Question

3. The plot below shows fractional saturation of proteins A and B at different ligand C concentrations. 10 07 06 05 04 02 01 0 20 4 0 100 a. Estimate the K, for the B-C complex b. If the forward rate constant k, for formation of the B-C complex is 10'M 's', calculate the dissociation rate with 50 mM B-C complex. Please make sure your units makes sense. c. Estimate [C], for the A-C complex d. Is the value from part c a K? Why or why not? e. What can you deduce about the subunit structure of protein A, based on the shape of the curve?

Answer 1

Dissociation constant = concentration of products / concentration of reactants

a.       The dissociation constant Kd for the B-C complex is concentration of products / concentration of reactants. In the equation B ó B-C, Kd will be equal to [B-C] / [B].

b.      Forward rate constant is 10^9 /M-s which is equal to [B-C] / [B].

Dissociation rate at the concentration of 50mM B-C complex

The dissociation rate of B-C when [C] is 100mM is 10^9 /M-s. If the complex is 50mM, then the dissociation rate is 10^9/2 = 0.5 * 10^9/M-s.

c.       A is able to bind to C to the maximum with 0.9 of fractional saturation at 100mM concentration of C. The concentration of C when 50% of the complex is made is according to the graph, it falls at around 30mM.

d.      Kd is the dissociation constant which is the ratio of the concentrations of products to the reactants. But, the value in part c is the concentration of ligand C to form 50% of the A-C complex.

e.      The fractional saturation of A is higher than that of B with ligand C. The fraction of A protein molecules that are saturated with ligand C, are more than that of B.

​