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Biochemical pathways

"A branched biochemical pathway synthesizes two related essential amino acids D and F. A mutant defective for enzyme 2 will grow on minimal medium supplemented with which compounds?"

The picture drawn with this problem looks something like this (I'll also attach a Word file in case this is unclear:

1 2 4
precursor ---> A ---> B ---> C ---> D
3 5
---> E ---> F

So that A leads to B, and B leads to either C then D (on one pathway), or to E then F (on a different pathway).

I thought the answer would have been B. Mutant 2, if given compound B, can synthesize its own C, then D. But the answer given in the answer key is "C and D." I don't understand that. At the very least, I would think the answer would be B, C, or D -- that mutant 2 could grow if any of those were added to minimal media.

Also, if I'm totally wrong in my thinking here, and C and D are the right answers, then shouldn't the answer be C OR D, not C AND D. Mutant 2 should only need one or the other, not both. If given C, it can make its own D; if given D, it doesn't need the intermediate C.

Can you please clear this up for me?

Thank you!

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Hello,
<br>
<br>I think that you have explained your thinking on this problem very clearly. I just want to clarify the terminology with regards to enzymes to make sure that we are saying the same thing. An enzyme catalyses a reaction by overcoming the activation energy for that reaction under physiological conditions (i.e. in a way that does not harm the cell). In catalysing a reaction, the enzyme acts on one product, called the substrate, to assist in converting it to the product of the ...

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