The conversion of molecule S to product P has a Keq = 2.0. Two different, but related enzymes, A and B, can catalyze the reaction. The product P serves as an allosteric inhibitor of enzyme B, but not of enzyme A.
A. 10 mM S is placed into three separate test tubes containing, respectively, no enzyme, enzyme A, and enzyme B. Describe the endpoint of the reaction in each tube, and tell why that is the endpoint. What are the final concentrations of S and P?
B. Which tube (uncatalyzed, enzyme A, or enzyme B) will get to the endpoint first, second and third? Explain.
C. Describe the endpoint of this reaction if the enzymes A and B were found in a cell.
A. In the tube without any enzyme, there will be no catalysis, and therefore no conversion of molecule S to product P. The endpoint in this tube will be the initial concentration of S, which is 10 mM.
In the tube with enzyme A, the endpoint will be determined by the equilibrium constant (Keq) of the reaction. Since product P does not inhibit enzyme A, it can continue catalyzing the reaction until equilibrium is reached. The reaction will continue until the concentrations of S and P reach a point where the ratio of [P]/[S] is equal to the Keq value of 2.0. The final concentrations of S and P at this endpoint will depend on the initial concentration of S and the stoichiometry of the reaction.
In the tube with enzyme B, the presence of product P as an allosteric inhibitor will stop the reaction before reaching equilibrium. This allosteric inhibition will prevent the conversion of S to P, resulting in a lower concentration of P compared to the tube with enzyme A. The endpoint in this tube will be determined by the extent of allosteric inhibition and the initial concentration of S.
B. Enzyme A, which is unaffected by product P, will catalyze the reaction until the equilibrium point is reached. Therefore, the tube with enzyme A will reach the endpoint first.
The tube without any enzyme will not exhibit any catalytic activity and will not reach a specific endpoint.
Enzyme B, being inhibited by product P, will not be able to catalyze the reaction effectively. It will reach the endpoint last, or not at all, depending on the degree of allosteric inhibition. It will have a lower concentration of product P compared to the tube with enzyme A.
C. If enzymes A and B were found in a cell, the endpoint of the reaction would depend on the regulation and kinetics of the enzymes. Enzyme A, being unaffected by product P, can continue catalyzing the reaction until equilibrium is reached. Enzyme B, on the other hand, being allosterically inhibited by product P, would be regulated and controlled by cellular factors, such as feedback inhibition or other mechanisms. The endpoint in a cell would be determined by the balance between enzyme regulation and the concentrations of S and P, which would be dynamic and influenced by cellular metabolism and homeostasis.