Biochem Qbook

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khadeja
Posts: 85
Joined: Wed Feb 07, 2018 10:42 am

Biochem Qbook

Post by khadeja » Sat Apr 14, 2018 1:39 pm

I don't know the exact question; but I have a question somewhat unrelated; how are higher levels of Acetyl CoA going to inhibit the pentose phosphate pathway?
khadeja
Posts: 85
Joined: Wed Feb 07, 2018 10:42 am

Re: Biochem Qbook

Post by khadeja » Sat Apr 14, 2018 1:55 pm

also q 794 doesn't make a lot of sense; could you please explain that?
NS_Tutor_Andrew
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Re: Biochem Qbook

Post by NS_Tutor_Andrew » Mon Apr 16, 2018 1:18 pm

Hi khadeja,

The rate-limiting step of the PPP is when glucose 6-phosphate is converted to 6-phosphoglucono-δ-lactone by glucose 6-phosphate dehydrogenase. This is upregulated by NADP+, high levels of which essentially serve as a signal that the cell needs more NADPH, which is one of the points of the PPP. Some sources I've found suggest that acetyl-CoA might affect the amount of NADP+ present, but the details seem elusive to track down, which generally means it's unlikely to be within the scope of the MCAT. (BTW, the Q you're referring to is 774, and even the wording of it suggests that it's a hypothetical question ("would you expect...") -- so I would suggest just understand their reasoning. I'm not totally sure how correct this reasoning is—and if nothing else, it's a problem if it's hard to confirm and not usually w/in the scope of MCAT textbooks—so I'll flag it for review).

Q794 has a lot of complicated window-dressing but boils down to relatively simple math, which is the logarithm power rule, which states that log(x)^2 = 2 log (x). The Nernst equation is just a way of introducing a logarithmic relationship, and the detailed explanation explains why you can cancel out certain terms from the full version of the Nernst equation. The takeaways IMO would be (1) to know what the Nernst equation is and be prepared to apply it conceptually, and (2) to be familiar w/ log rules just in case.

Hope this helps clarify these questions!
Andrew D.
Content Manager, Next Step Test Prep.
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