Dihydrofolate reductase (DHFR) is an enzyme that converts dihydrofolate (DHF) into tetrahydrofolate, a methyl group shuttle required for the de novo synthesis of purines, pyrimidines and certain amino acids

Dihydrofolate reductase (DHFR) is an enzyme that converts dihydrofolate (DHF) into tetrahydrofolate, a methyl group shuttle required for the de novo synthesis of purines, pyrimidines and certain amino acids. This enzyme has been targeted in the treatment of some types of cancer, since a disruption in its catalytic function would inhibit the synthesis of DNA, RNA, thymidylates, and proteins needed in the replication of cancer cells. The following data summarize a study with 2 types of inhibitors (methotrexate and folinate) for this enzyme.

Solution with substrate and enzyme. No inhibitor: 

[DHF](mM) 10.00 20.00 40.00 60.00 90.00 

Relative reaction rate 0.45 0.72 0.88 1.05 1.13 

Solution with substrate and enzyme. Methotrexate concentration was 2.5 mM: 

[DHF](mM) 12.00 25.00 35.00 50.00 85.00 

Relative reaction rate 0.17 0.31 0.39 0.51 0.75 Solution with substrate and enzyme. Folinate concentration was 1.5 mM:

 [DHF](mM) 10.00 30.00 50.00 100.00 120.00 

Relative reaction rate 0.10 0.18 0.22 0.24 0.27 

NOTE: The rates in this problem were all measured using the same concentration of the enzyme and are relative to the rate measured when [DFHR] = 50.5 mM in the absence of inhibitor. 

a) Use a graphical approach to determine the modes of inhibition of DHFR with the 2 drugs. 

b) Based on the data from the uninhibited reaction, determine the Michaelis constant and the maximum rate. 

c) Which one has a stronger inhibition effect? Briefly explain.