[Bio_student]

Hello Folks,

 

I'm a Biochemistry student trying to do some extra work over the summer and to get ahead in my next year at university. I've been doing some problems/questions based on what is available for download at my university intranet site/black board. One of the practice questions has me stumped and I hoped to ask for some help if at all possible?

 

The whole point of the exercise is to calculate the maximum realizable ATP yield per mole of myristic acid being fully oxidized to CO2 and H2O in a skeletal muscle cell under aerobic conditions. I'm trying to understand the step-by-step calculation so as to compare to other substrates such as glucose if that makes sense? I've never really done anything like this before and Biochemistry isn't my field (I'm more animals, plants and ecology).

So far what I know is that Myristoyl CoA is the product of the activation of myristic acid (C14:0) in the cytoplasm of a cell with a low energy charge, prior to transport to the mitochondria for beta-oxidation. The overall reaction of beta-oxidation of myristoyl CoA in the mitochondrial matrix of an animal cell is:

6 FAD + 6 NAD+ + 6 CoASH + 6 H2O + H(CH2CH2)6CH2CO-SCoA  -->  7 CH3CO-SCoA + 6 FADH2 + 6 NADH + 6 H+

[Bio_student]

I'm sitting trying to work this all out.

Basically, what I have for this so far is that Mistric Acid has 14 Carbons and goes through 4 rounds to be fully converted. After which it produces 7 CH3CO-SCoA + 6 FADH2 + 6 NADH + 6 H+ right?

I think I'm missing some information because it doesn't tell me whether the yields of FADH2 and NADH+H are 2.5 ATP and 1.5 ATP or 3 ATP and 2 ATP. This makes all the difference when trying to do the calculation I think?

[Bio_student]

So far for my answer I have the following:

It is first important to consider that ATP is required for the activation of the fatty acid. This is usually 1 molecule of ATP where two energy rich bonds are hydrolyzed, resulting in AMP and 2P. This isn't considered balanced in terms of energy consumption so sometimes it is assumed that 2 ATP are required but this isn't always the case. Looking at the overall reaction of beta-oxidation, where molecules are broken down in the mitochondria to generate acetyl-CoA (enters and is used in Citric Acid Cycle) and NADH+H and FADH2 (used in the Electron Transport Chain), we can see already that an accurate calculation based on the number of Carbons conforms to what we see in the overall reaction: 14/2 (2 Carbons required to form the Acetyl group) = 7 Acetyl CoA (7 CH3CO-SCoA) and because Beta oxidation produces 1 NADH+H and 1 FADH2, which produces 5 ATP (1 x 3) + (1 x 2). Then running through each round of the Citric Acid Cycle for each 2 carbon Acetyl CoA formed (4 times in this case) there is a further yield of 1 ATP directly, plus 3 from NADH+H and 1 FADH2 which subsequently produce (3 x 3) + (1 x 2) = 11 ATP (keep in mind we already have one Acetyl CoA from the first round which is where the 5 ATP comes from). These acetyl CoA molecules are then oxidized up to CO2 and H2O in the Citric Acid Cycle, and in terms of ATP yield per mole of myristic acid NADH.H+ and FADH2 yield 3 ATP and 2 ATP respectively.

The formula to use here is (n - 1) x 5 + (n x 12) - 2, where n is the number of Carbons. So for Mistric Acid it would be (7 - 1) x 5 + (7 x 12) - 2 = 30 + 84 - 2 and that in turn equals a total of 112 ATP. Compare this to Glucose (6 Carbons) we would have a calculation of (3 - 1) x 5 + (3 x 12) - 2 = 44 ATP if fully oxidised (usually maximum of 36 or 38). Essentially Glucose has less Carbon molecules (6) than that of Mistric Acid (14) and that is why it yields more ATP.

I'd appreciate anyone telling me if this is anywhere near correct or if I'm on the right path to understanding this biochemistry. Thank you!

[Babcock_Hall]

One, each round of beta-oxidation peels off one acetyl CoA; therefore, I think your counting of the number of rounds of beta-oxidation is too small.  Two, each NADH is typically thought to yield 2.5 ATP upon oxidation, not 3, as some old textbooks say.  IIRC the yield might be different in bacteria, but that is not germane to your question as I understand it.  Can you tell me what you think the yield of ATP for each turn of the citric cycle is?  I am having a hard time following you there.

[Bio_student]

One, each round of beta-oxidation peels off one acetyl CoA; therefore, I think your counting of the number of rounds of beta-oxidation is too small.  Two, each NADH is typically thought to yield 2.5 ATP upon oxidation, not 3, as some old textbooks say.  IIRC the yield might be different in bacteria, but that is not germane to your question as I understand it.  Can you tell me what you think the yield of ATP for each turn of the citric cycle is?  I am having a hard time following you there.

Thanks - I'll amend that now. If I'm honest I'm unsure what the yield is for each turn of the cycle. I'm no good at any of this...

[Bio_student]

My answer as it stands is as follows:

When considering the question it is first important to consider that ATP is required to activate the fatty acid for use in respiration. The hydrolysis of the relatively energy rich bonds carbon to carbon bonds requires ATP resulting in AMP and 2Pi, however this energy consuming process is offset when looking at the overall metabolic breakdown of the molecule. In beta-oxidation molecules are broken down in the mitochondria to generate acetyl-CoA this enters and is used in Citric Acid Cycle and NADH+H and FADH2 used in the Electron Transport Chain. An accurate calculation based on the number of Carbons conforms to what we see in the overall reaction: 14/2 (2 Carbons are required to form the Acetyl group) = 7 Acetyl CoA (7 CH3CO-SCoA) and because Beta oxidation produces 1 NADH+H and 1 FADH2, which produces 4 ATP (1 x 2.5) + (1 x 1.5). Each round of the Citric Acid Cycle coverts 2 molecules of acetyl CoA (4 times in this case) there is a further yield of 1 ATP directly, plus 3 from NADH+H and 1 FADH2 which subsequently produce (3 x 2.5) + (1 x 1.5) = 9 ATP (keep in mind we already have one Acetyl CoA giving 4 ATP on top). The acetyl CoA molecules are then oxidized to CO2 and H2O in the Citric Acid Cycle, and in terms of ATP yield per mole of myristic acid NADH.H+ and FADH2 yield 2.5 ATP and 1.5 ATP respectively.

