Question

Protein A and B are stably bound in a complex that you expect to last for 10's of minutes. Kd = 10E-9M. A reg protein binds and Kd = 10E-7 now. Which of the following describes this event?

1) The binding affinity of protein A and B has increased
2) The binding affinity of protein A and B has decreased
3) The binding affinity of protein A and B has remained the same
4) The binding affinity of protein A and B cannot be determined

Answer 1

The binding affinity of protein A and B has decreased with the introduction of the reg protein, as indicated by the change in the Kd value from 10E-9M to 10E-7M.

Step-by-step explanation:

When analyzing the interaction between protein A and B in the presence of a third protein (reg protein), we focus on the dissociation equilibrium constant (Kd). The Kd value is inversely proportional to the binding affinity; a low Kd indicates a high binding affinity, whereas a high Kd indicates a low binding affinity. Initially, protein A and B have a Kd of 10-9M, which signifies a strong interaction. After the reg protein binds, the Kd value changes to 10-7M, indicating that the affinity between protein A and B has decreased.

In the context of binding affinity, the introduction of the reg protein with a Kd of 10-7M suggests that it requires a higher concentration of A and B to maintain 50% of their complex, when compared to the original Kd of 10-9M. The higher Kd value reflects this reduction in binding affinity, meaning that the affinity between protein A and B has indeed decreased in the presence of the reg protein.

Therefore, the correct answer to describe the event when the reg protein binds with a Kd of 10-7M is that the binding affinity of protein A and B has decreased.