NBME biochem q

[MudPhud20XX]

A diagram of a pathway of amino acid biosynthesis is shown. Enzymes are indicated by letter, and intermediates of the pathway are indicated by numbers. Compound 5 most likely regulates which of the following labeled enzymes? My guess is enzyme D, any thoughts???

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[EazyE1907]

ok. why did u pick d?

 

[seminoma]

More likely that it inhibits B imo. Product usually feedsback to inhibit the committed step (B in this case).

 

[MudPhud20XX]

ok. why did u pick d?

I chose D b/c of the simple negative feedback. I did think it could be B as "seminoma" pointed out, but I wasn't too convinced about it. So is it B?

 

[EazyE1907]

answer is b. Just like seminoma explained it. Tricky question.

 

[thehundredthone]

Sure it could be D, but then what would regulate the branch in synthesis? You could think of this case as a railway track, if the destination is not 5 then it's 7, and to get to 7 you have to switch at 2.

 

[notbobtrustme]

the first committing enzyme is usually rate-limiting, meaning it'll be affected most by negative feedback. Pathway B makes the most sense because then you wouldn't have a buildup of intermediates as in D and compound 7 will be produced instead, which is subject to it's own rules of production.