01.10 Lipid Metabolism
- Types and sources of lipids
- Lipid transport
- Fatty acid breakdown & build-up
- Fat storage
- Types of fatty acids
- Lipids are non-polar molecules
- They don’t mix with water
- Types: fatty acids, triglycerides, phospholipids & sterols
- Dietary sources of fat
- Naturally fat-containing foods have all three types of fatty acids
- Animal foods also contain cholesterol, which is a type of lipid
- Fatty acid categories
- Saturated fatty acids have no double bonds
- Monosaturated have one double bond
- Polyunsaturated have more than one double bond
- Triglyceride structure
- 3 fatty acids are attached to a glycerol backbone
- Note double bonds create kinks, thus they are somewhat helical in structure
- Lipids are non-polar molecules
- Lipid particles
- Fats do not dissolve in blood since blood is mostly water
- Triglycerides and cholesterol are transported by lipid particles: HDL, LDL, VLDL, etc.
- Fatty acids are released from the triglyceride prior to uptake by cells
- Fatty acids passively enter cells and travel to the mitochondria where they are broken down for energy
- Fatty acid breakdown
- Also known as “beta-oxidation” because the fatty acid is cleaved at the carbon “beta” to the carbonyl of the acylCoA
- Occurs in the mitochondria
- Two carbon chunks are removed from the acylCoA, eventually turning all carbons of the fatty acid into acetylCoA
- Note: acyl = long chain whereas acetyl = two carbons
- This process produces FADH2 and NADH which provide energy via the electron transport chain
- The resulting acetylCoA molecules enter the Kreb’s Cycle to produce CO2 and more NADH & FADH2.
- Fatty acid synthesis
- Basically the reverse of beta-oxidation
- Build-up occurs 2 carbons at at time
- Occurs in the cytoplasm
- NADPH is needed to make new fatty acids
- Fat Storage
- When the body doesn’t need energy, acetylCoA molecules from glucose & fat (and some amino acids) get turned into palmitic acid (a 16 carbon fatty acid)
- Happens 2 carbons at a time via chain elongation
- Fatty acids not needed for energy are stored in adipocytes (aka fat cells)
- Lipids are nonpolar and they include fatty acids, triglycerides, phospholipids and cholesterol.
- Lipids are transported throughout the body via particles
- When fatty acids enter a cell, they can be broken down via beta-oxidation in the mitochondria
- When a body has excess acetylCoA it gets converted to palmitic acid and is stored in triglycerides in fat cells.
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Hey there. What we’re going to talk about today is lipid metabolism. We’re going to focus on breakdown and build up and also storage. First I would like to talk a little bit about the types of lipids. There are things like fatty acids and there are triglycerides which I’m going to abbreviate that as a TAG, and there’s another way of saying triglyceride, and that is triacylglycerol. I actually like drawing just a little symbol of it…this is what I make a triacylglycerol look like. And we’ll talk a little bit more about that in a bit. There’s also phospholipids, so PL for phospholipid, and also cholesterol. What I’m going to focus on in this video are going to be the fatty acids and the triacylglycerols. What these guys have all in common is that they are non-polar…these things do not dissolve well in water, right? And so that’s what makes them lipids. Because of that, they have to have a special transport system through our blood. We’ll talk about that.
We will also talk about how fatty acids get broken down for energy and how they get built back up. And when they get built back up, they’re often put into storage. Here’s a triglyceride and it is showing the three types of fatty acids. But before talking about those different types of fatty acids, I’d like to bring your attention to the glycerol backbone over here. Remember, we can say a triglyceride? We can also call that a triacylglycerol. The glycerol is these three carbons plus OHs coming off of each; you don’t see the Hs there because the Hs get replaced with these fatty acid chains coming off the ends here. So that’s what makes it, triacylglyceride.
Now let’s look at these different types. So in this one…this is what we call a saturated fatty acid. What we’re really saying is that it’s saturated with hydrogens. So off of each of these carbons is two hydrogens. Okay? So it’s saturated with H’s…that’s in contrast to this guy down here…because carbon can only have four bonds around it. Each of these carbons…see how we have one, two, three, four… they can only have one H. That’s why that’s called an unsaturated fat. Now, in this case, we’ve just got one double bond, so it is called a monounsaturated fat. Whereas down here, we’ve got one, two, three double bonds, so that’s why it’s called a polyunsaturated fat.
I want to bring your attention to this little guy here. This is an omega. Omega is the last letter in the Greek alphabet, so it’s used to describe fatty acids because it tells us where the last double bond is. If this is the last carbon, second to last carbon, third to last carbon, and that’s where the last double bond is, we would call this an omega-3 fatty acid. Similarly, if we come over here…this is four, five, and six…this is going to be an omega-6 fatty acid. These guys are super important. These are what we call essential fatty acids. The word essential actually means that ‘we need to eat’–they have to come in through our diet. Our bodies can’t make these guys, whereas up here, this saturated fat like this, our bodies can make this; our bodies make something called palmitic acid, that’s got 16 carbons in it. We’ll talk about that in a couple of slides.
