AAMC Sample C/P Questions

[kryptonxenon]

I don't quite understand 38, 48, and 55 on this section.

38) What is the expected 14C labeling pattern found in isopentenyl phosphate produced this way? Labeled carbon atoms should be found at...
I understand the mechanism that theonlytycrane drew here: http://forums.studentdoctor.net/threads/aamc-unscored-38.1191909/#post-17600662
But how can you follow a radioactively labeled Carbon without having to draw the mechanism? These type of problems have been challenging for me. It would be great if someone could provide me with a resource to master this.

48) What are the components that comprise Coenzyme A (Figure 1)? Why isn't A correct?

55) The advantage of the Doppler ultrasound technique over the standard ultrasound technique is that it also allows:
I basically just guessed on this question. How do you approach it?

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[theonlytycrane]

38) I'll let others chime in. I didn't really like this question- I think it was a time sink. I think it would be more fair if they showed the labels on Compound 2, not only back on acetoacetyl-coa.

48) dAMP stands for "deoxy", which means no -OH on carbon 2 of the ribose ring. CoA shows -OH on carbon 2 of the ribose, so this eliminates choices A and C.

55) Doppler ultrasound is a technique to image stuff that is moving like blood or a developing fetus. Without that knowledge, the first paragraph mentions that doppler ultrasound indicates that a vein is constricted. From this, it could be inferred that they must have measured blood flow somehow to determine this which is most related to choice B.

 

[aldol16]

But how can you follow a radioactively labeled Carbon without having to draw the mechanism? These type of problems have been challenging for me. It would be great if someone could provide me with a resource to master this.

You have to draw the mechanism. At least, I had to. So this is one where I would mark and then come back to it if I have time, after answering all the easy ones.

 

[kryptonxenon]

48) dAMP stands for "deoxy", which means no -OH on carbon 2 of the ribose ring. CoA shows -OH on carbon 2 of the ribose, so this eliminates choices A and C.

How were you able to tell that "deoxy" is at the C-2 position? Is it because of the structure provided or because deoxy always happens at the C-2 position?

You have to draw the mechanism. At least, I had to. So this is one where I would mark and then come back to it if I have time, after answering all the easy ones.

Looks like I am going to have to skip the question if something similar pops up on the exam!

 

[theonlytycrane]

How were you able to tell that "deoxy" is at the C-2 position? Is it because of the structure provided or because deoxy always happens at the C-2 position?

oh yeah that notation is specific to the ribose sugar. It's used in RNA / DNA. RNA has a hydroxy (-OH) group on 2' and DNA is deoxygenated (deoxy) on 2'

 

[Domepiece]

The mechanism for 48 is weird, enzyme has to be hella strong to deprotonate the methyl group on acetyl-CoA. Agree that question is a time sink; knowledge of malonic ester synthesis and Claisen condensation will help with elucidation of the mechanism if you encounter questions with similar mechanisms on the actual exam.

 

[aldol16]

The mechanism for 48 is weird, enzyme has to be hella strong to deprotonate the methyl group on acetyl-CoA.

Yes, I believe the active site plays with pH to modulate the basicity of the relevant residues. That's why nature usually uses malonyl-CoA instead of acetyl-CoA. Terpene biosynthesis is such an interesting pathway because with tiny modifications of enzyme structure, one can produce all the natural terpenes (and there are a lot of them!).