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Old 03-14-2013
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Default Km and affinity for enzyme - Biochem Q

Kaplan QBook, p 156:

Methotrexate is used as therapy for rheumatoid arthri-
tis, but has many side effects related to competitive
inhibition of the enzyme dihydrofotate reductase.
These side effects can be decreased without interfering
with the efficacy of methotrexate by ingestion of addi-
tional folate. If patients treated with methotrexate are
given sufficient fotate, which of the following would
most likely occur?

(A) Dihydrofolate reductase will have a higher apparent
Km for dthydrofolate

(B) Dihydrofolate reductase will have a higher Vmax
than it would in the absence of methotrexate

(C) Dihydrofolate reductase will have a lower apparent
Km for dihydrofolate

(D) Dihydrofolate reductase will increase its affinity
for methotrexate

(E) Dihydrofolate reductase will reach the same Vmax
as it would in the absence of methotrexate


What is the correct answer in your opinion?

If it's (E), then how come the addition of extra substrate doesn't affect the Km?
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Old 03-14-2013
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Quote:
Originally Posted by singular View Post
Kaplan QBook, p 156:

Methotrexate is used as therapy for rheumatoid arthri-
tis, but has many side effects related to competitive
inhibition of the enzyme dihydrofotate reductase.
These side effects can be decreased without interfering
with the efficacy of methotrexate by ingestion of addi-
tional folate. If patients treated with methotrexate are
given sufficient fotate, which of the following would
most likely occur?

(A) Dihydrofolate reductase will have a higher apparent
Km for dthydrofolate

(B) Dihydrofolate reductase will have a higher Vmax
than it would in the absence of methotrexate

(C) Dihydrofolate reductase will have a lower apparent
Km for dihydrofolate

(D) Dihydrofolate reductase will increase its affinity
for methotrexate

(E) Dihydrofolate reductase will reach the same Vmax
as it would in the absence of methotrexate


What is the correct answer in your opinion?

If it's (E), then how come the addition of extra substrate doesn't affect the Km?


The key word here is Competitive inhibitors. Next, we are comparing Folate, which is agonist alone VS. Methotrexate, which is agonist + competitive inhibtors. With this mind, we look at properities of competitive inhibitors and the answer will be E. Vmax will the same, unchange. You are right about Km and it should increase; but we are comparing folate vs. methotrexate, and not folate vs. dihydrofolate.

Also, I do believe folate and dihydrofolate are same. One converts into other. Methotrexate inhibits reductase.
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Old 03-14-2013
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Initially i thought maybe the answer is C but thats wrong the correct answer is F competitive inihibitors is the key.

Some please explain why C is not the right answer.
I know that with lower km, it means increased affinity ,I would therefore imagine that DHF would have a lower km than methotrexate
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