Before we go there, let’s talk about how these very non polar molecules end up traveling through our blood. Remember our blood–a large portion of it is water. So these guys would not mix. They wouldn’t be able to dissolve in the water. And so they need to travel in particles. So these particles have phospholipids here on the outside, a variety of different proteins embedded in that phospholipid layer. And on the inside are triglycerides. That’s what these little T’s are and cholesterol molecules (C). So these triglycerides, those are essentially these little E-like things that I’ve been drawing. Those things can release the fatty acids from the glycerol backbone. You can have it released from the glycerol backbone, and then those fatty acids could go and enter into a cell. If this is a cell, then the fatty acids get into the cell. They can go into the mitochondria, right? (It’s kind of a skinny one.) The fatty acids can come in here and then get broken down for energy through a process called beta oxidation. That’s going to give energy right now. We’ll talk about that on the next slide.
I do want to point out…you’ve probably heard of these particles before: things like HDL, LDL, VLDL. Those are some different names of what these types of particles are and the differences between them. It’s just going to be the ratio of how many proteins there are to the lipids that are in here and what the specific proteins are that make up the outside. That’s what makes a difference in these different particle names.
This is beta oxidation. This is what the metabolic pathway looks like. The basic idea is that here we can take this 16 carbon chain…it’s called an acylCoA. Remember, this is acetylCoA…it’s got two carbons, one here, one here. We use the word acetyl to describe it. If there’s more than two carbons, if it’s some number of carbons, there a general term for it: an acylCoA…some number of carbons that are in there. What’s going to happen is this molecule is going to go through a series of chemical reactions and two carbons are going to get chunked off of it. This carbon and this carbon are going to disappear. And now this one’s down to being 14 carbons. It’s going to go back through the cycle and the cycle is going to keep continuing. Each time as you go around, two carbons are going to get off of it. Just to let you know where the naming of this comes from it’s because this carbon right here, that guy right there, that’s what we call the beta carbon. And so there’s oxidation taking place at this carbon. There’s going to be a new one over here, and that’s this one that we see here. And so we’re getting oxidation taking place at that carbon. That’s why it’s called beta oxidation. So two by two, we’re going to break all these things down, creating acetylCoA, and so 8 acetylCoAs are going to get made from the 16 carbon acylCoA. In the process, we’re going to generate some FADH2 and we’re going to generate some NADH’s. These acetylCoAs can come down here and go through the Kreb’s cycle. And then, as you know, the Kreb’s cycle is going to generate some CO2 and some NADH and some FADH2. They can all go to the electron transport chain, and that’s where lots and lots of energy is going to come from. So it’s a very energy producing process that we’re looking at here.
Now this is the reverse of this, and I say approximate reverse of this because there are some differences. But the reverse of this is what we can think of as the biosynthesis of a fatty acid. Recall what I had said before that beta oxidation takes place in the mitochondria. In contrast to that, synthesis of fatty acids occurs in the cytoplasm. Like the breakdown, what we’re seeing here is buildup of a fatty acid, two carbons at a time. What that means is that you can take eight acetylCoAs and they can get turned into one palmitic acid. Let’s take a look at how that happens here. Here’s our acetylCoA that we’re starting with. We go through some chemical reactions and then it comes into the cycle part of it. So here it’s going to go through multiple cycles, eventually getting to the point of having a C-16, and if that’s the case, we come over here and that is our 16-carbon acylCoA. This is just a different way of saying it here, but that’s the palmitoylCoA, that’s what that guy is there right now. One other difference I want to point out to you is that there’s a thing called NADPH that is getting used in this. This is NADH with a phosphate (P) added onto it, and that’s what’s used in this process–it’s called reductive biosynthesis. The idea is that this H right here, that H is what’s getting added to these carbons here. And that’s, what’s helping to create that long chain so that we’ve got all those Hs in there. We’re getting Hs added on so that we do reductive biosynthesis.
The last thing I want to focus on here is what happens if we have a bunch of acetylCoA. Let’s say we have an excess, as in we’ve got too much acetylCoA. Those guys are going to get converted into fatty acids; those guys are going to get linked onto that glycerol backbone to make triacylglycerols. And then those are going to get stored in your fat. So the way we talk about fat is it’s adipose tissue…that’s basically saying fat tissue. Or fat cells are called adipocytes. That’s how we store excess acetylCoA. It’s important to know acetylCoA can come from glycolysis. If you recall, you can have glycolysis go to pyruvate, pyruvate to acetylCoA. We just saw how fatty acids can get broken down into acetylCoA, and it ends up that some amino acids can also get turned into acetylCoA. So here’s your carbs, here’s your fats, here’s your protein. Anything that we eat too much of…we generate too much acetylCoA…our bodies are going to store as fat. We were evolutionarily designed to do this…this is how we can make it through hard times is because of our fat stores…all this fuel to be used at a later time when we don’t have enough food around.
To summarize: We looked at some different types of lipids. We focused on fatty acids and triglycerides, and we also talked about how fats are going to be transported through the blood since they’re so non-polar, so we’ve got particles like LDL, HDL, VLDL. And then we talked about how the breakdown process that is called ‘beta oxidation’ breaks down fatty acids into acetylCoA. And build up can happen by linking acetylCoAs together. These processes both involve NADPH or NADH and also some FADH2. Lastly, if we have an excess of acetylCoA, they’re going to get stored as triglycerides in your fat cells, which are also known as a dipocytes.
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