Q 115: The likely genetic basis of increased levels of uric acid is a mutation:
The answer is: affecting an allosteric site of PRPP synthetase.
Can anyone help clarify this? I didn't really understand the AAMC answer
So let's talk about that patient:
w) blood levels exhibited markedly increased levels of 5-PRPP
x) patient has normal levels of PRPP synthetase
y) enzyme activity was three times normal levels in cultured cells
z) pH optimum and the enzyme activity of purified enzyme are normal
Answer choices:
A) affecting an allosteric site of PRPP synthetase
B) affecting the active site of PRPP synthetase
C) in a promoter gene regulating the transcription rate of PRPP
D) a gene coding for a transcription factor for the PRPP synthetase gene
Now, answer choices (C) and (D) would imply that the patient would NOT have normal levels of PRPP synthetase... however since according to information (x), the patient does, those answer choices are eliminated.
So we're choosing between a mutation at the active site or the allosteric site. Mutations typically result in a loss of function, so either we lost function at the active site (which would mean that no product would be formed), or we lost function at the allosteric site (which would mean the enzyme is not being regulated properly).
Loss of function at the active site would typically result in no 5-PRPP being formed, however according to (w) and
, that's not the case. Plus (z) tells us that the activity of the enzyme is normal; that is, it isn't more active than it should be in terms of being able to convert substrate into product.
If we lost function at the allosteric site... maybe it acts as an allosteric feedback inhibitor, then that inhibitor or PRPP synthetase activity is not functioning properly. That would result in increased levels of PRPP in the patients as well as in cultured cells.
Answer choice A makes much more sense then the rest based on the information provided.