biochem: a Q about ATP!

[allylz]

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hi, i'm new here.

sorry for asking a question about biochem because i know it's frustrating subject and i don't want to drive anyone crazy, but this is killing me.

i have found various websites/books giving me different information on how many ATPs are yielded by an acetyl CoA going through the TCA cycle. the main reason i was trying to figure this out is to answer a question on ATP yield of fatty acid oxidation on qbank, i know there are 4 ATP per cycle of fatty acid oxidation but then the number coming from the acetyl CoA seems to vary to either 10 or 12.

therefore, the question is about lauric acid (12carbons) and i keep getting 78 ATPs but the qbank answer is 95 ATPs. help! i think this has something to do with various people's rounding of FAD/NAD ATP yields (is it 3 and 1? 2.5 and 1.5????)
if anyone can help it would be much appreciated.
sincerely, alison

 

[allylz]

hey, i've been surfing around and i noticed most questions here are more administrative in nature. so i just wanted to apologize if my question doesn't fit the mission here, i think you guys have a very helpful forum going much more literate than the others i've visited, so keep up the good work and if anyone knows the answer to my question i would certainly appreciate it and i will try to be a good forum member from now on!

 

[Linus2807]

the way i do it
divide amount of carbons by 2
this equals amount of acetyl CoA, so in this case there are 6 AcetylCoA
Each acetyl CoA will give you 12 ATP, due to 3 ATPs per NADH, 2 ATPs per FADH, and the 1 GTP
so if you have 6 acetyl CoA and multiply by 12 you get 72 ATPs
you also get NADH and FADH from the oxidation, the easy way to figure that out is to subtract 1 from the amount of acetyl CoA---so 6 acetyl coA then there will be 5 of NADH and 5 of FADH
so in summary
6 acetly coa X 12= 72 ATP
5 additional NADH X 3= 15 ATP
5 additional FADH X 2= 10 ATP
this gives you 97 ATPs and you then subtract 2 ATPs since they were used in the breakdown for a net of 95ATPs

sorry if confusing

 

[allylz]

no it makes sense, if i use those numbers it comes up right i just wasn't sure what the "official" number was on the NADH and FADH2 (so it's 3 and 2 then, i'll just go by that!)

thanks for the help!

 

[hdu]

You can find different values depending on the book you are using.
In some books, 1 FADH yields 1.5 ATP, and 1 NADH.H+ yields 2.5 ATP, while other books consider that 1 FADH2 yields 2 ATP and 1 NADH.H+ yields 3ATP.
In a fair question about energetic balance of a metabolic pathway, the question should state the ATP equivalence of the reduced cofactors that are used.

Other source of variation in results is the use of different shuttles for the NADH.H+ that comes from glycolisis. (Remember: Glycolysis happens in cytosol, but the reduced cofactors are oxidized in mitochondria, and the internal membrane is impermeable to NADH.H+, that is why the shuttles are necessary). If the NADH.H+ produced in the cytosol enters the mitochondria using the malate-aspartate shuttle, then the equivalence is the same as the one used for NADH.H+ produced inside the mitochondria. If the NADH.H+ produced in the cytosol enters the mitochondria using the glicerophosphate shuttle, then the yields in ATP will be the same as the yields in ATP for a FADH2.

So, you can obtain at least four different values for the total oxidation of glucose up to CO2 and water, depending of the NADH.H+ equivalence in ATP and the kind of shuttle used.

As in life, different does not mean wrong!

 

[allylz]

glad to hear it. i had a feeling the story was going to be something like that. figured if there were at least a few fairly reputable sources saying different things that it wasn't a set thing (a strange thing not to be set though, in my mind!)

thanks for the help!