Im having trouble with the tally of ATP for B-Oxidation (TBR Test8 Q106). During the first activation step, we convert ATP into AMP and PPi. Shouldnt that only account for 1 ATP loss because Pyrophosphate can be hydrolyzed again into 2xPi? The answer explanation counts it as the equivalent of 2 ATP....
ATP tally for B-Oxidation (TBR CBT8)
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I don't have that CBT but I believe for fat oxidation you do:
(# of C-2)/2 = number of turns for beta oxidation. each turn = 1 FADH2 1 NADH (which is 2 atp and 3 atp respectively)
and then you also do #C/2 = number of acetyl CoA for krebs = number of turns for krebs (1 gtp, 3 NADH, 1 FADH2). And then yes you subtract 2 for activation.
(# of C-2)/2 = number of turns for beta oxidation. each turn = 1 FADH2 1 NADH (which is 2 atp and 3 atp respectively)
and then you also do #C/2 = number of acetyl CoA for krebs = number of turns for krebs (1 gtp, 3 NADH, 1 FADH2). And then yes you subtract 2 for activation.
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That's so more than you need to know for the MCAT
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While others are right that you don't need to know background info on beta oxidation for the MCAT, it looks like the OP is asking about a question that's in a practice MCAT. It's likely to be passage-based as well. Something like this is totally fair game on the real MCAT.
OP, the activation step counts as two ATP even though only one ATP molecule is really used because the formation of the fatty acyl-CoA involves the hydrolysis of two high energy bonds in ATP: 1) the hydolysis of ATP to AMP and PPi, and 2) the hydrolysis of PPi into two Pi. This hydrolysis of two high energy bonds releases much more energy than just the hydrolysis of ATP to ADP and Pi. That's why it's counted as 2 ATP, along with the fact that it'll take more energy to regenerate ATP from AMP and 2 Pi compared to ADP and one Pi.
Hope this helps.
OP, the activation step counts as two ATP even though only one ATP molecule is really used because the formation of the fatty acyl-CoA involves the hydrolysis of two high energy bonds in ATP: 1) the hydolysis of ATP to AMP and PPi, and 2) the hydrolysis of PPi into two Pi. This hydrolysis of two high energy bonds releases much more energy than just the hydrolysis of ATP to ADP and Pi. That's why it's counted as 2 ATP, along with the fact that it'll take more energy to regenerate ATP from AMP and 2 Pi compared to ADP and one Pi.
Hope this helps.
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Exactly what I was thinking. Just started taking a few AAMC full lengths for my MCAT on Thursday...got a 37 on AAMC9...very, very surprised at how much easier the real thing is than the "practice" that I've been getting here on the forum as well as from TPR.
But then again, with the knowledge of the idiosyncratic situations and ways I can be tested on a concept, I find myself leaving each section with much more confidence--something I didn't have before.
I dunno I just feel that in most questions, the other answer choices are so bad that I don't really need the fine tuned knowledge of the concept in order to answer it correctly...
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I agree, I just posted the formula to see if that's what he was looking for. Anyways though all you really need to know for beta oxidation I guess is that it is the break down of fats, and occurs in the mitochondria.
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There's an enzyme in solution that catalyzes the hydrolysis of pyrophosphate. The energy released isn't coupled to the reaction, but the reaction is driven forward by LeChatlier's
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Not sure if you're still talking about beta oxidation but the hydrolysis of pyrophosphate is definitely coupled to the activation step. The hydrolysis of PPi pulls the activation reaction towards the formation of the fatty acyl-CoA. Without this hydrolysis, fatty acyl-CoA formation is not really favorable. I could be wrong since I took biochem two years ago, but this is how I remember it.
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Lehninger runs the numbers for dG like it's a coupled reaction. I was confused by this, since I was under the impression that pyrophosphatase was a free enzyme. I asked my biochem prof whether pyrophosphatase interacted with the enzymes "using" the energy from the pyrophosphate hydrolysis so that the reactions could be coupled, since there wasn't any mention of this in Lehninger. Her reply was that she wasn't aware of any literature supporting any such interaction, and that LeChatlier's was how she saw the hydrolysis carrying the reaction.
A reaction doesn't have to be favorable to run to a significant extent or even completion if that reaction is irreversible (which rapid pyrophosphate hydrolysis would lead to).
That said, it's possible she's wrong too. She answered on the spot - didn't pull references or anything. And Lehninger gives the opposite impression.
A reaction doesn't have to be favorable to run to a significant extent or even completion if that reaction is irreversible (which rapid pyrophosphate hydrolysis would lead to).
That said, it's possible she's wrong too. She answered on the spot - didn't pull references or anything. And Lehninger gives the opposite impression.
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Gotcha. I based my answer on how I remembered the picture showing the activation step in Lehninger's. I remember both hydrolysis reactions being side-by-side, etc. Oh well, this is just really nitpicky stuff that will very likely be provided in the passage itself if a question's asked.
