Fatty Acid Oxidation 2: Study with Video Lessons, Practice Problems & Examples

Unsaturated fatty acids undergo beta oxidation, requiring isomerases to reposition double bonds for effective metabolism. Odd-numbered fatty acids yield propanoyl CoA, which is converted to succinyl CoA for entry into the citric acid cycle (TCA). For palmitic acid, the ATP yield from beta oxidation and subsequent TCA is significant, totaling 108 ATP. This highlights the energy efficiency of fatty acids compared to glucose, despite the latter having a higher energy density per weight. Understanding these metabolic pathways is crucial for grasping energy production in biological systems.

Fatty Acid Oxidation 2

Unsaturated fatty acids sometimes require a little helping hand with beta oxidation, and that is because they sometimes need to move that double bond around, right? So, remember trans conformations are okay but cis have to be rearranged. Furthermore, we're going to want that double bond like we saw before. We're going to want it between the 2₃ carbon on the molecule. Now, with what we're about to talk about, I just want to say that don't obsess over the details. Just try to come away with sort of like an understanding of the generalities. So, you know, let's take a look at this figure here. Basically, we're, you know, doing beta oxidation, chipping away at this until boom. We get right up close and personal with those double bonds. Now, those both those double bonds are, in the cis form. So both of them are unacceptable. Additionally, we want this double bond to be between the 2₃ carbon. Right now, it's between 3₄. So, we're going to use an isomerase to actually move that over into the right position and then from there, we can just do normal beta oxidation, right? Chop it off. Here is the catch. The catch is if you do this, you're not going to generate an FADH₂ from that round of beta oxidation, right? Because the first step of beta oxidation is introducing that double bond and reducing FAD, right? Well, you didn't introduce that double bond. You just moved it over. So you're not going to make an FAD if you do that. Alright. Just something to be aware of and keep track of if you're thinking about the beta oxidation of various molecules. Alright. So, moving on. If you have multiple points of unsaturation in a particular arrangement, it might actually prevent an isomerase from being able to take care of the issue, right? So right here, that can't be dealt with by an isomerase alone. So what's going to have to happen is we're going to have to use NADPH to reduce our double bond and then we can deal with it. So we're going to have NADPH come in and NADPH is just like NADH. It's an electron carrier. It's going to drop off its electrons to reduce that bond and we're going to be left with NADP⁺. And then, we'll go through, or depending on the conformation here, we can actually just move this bond over, right, and do beta oxidation. In some situations, you might not have an arrangement like that and you might reduce with NADPH and then actually have to create the double bond and generate an FADH₂. So odd-numbered fats, odd-numbered fatty acids will end up with, well, if we have an even number of fatty acid, right, in our last round of beta oxidation, we're gonna have a 4 carbon molecule that's gonna get chopped into 2 acetyl CoAs, right? Both of those 2 carbon molecules. If we have an odd number of fatty acid, that last step is going to have a 5 carbon molecule, and we're gonna chop it into an acetyl CoA, 2 carbons, and a propanoyl CoA, just 3 carbons. Now, acetyl CoA, that's fine. We can work with that. Right? But but propanoyl CoA, we're going to have to modify that molecule before we can do anything with it. And what we're going to do is we're going to add a CO₂ to it. It's actually going to cost ATP. And we're going to convert it into succinyl CoA. Hey, I remember that molecule. That's used in step 5 of the tricarboxylic acid cycle or the citric acid cycle. I abbreviated citric acid cycle here as TCA. But that's just the citric acid cycle. There's actually three well, there, wait. Let's get messy. There's actually three names for the citric acid cycle. You have the citric acid cycle, the Krebs cycle, and the tricarboxylic acid cycle. Same thing. TCA is often the abbreviation for this cycle though. Anyhow, moving on. The last thing we're going to do is we're going to take a look at this molecule, acid and we are going to put it through beta oxidation and see how much ATP we generate. This is going to be a good exercise for sort of tracing the logic of the various metabolic pathways we've been talking about. So I'm going to hop out of the image here, free up some space so you can see the molecule and here we go. So palmitic acid, happen. Now, if you're going to happen. Now if you remember before there's like a little simple rule you can think of to figure out how many acetyl CoAs you're going to produce, right, and how many rounds of beta oxidation you're going to go through and that little, trick is you do half the number of carbons in the molecule minus 1, right? So here we have 16 carbons divided by 2. That's 8 minus 1. So 7 rounds. And this is going to produce 7 acetyl CoAs. Alright. So from beta oxidation alone, we're going to generate 7 FADH₂s which will lead to 10.5 ATPs. And we're going to generate 7 NADHs which will lead to 17.5 ATPs. Now, these 7 acetyl CoAs are going to go through the citric acid cycle, right? When I'm sorry. Whoops. This is 8 acetyl CoAs. Sorry. Seven rounds of beta oxidation, 8 Acetyl CoAs. My bad, guys. My bad. Sorry if you caught that. Anyways, one acetyl CoA going through the citric acid cycle yields 1 FADH₂ and 3 NADH as well as one ATP or GTP. So 8 acetyl CoAs is going to make 8 FADH₂, right, which is 12 ATP. NADH, that's going to make 24 of those guys, right, which is going to lead to, let's see, 60 ATP. And don't forget that we're also going to be producing 8 ATP or GTP. Same thing. We're going to count them, right, from the citric acid cycle. So total, we are going to make how much ATP? 108 ATP by putting palmitic acid through beta oxidation. That's a ton of ATP, right? Pretty crazy. If you're wondering, you know, just if you're wondering like, hey, wait, that's so much energy. Why do we use glucose and not just use fat all the time? The answer is actually if you divide by the molecular weight, there's more energy per weight in glucose than there is in fatty acids. Even though one fatty acid can generate like, you know, a ton of ATP. Alright. That's all I have for this, exam review. Let's move on to some exam practice questions.

