# Protein X has a Kd of 0.25 mic

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

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 proteinX.

Answer: Option (1) is thecorrect answer. Protein X has the highest affinity for ligandA.

Formula for affinityestimation is as such:

Ka = 1/Kd,

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

Kd of protein X is = 0.25 µM (2.5 x10-7M)

Kd of protein Y is = 0.5 µM (5.0 x10-7M)

Kd of protein Z is = 0.75 µM (7.5 x10-7M)

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

Detail explanation:

Kd is called an equilibriumdissociation constant. The equilibrium concentrations of reactantsand products could also be characterized by an equilibriumassociation constant (Ka) which is simply the reciprocal of Kd (asshown in above formula).

Binding affinity is the strength ofthe binding interaction between a single biomolecule (e.g. proteinor DNA) to its ligand/binding partner. Binding affinity istypically measured and reported by the equilibrium dissociationconstant (KD), which is used to evaluate and rank order strengthsof bimolecular interactions. The smaller the KD value, thegreater the binding affinity of the ligand for its target.The larger the KD value, the more weakly the target molecule andligand are attracted to and bind to with each other. Bindingaffinity is influenced by non-covalent intermolecular interactionssuch as hydrogen bonding, electrostatic interactions, hydrophobicand Van der Waals forces between the two molecules. In addition,binding affinity between a ligand and its target molecule may beaffected by the presence of other molecules too.

Many technique and assays can beused to measure binding affinity and dissociation constants, forexample: ELISAs, gel-shift assays, pull-down assays, equilibriumdialysis, analytical ultracentrifugation etc.

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