Im having trouble answering this question can someone explai

10 Protein E can bind to two different proteins,s and I The binding reactions are described by the following equations and values: E +S ES Kes for ES 10 E t I El K for El 20 Given the equilibrium constant values, which one of the following statements is true? (a) E binds I more tightly than S (b) When S is present in excess, no I molecules will bind to E. (c) The binding energy of the ES interaction is greater than that of the El interaction. (d) Changing an amino acid on the binding surface from a basic amino acid to an acidic one will probably make the free energy of association

Solution

Ans. Given,

I. K_eq = 10 for the reaction E+S ----> [ES]

II. K_eq = 20 for the reaction E+I ----> [EI]

The equilibrium constant, K_eq is given by the ratio [ES] / ([E] [S]) for I and [EI] / ([E] [I]) for II.

A greater value of K_eq indicates that the reaction proceeds with greater speed, i.e. the rate of reaction (forward direction) of (E+I) is greater than that of (E+S).

Option a: Correct. A higher rate constant indicates that the enzyme binds more tightly to the specified substrate compared to the other substrates that result lower rate constant. Thus, I with greater k_eq has greater rate constant and greater binding affinity for the enzyme- when compared to S.

Option b: Incorrect. S binds less tightly to the enzyme than I. So, in a reaction mixture where both S and I are present in equal concertation, the enzyme preferably binds to I because it has greater binding affinity for the same. So, even if S is present in excess, I may bind to E.

For example, in case of competitive inhibition, the statement is true than “I (inhibitor) does not bind to E when S is in excess”. However, the same statement is NOT true for uncompetitive/ non-competitive inhibitions. Since, I is not specified, the statement can’t be hold true in general.

Option c: Incorrect. Binding energy is defined as the amount of free energy released during stabilization of (Enzyme-Substrate) transition state complex formation. Greater is the binding energy, greater is the rate of reaction.

Since, I has greater rate of reaction, it has higher binding energy than S.

Option c: Incorrect. Since the nature of E, I and S and the mechanism of catalysis are not species, the statement can’t be generalized. An approximation can only be made once these parameters are defined.