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nbme7 blk3 q27 - lady
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07-13-2010, 01:10 PM
diagram of amino acid synthesis is showm. letters are enzymes and numbers are intermadiates of the pathway. compand 5 regulate which of the enzymes?
1 --a-> 2{--b-> 3 --c-> 4 --d-> 5 --e-> 6 --f->7 1.a 2.b 3.c 4.d 5.e 6.f
07-13-2010, 01:49 PM
i CHOOSED BB
most likely B is the rate limiting enzyme ,,, will be inhibited by the product (5)
07-13-2010, 01:52 PM
the offline answer is B too, but why A could not be the answer?
07-13-2010, 01:56 PM
A is not the rate limiting ,, because it may take another pathway
1 to 2 to 7
07-13-2010, 03:08 PM
I think that the given pathway is the synthesis of norepinephrine from tyrosine, where 1 is phenyl alanine. The side track is pathway where tyrosine converts to homogentisate and then joing TCA cycle.
The end product 5 - norepinephrine, controls the rate limiting step catalyzed by tyrosine hydroxylase which converts tyrosine to DOPA. hope this helps.
07-13-2010, 03:27 PM
I think the question is not about control of rate limiting enzymes by substrate level(concentration) and also we cant know for sure which enzyme in the pathway is rate limiting enzyme ...also we cant assume this reaction is for the synthesis of NE and Epi ...in the boards we should answer from the given facts ...and the given fact is compund 5 is made from compound 4 by the enzyme D ..hence as 5 is the product of enzyme d ...enzyme d can be regulated by its product (which is 5 )....
--HENCE MY Answer is enzyme D
07-13-2010, 04:48 PM
@yeabiruh
Actually I picked D, when I did the test online, but on a second thought as the question mentioned amino acid biosynthesis, I thought norepinephrine synthesis path could be a possibility. And I dont think they will give us a question that can be so easy with D as answer. Wish NBME gives us answers!!! |
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