USMLE Forums banner
1 - 10 of 10 Posts

·
Registered
Joined
·
590 Posts
Discussion Starter · #1 ·
A patient's acute lymphoblastc leukemia (ALL) has finally been beaten into remission, and you begin maintenance therapy with weekly doses of methotrexate. Methotrexate is a reversible, competitive inhibitor of dihydrofolate reductase, so it will:
  • Decrease the Km of the enzyme but not affect the Vmax
  • Decrease the Vmax of the enzyme but not affect the Km
  • Increase the Km of the enzyme but not affect the Vmax
  • Increase the Vmax and the Km of the enzyme
  • Increase the Vmax of the enzyme but not affect the Km
 

·
Registered
Joined
·
487 Posts
C) Increase the Km of the enzyme but not affect the Vmax

My ans is C) Increase the Km of the enzyme but not affect the Vmax

Competitive inhibition increases Km (i.e., the inhibitor interferes with substrate binding), but does not affect Vmax (the inhibitor does not prevent catalysis in enzyme+substrate complex because it cannot bind to enzyme+substrate complex).
 

·
Registered
Joined
·
590 Posts
Discussion Starter · #9 ·
Correct!

As a competitive inhibitor of dihydrofolate receptor, methotrexate will increase the Km of the enzyme but not affect the Vmax (C). An increase in the concentration of substrate can overcome competitive inhibition. Vmax is an expression of moles of product produced per unit of time per amount of enzyme. The concentration of substrate necessary to reach Vmax (i.e. to saturate the enzyme with substrate), of which the Km is a function, will increase in the presence of competitive inhibition, but not the production capacity of the saturated enzyme. In noncompetitive inhibition, adding substrate will not overcome the inhibition, since it cant win out over the inhibitor by sheer probability - therefore, Km does not change. Rather, the Vmax is lowered, since some fraction of the enzyme is rendered inactive or less active.

Michaelis_Menten2.gif
click image to enlarge

Lineweaver_Burk2.gif
click image to enlarge
 
1 - 10 of 10 Posts
Top