Triacylglycerol vs. Glucose

[Franksta1118]

Can somebody provide an explanation of why Triacylglycerols provide more ATP per carbon when fully oxidized to ATP than does Glucose. I'm thinking along the lines of B-Oxidation and such but I'm not too sure how the numbers work out. Thanks a lot!

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[BloodySurgeon]

In order for a n-carbon fatty acid to be completely oxidized, (n/2)-1 turns of B-oxidation cycles must be completed, which produces (n/2)-1 FADH2, (n/2)-1 NADH, and n/2 acetyl-CoA. Each acetyle-CoA then goes through the citric acid cycle and produces the same amount of ATP as half a glucose. So as long as a fatty acid is 4 carbons or longer, it will produce more ATP than glucose.

Also remember a triglyceride has three fatty acid chains and have chains usually around 16 carbons long in humans. So if you do the math, there are usually around 9x more ATP in triglycerides.

 

[BloodySurgeon]

This of course is unless information for the mcat... all you need to know is that triglycerides have more energy, but ATP are more usable energy sources by breaking the high energy phosphate bonds.

 

[RSAgator]

Triacylclycerol is a glycerol backbone plus 3 variable length fatty acid chains. Essentially every 2 carbons in a fatty acid side chain goes on to produce an acetyl-CoA, an NADH, and an FADH2. Aside from the ATP produced by the initial NADH/FADH2, the ACoA also goes down the citric acid cycle where it is converted into more energy (ala typical glucose metabolism, 3 NADH, 1 FADH2, 1 ATP).

Most of the energy in glucose metabolism is produced in the citric acid cycle. After glycolysis each glucose molecule is converted into 2 ACoA, which go through TCA. Compare the 2 ACoA produced from 1 molecule of glucose to the many ACoA that could potentially be formed by a triacylglycerol and you have the reason why it produces so much more energy.

Say you had a triacylglycerol of of 3 12 chain fatty acids, that would be a total of 16 ACoA produced from that triacylglycerol alone compared to the 2 ACoA from 1 molecule of glucose.

edit: I mean most of the energy is produced in the electron transport chain by the reducing equivalents that are created in the citric acid cycle. I'm sure someone's going to point out to me how I'm wrong because energy technically isn't produced in the citric acid cycle aside from the 1 ATP so here's my pre-emptive clarification.

 

[BerkReviewTeach]

Can somebody provide an explanation of why Triacylglycerols provide more ATP per carbon when fully oxidized to ATP than does Glucose. I'm thinking along the lines of B-Oxidation and such but I'm not too sure how the numbers work out. Thanks a lot!

The methods listed above work well for precise determination.

If you are looking for a simple answer it's as follows: We get energy from oxidation, so the compound with carbon in its most reduced state is the one that has the most potential to be fully oxidized (to CO2), and thus release the most energy.

In sugars, CnH2nOn, the average oxidation state of carbon is 0. In saturated fatty acids, the oxidation state of carbon is -2 for all carbons except the terminal carbons, which are -3 and +3. In glycerol, the carbons have oxidation states of -1, 0, and -1.

Triacylglycerides have carbons with more room to be oxidized than sugars, so they can release more energy.

 

[Kaustikos]

Triacylclycerol is a glycerol backbone plus 3 variable length fatty acid chains. Essentially every 2 carbons in a fatty acid side chain goes on to produce an acetyl-CoA, an NADH, and an FADH2. Aside from the ATP produced by the initial NADH/FADH2, the ACoA also goes down the citric acid cycle where it is converted into more energy (ala typical glucose metabolism, 3 NADH, 1 FADH2, 1 ATP).

Most of the energy in glucose metabolism is produced in the citric acid cycle. After glycolysis each glucose molecule is converted into 2 ACoA, which go through TCA. Compare the 2 ACoA produced from 1 molecule of glucose to the many ACoA that could potentially be formed by a triacylglycerol and you have the reason why it produces so much more energy.

Say you had a triacylglycerol of of 3 12 chain fatty acids, that would be a total of 16 ACoA produced from that triacylglycerol alone compared to the 2 ACoA from 1 molecule of glucose.

edit: I mean most of the energy is produced in the electron transport chain by the reducing equivalents that are created in the citric acid cycle. I'm sure someone's going to point out to me how I'm wrong because energy technically isn't produced in the citric acid cycle aside from the 1 ATP so here's my pre-emptive clarification.

At this point, you would either have to be a complete a**hole or whatever to even try to point that out. But the idea is that you get more ACoA for citric acid cycle to make more of those reducing agents versus glucose alone. Every NADH you make, you have 3 ATP generated in the ETC and 2 ATP for every FADH. If you can get more CoA from a carbon chain, you're going to end up making more energy.

One question I would like to add though. This is beyound the scope, I think, but it bothers me. I remember reading in my graduate level physiology course that, on average, when fasting, your body tends to use up energy stores from muscle much faster than fat; does this mean you use up proteins as well or just the glucose stores? I always remembered it as your body using proteins in the last possible way because of the process of having to deaminate the protein and then using the carbon skeleton. So I think I have two questions;
Am I right in that proteins are the VERY last stores used and that the book meant that the body prefers muscle glucose stores over fat/liver in fasting?
And what is it about muscle stores that are made more preferable over fat and liver?

 

[RSAgator]

At this point, you would either have to be a complete a**hole or whatever to even try to point that out. But the idea is that you get more ACoA for citric acid cycle to make more of those reducing agents versus glucose alone. Every NADH you make, you have 3 ATP generated in the ETC and 2 ATP for every FADH. If you can get more CoA from a carbon chain, you're going to end up making more energy.

One question I would like to add though. This is beyound the scope, I think, but it bothers me. I remember reading in my graduate level physiology course that, on average, when fasting, your body tends to use up energy stores from muscle much faster than fat; does this mean you use up proteins as well or just the glucose stores? I always remembered it as your body using proteins in the last possible way because of the process of having to deaminate the protein and then using the carbon skeleton. So I think I have two questions;
Am I right in that proteins are the VERY last stores used and that the book meant that the body prefers muscle glucose stores over fat/liver in fasting?
And what is it about muscle stores that are made more preferable over fat and liver?

Well I think that in reality you are constantly undergoing catabolic and anabolic processes as far as all metabolites are concerned. Your body is constantly using amino acids for energy, perhaps not preferentially but to some extent at least, much like it is constantly using your fat stores. To say the body prefers one source of energy over another is somewhat incorrect because in reality everything is constantly in use. If you have too much of one thing it is broken down into another, too little of one thing it is made into something else. Aside from the urea by product, some amino acids are actually incredibly energy rich.

One thing to remember also is that fats are much more energy dense than glucose stores. Saying the body uses up the muscle energy stores faster than fat may very well be true simply because there is so much more energy stored in the fat.

Also as far as amino acid degradation goes, deamination is a very easy process and in many cases immediately produces central metabolites (glycine becomes pyruvate via a simple deamination for example. Glutamate becomes alpha keto glutarate, etc). One possible disadvantage of using fats is that to my knowledge they are not gluconeogenic. Not sure what the implication is in terms of fasting though.

 

[RSAgator]

Also certain coenzymes need to be regenerated. NADPH is often involved in beta oxidation and so in order to regenerate that to undergo further beta oxidation you may need to break down some other energy sources.

 

[canuckythe]

I would think that triglyceride (with a 12 carbon chains) produces 6 ACoA in each fatty acid, there are 3 chains so that's 18 ACoA. plus 2 more from the glycerol that makes 20. why does everyone say it's only 16??