You isolate a crude fraction from the alien cells containing receptors for XFF. You perform a binding assay and plot the XFF receptors occupied (y-axis) at several XFF concentrations. You obtain a Michaelis–Menten-like simple binding curve that indicates a 1 XFF:1 receptor interaction and approaches a maximal level of binding you define as Bmax. From the curve you extrapolate the Kd for XFF binding as the concentration at which 50% of the receptors are bound. What value reflects the affinity of the receptor for the ligand?
You have isolated a primary signal factor from the blood of a space alien, which you name XFF (for X-files factor). You also have isolated various types of alien cells which appear to interact with XFF. Based on preliminary data, you think that XFF could be similar to one of three compounds normally found in human blood: epidermal growth factor (EGF), epinephrine, or estrogen.
A) Kd
B) Bmax
C) Kd − Bmax
D) 1/Bmax
E) The affinity cannot be determined from the information available.

When a chemical compound has a high affinity for a receptor, the Kd in the michael menten curves is usually very close to the 0 value of the Y axis. What does this mean? that at a lower concentration of the chemical compound the receptor ligand binding reaches 50%, a clear example of this is insulin that has more affinity for CO2 than for O2, therefore the curve of michaels menden in the case of CO2 manifests its Kd more counterclockwise compared to oxygen.

Explanation:

The affinity in this case cannot be calculated, that is, the correct option is the last one, E, since we need to know the concentration of the ligand, in order to know that the higher the concentration and the more to the right the value of Kd is in the curve of michaels with less affinity will have between them, otherwise, in the case that a lower concentration of the ligand the Kd value is more to the left, this will indicate a greater affinity for the receptor.