The formula to use here is (n - 1) x 4 + (n x 9) - 2, where n is the number of carbons. So for myristic acid it would be (7 - 1) x 4 + (7 x 9) - 2 = 24 + 63 - 2 and that in turn equals a total of 85 ATP. Compare this to glucose (6 Carbons) we would have a calculation of (3 - 1) x 4 + (3 x 9) - 2 = 33 ATP if fully oxidised (usually maximum of 36 or 38 because glucose goes through a different series of processes). Glucose has less carbon molecules (6) than that of myristic Acid (14) and that is why it yields more ATP.

[Babcock_Hall]

One, glucose and myristic acid converge at acetyl CoA.  Therefore, only the yield from glycolysis plus the conversion of pyruvate to acetyl CoA set glucose apart.  Two, the total yield of ATP for each acetyl CoA is 10, not 9 (at one point you mention that it produces one ATP "directly," presumably meaning through substrate level phosphorylation.  9 + 1 = 10.  Generally when one speaks of rounds of the citric acid cycles, one is thinking in terms of one acetyl CoA entering, and two CO₂ leaving.  Three, with respect to myristic acid, I calculated a number that was close to yours, but not identical.  Did you remember to account for the ATP used in converting myristic acid into myrisoyl CoA?  Four, your value of ATPs per glucose looks to be a bit inflated, if I understand what you wrote correctly.

i find it helpful to think in terms of the oxidation states of the carbon atoms in fatty acids versus glucose.

[Bio_student]

One, glucose and myristic acid converge at acetyl CoA.  Therefore, only the yield from glycolysis plus the conversion of pyruvate to acetyl CoA set glucose apart.  Two, the total yield of ATP for each acetyl CoA is 10, not 9 (at one point you mention that it produces one ATP "directly," presumably meaning through substrate level phosphorylation.  9 + 1 = 10.  Generally when one speaks of rounds of the citric acid cycles, one is thinking in terms of one acetyl CoA entering, and two CO₂ leaving.  Three, with respect to myristic acid, I calculated a number that was close to yours, but not identical.  Did you remember to account for the ATP used in converting myristic acid into myrisoyl CoA?  Four, your value of ATPs per glucose looks to be a bit inflated, if I understand what you wrote correctly.

i find it helpful to think in terms of the oxidation states of the carbon atoms in fatty acids versus glucose.

I think I've got it:

When considering the question it is first important to consider that ATP is required to activate the fatty acid for use in respiration. The hydrolysis of the relatively energy rich bonds carbon to carbon bonds requires ATP resulting in AMP and 2Pi, however this energy consuming process is offset when looking at the overall metabolic breakdown of the molecule. In beta-oxidation molecules are broken down in the mitochondria to generate acetyl-CoA this enters and is used in Citric Acid Cycle and NADH+H and FADH2 used in the Electron Transport Chain. An accurate calculation based on the number of Carbons conforms to what we see in the overall reaction: 14/2 (2 Carbons are required to form the Acetyl group) = 7 Acetyl CoA (7 CH3CO-SCoA) and because Beta oxidation produces 1 NADH+H and 1 FADH2, which produces 4 ATP (1 x 2.5) + (1 x 1.5). Each round of the Citric Acid Cycle converts 1 molecule of acetyl CoA (2 Carbons and so 4 rounds in this case) there is a further yield of 1 ATP directly, plus 3 from NADH+H and 1 FADH2 which subsequently produce (3 x 2.5) + (1 x 1.5) = 10 ATP (keep in mind we already have one Acetyl CoA giving 4 ATP on top). The acetyl CoA molecules are then oxidized to CO2 and H2O in the Citric Acid Cycle, and in terms of ATP yield per mole of myristic acid NADH.H+ and FADH2 yield 2.5 ATP and 1.5 ATP respectively.

The formula to use here is (n - 1) x 4 + (n x 10) - 2, where n is the number of carbons. So for myristic acid it would be (7 - 1) x 4 + (7 x 10) - 2 = 24 + 70 - 2 and that in turn equals a total of 92 ATP. Compare this to glucose (6 Carbons) we would have a calculation of (3 - 1) x 4 + (3 x 10) - 2 = 36 ATP if fully oxidised. Glucose has less carbon molecules (6) than that of myristic Acid (14) and that is why it yields more ATP.

[Babcock_Hall]

Your first couple of sentences do not look quite right.  Activation of a fatty acid to a fatty acylthioester of coenzyme A does not involve the making or breaking of carbon-carbon bonds.  Also your yield of ATP looks too high for both myristic acid and glucose.  How many rounds of β-oxidation are you assuming?  The value of 70 ATP from acetyl CoA is correct.

The calculation for glucose depends upon making one of two assumptions about the cost of transporting electrons from the cytoplasm into the mitochondria, and the numbers one calculates differ by two ATP per glucose IIRC.  Again going on memory, it is 30 versus 32 ATP per glucose, depending on how one deals with the cytoplasmic electrons (in the form of cytoplasmic NADH).  There might have been a thread on this about a year ago here about this question, which may be outside what you need to know.