Structural isomer quiz

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lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Structural isomer quiz

Post by lotus0618 » Sun Jan 19, 2020 4:22 pm

I was debating between the first option and the last option. Why is the first option wrong? isn't it also correct because the repulsive charges of those oxygen makes the molecule more unstable and easier for the phosphate group to be expelled, thus energy being released.
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NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: Structural isomer quiz

Post by NS_Tutor_Mathias » Sun Jan 19, 2020 8:03 pm

This is a kind of question that is rare on the MCAT, but can still happen. It specifically ask what best explains the deltaG of dephosphorylating PEP.

So both a) and d) are true, but d) explains why PEP in particular - because the enol tautomer is significantly unfavorable at physiological pH. The modern MCAT strongly prefers wrong answer choices to be actually wrong to avoid subjectivity ("best" explains?!), but this can still happen - so read carefully!
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: Structural isomer quiz

Post by lotus0618 » Mon Jan 20, 2020 4:06 pm

ohh so A is correct but it can also be applicable to other high energy compounds, such as ATP, while option d is specific to PEP?
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: Structural isomer quiz

Post by NS_Tutor_Mathias » Mon Jan 20, 2020 4:30 pm

Yupp.

(Also note that there are no phosphodiester bonds here, so the stability of the enol may be a greater concern than the electrostatic repulsion between two phosphate groups separated by 3 carbons. This isn't as intense as what happens in phosphodiester bonds!)
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