate and rank order strengths of bimolecular interactions. The smaller the KD value, the greater the binding affinity of the ligand for its target. The larger the KD value, the more weakly the target molecule and ligand are attracted to and bind to with each other. Binding affinity is influenced by non-covalent intermolecular interactions such as hydrogen bonding, electrostatic interactions, hydrophobic and Van der Waals forces between the two molecules. In addition, binding affinity between a ligand and its target molecule may be affected by the presence of other molecules too.

Many technique and assays can be used to measure binding affinity and dissociation constants, for example: ELISAs, gel-shift assays, pull-down assays, equilibrium dialysis, analytical ultracentrifugation etc.