Here’s what students ask on this topic:

What role do isomerases play in the beta oxidation of unsaturated fatty acids?

Isomerases are crucial in the beta oxidation of unsaturated fatty acids because they reposition double bonds to make them suitable for further oxidation. Unsaturated fatty acids often have double bonds in the cis configuration, which are not ideal for beta oxidation. Isomerases convert these cis double bonds to trans configurations and move them to the correct position, typically between the 2nd and 3rd carbon atoms. This adjustment allows the fatty acid to continue through the beta oxidation pathway. However, this process does not generate FADH₂ because the double bond is not introduced but merely repositioned.

How is propanoyl CoA from odd-numbered fatty acids metabolized?

Propanoyl CoA, produced from the beta oxidation of odd-numbered fatty acids, is metabolized by converting it into succinyl CoA. This conversion involves adding a CO₂ molecule to propanoyl CoA, a process that requires ATP. The resulting succinyl CoA then enters the citric acid cycle (TCA cycle), where it can be further oxidized to produce ATP, NADH, and FADH₂. This pathway ensures that the energy stored in odd-numbered fatty acids is efficiently utilized in cellular metabolism.

Why does beta oxidation of unsaturated fatty acids sometimes not produce FADH₂?

During the beta oxidation of unsaturated fatty acids, FADH₂ is typically produced in the first step when a double bond is introduced between the 2nd and 3rd carbon atoms. However, if the fatty acid already has a double bond that needs to be repositioned by an isomerase, this step is bypassed. Since the double bond is not newly introduced but merely moved, no FADH₂ is generated in this round of beta oxidation. This results in a lower ATP yield compared to the oxidation of saturated fatty acids.

How much ATP is generated from the complete oxidation of palmitic acid?

The complete oxidation of palmitic acid (a 16-carbon fatty acid) generates a significant amount of ATP. Beta oxidation of palmitic acid produces 8 acetyl CoA, 7 FADH₂, and 7 NADH. Each acetyl CoA entering the citric acid cycle yields 1 FADH₂, 3 NADH, and 1 ATP (or GTP). Summing up, beta oxidation and the citric acid cycle together produce 108 ATP from one molecule of palmitic acid. This high ATP yield underscores the energy efficiency of fatty acids as a fuel source.

Why is glucose preferred over fatty acids for quick energy despite fatty acids producing more ATP?

Although fatty acids produce more ATP per molecule compared to glucose, glucose is preferred for quick energy because it has a higher energy density per weight and can be rapidly mobilized and metabolized. Glucose metabolism via glycolysis and the citric acid cycle is faster and more efficient in terms of speed, making it ideal for immediate energy needs. Additionally, glucose can be utilized anaerobically, providing energy even when oxygen levels are low, unlike fatty acids which require oxygen for beta oxidation.