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Overview of Fatty Acid Oxidation

Video transcript
- [Instructor] What I've drawn here is the chemical structure for a triacylglyceride and recall that this chemical structure is commonly what we are referring to when we talk about the type of fat found in our food as well as how fat is stored in our body. Now the question I wanna begin to answer in this video is how do we extract ATP, the chemical energy, from this molecule because you've probably heard that fats are a very rich source of energy but how exactly do we get ATP from a structure like this? Well the first thing I wanna point out is that 95% of the chemical energy that we can extract from this molecule comes from these carbon, hydrogen-rich chains that we usually refer to as fatty acid chains. Now I'm gonna put under or beside this, that 95% of our chemical energy, I'll just make symbol here, of the energy that we can extract comes from these fatty acid chains. Of course that means the remaining 5% of chemical energy that we can extract from this molecule comes from this glycerol backbone right here, so this tiny portion of the molecule is not gonna really contribute a lot and it essentially can enter actually glycolysis, potentially, where it can be oxidized further to produce a little bit of chemical energy. So because these fatty acid chains are contributing to the bulk of the energy that we're extracting, we're gonna focus on how we extract ATP from these fatty acid chains in particular. And now to help us kind of get a bird's eye view of how we're able to extract ATP from these fatty acid chains, I've actually went ahead and drawn out a 16-carbon saturated fatty acid that our body can synthesize, which is called palmitic acid. Now if you think back to how we extracted chemical energy or ATP from glucose, you might remember that we oxidized glucose, we essentially stripped it of its electrons and we transferred those electrons to electron carrier molecules to form reduced intermediates like NADH and FADH2 and these carried the electrons found in that glucose to the electron transport chain, where we were able to produce ATP quite efficiently. Ultimately, we just simply wanna do the same thing with our fatty acid, we wanna be able to oxidize it, extract all of those electrons, transfer them to those electron carrier molecules, NADH and FADH2, to be able to be used to produce ATP in the electron transport chain. And from a bird's-eye view, I think the big picture takeaway is to realize that we wanna do is essentially the reverse of fatty acid synthesis, we wanna be able to take this long string of carbons and hydrogens and essentially break them down into two carbon sub-units each and as we break them up into these two carbon sub-units, we're also simultaneously oxidizing them to release all of this energy, and ultimately what we're doing is we're breaking up this large fatty acid into single molecules of acetyl-coA, and if you remember the structure of acetyl-coA looks something like this, so two carbons, one attached to an oxygen, and of course we have our co-enzyme A group, which I'm abbreviating like this. Now notably, the energy extraction process doesn't stop there, remember that acetyl-coA is quite a versatile metabolyte when it comes to metabolism. Remember that this is what can enter the Krebs cycle, so it can enter the Krebs Cycle in the mitochondria and when it enters the Krebs cycle, even more electron carrier molecules like NADH and FADH2 can be produced by further oxidizing this molecule and so altogether, you can see that the amount of ATP that's gonna be produced is gonna be enormous because we're getting electron carrier molecules both from the direct oxidation into acetyl-coA as well as acetyl-coA's own oxidation in the Krebs Cycle. And to give you an idea of how much ATP we're really talking about, I've went ahead and calculated that for each run through the Krebs Cycle, we can produce about a net of 10 ATP per acetyl-coA molecule and we're producing one, two, three, four, five, six, seven, eight, eight of these pairs of acetyl-coA carbons and so altogether, we're producing about 80 ATP in the Krebs Cycle alone and then add that to the amount of ATP that's produced in this direct oxidation into acetyl-coA and that happens to be about 27 ATP. And I've calculated these numbers based on counting up how many NADH and FADH2 molecules are produced at each step and then multiplying that by a conversion factor and the commonly accepted conversion factor is that they're about 2.5 ATP per molecule of NADH produced and 1.5 ATP per FADH2, but the big point that I really wanna drive home here is that in the end, ultimately, we are producing 80 plus 27, which is 107 ATP molecules in total just from the oxidation of one 16-carbon fatty acid. Now compare that to the amount of ATP that we produce from one molecule of glucose, one molecule of glucose gives us about 30 to 32 molecules of ATP so that's per one molecule of glucose. So you can see here how much more ATP that we can extract from this fatty acid.