Hello, everyone! It’s only been a day, but it feels like it’s been forever. That’s probably because I’ve been procrastinating mightily in order to avoid studying this stuff, even though I really like it quite a lot. (You think I’m kidding. I just spent the last twenty minutes trying to learn the words to 那些花儿, a song my Chinese partner is learning on ukulele…)
Anyway, today we’re going to be talking about another important topic involving our favorite lipids—fatty acid biosynthesis! Aren’t you psyched? I sure am! I just love it when our bodies decide to make fat!
As we know from high school biology or chemistry (or both—I’m looking at you, Katniss), fatty acids are the building blocks of triacylglycerides (TAGs, or, as we often say, “triglycerides”). Our bodies make them into TAGs instead of keeping them floating around as free fatty acids because free fatty acids love to make these little things called micelles. These have “detergent properties” (read: they’re soap), which, shall we say, is a bit not good as far as our insides are concerned.
All right, that’s all well and good, but what on earth would posses our bodies to make these things? Well, remember when we were talking about the synthesis and catabolism of glucagon? Yep, same basic concept. As we saw in the post on fatty acid catabolism, these suckers have a lot of energy in them. Thus, our body likes to build them up and save them for rainy days.
Under what circumstances, then? Well, as you’d expect, our bodies prefer to store up energy when they have a lot to spare. In other words, if we have a lot of glucose in our blood, our bodies start to suck it up and use it in fatty acid biosynthesis. As you know, this is a good idea, since fatty acids are a really efficient way to store energy in relation to glucose.
Triacylglyceride synthesis takes place in the cytosol of liver cells, and uses fatty acids from two sources: those taken in directly from the blood (released by lipases breaking down TAGs in adipose tissue), and those synthesized on the spot. Carbons for those spiffy new fatty acids come from a lot of different places: glucose, fructose, amino acids (either directly or through interconversion with glycolytic intermediates), and glycerol are all viable sources.
As you might expect, under these same conditions, the citric acid cycle and oxidative phosphorylation slow down. (After all, this isn’t organic—What’s gots doesn’t gets.) This means that acetyl-CoA (our starting material in the citric acid cycle, remember) accumulates in the mitochondrial matrix. Now, that’s a lovely source of carbon for our synthesis of fatty acids, but we have a slight lil’ problem: fatty acid synthesis takes place in the cytosol.
Transport of acetyl-CoA from the matrix to the cytosol is accomplished using the “citrate-pyruvate cycle.” Firstly, oxaloacetate reacts with acetyl-CoA to make citrate (step one of the citric acid cycle) by citrate synthase. Next, a transporter protein called the citrate transporter moves this into the cytosol. Finally, an enzyme called citrate lyase undoes the first reaction, converting citrate back into acetyl-CoA and oxaloacetate in the cytosol.
This becomes a proper cycle after fatty acids are made using the acetyl-CoA. An enzyme called malate dehydrogenase reduces oxaloacetate to malate using NADH (the reverse of the last reaction of the CAC). Malic enzyme then oxidizes this to pyruvate and carbon dioxide using NADP+. The pyruvate enters the matrix using a pyruvate transporter, and is then converted into oxaloacetate using ATP and a biotin-dependent enzyme called pyruvate carboxylase.
All right, now that we’ve got plenty of acetyl-CoA in the cytosol, how are we going to make it into a fatty acid? Well, in keeping with the example we used in the last post, here’s the net reaction for the conversion of acetyl-CoA into palmityl-CoA:
8 Acetyl-CoA + 7 ATP + 14 NADPH —> 1 Palmityl-CoA + 7 ADP + 7 Pi + 14 NADP+ + 7 CoASH
Now, you’ll notice that there’s a lot of energy being consumed in this reaction. We’re using 7 ATP, plus the energy in the thioester bonds of the acetyl-CoAs. Thus, this is an anabolic reaction that’s very spontaneous. As you’ll remember from our studies of glycolysis and gluconeogenesis, both the catabolic and anabolic metabolisms of fatty acids are made spontaneous because they aren’t identical in both directions (after all, we didn’t make ATP directly in our catabolism, did we?).
All right, now it’s time for us to go into a little more detail. You know what that means! It’s time to get mechanistic~!
The first step in the synthesis of fatty acids from acetyl-CoA is the addition of two carbons by carboxylation. This is accomplished by the enzyme Acetyl-CoA Carboxylase. This enzyme is ATP-dependent and biotin-containing. Here’s what that looks like:
Now, we still have quite a bit to go, but before we look into the details of those reactions, we have to take a step back and look at the structure of a very important little thing called Fatty Acid Synthase.
As you might expect from its name, this lil’ guy has enzymatic properties. However, it’s a complex of not two or three, but six enzymes (and one non-enzymatic protein). These are arranged into two dimers that form a structure that looks kind of like a body.
Now, fatty acid synthase (FAS) contains six enzyme domains, which are found, from amino to carboxyl end, in this order: beta-Ketoacyl-ACP Synthase (KS), Malonyl/Acetyl Transacetylase (MAT), Hydroxyacyl-ACP Dehydrase (DH), Enzyme-ACP Reductase (ER), Ketoacyl-ACP Reductase (KR), and Thioesterase (TE). A carrier protein (not an enzyme), Acyl Carrier Protein (ACP) is found between the last two enzymatic domatins.
You may ask, what the heck is the advantage of smooshing all of these enzymes into one place? Well, this thing works by picking up a substrate (using ACP) and performing each chemical transformation on it before finally releasing it. The advantage of this is what you’d expect: the rate increases greatly, since each particular enzyme doesn’t have to wait around for its substrate to bump into it as it’s wandering around the liver cell’s cytosol.
In case you’re wondering, here’s a beautiful and entirely Public Domain picture of the “head-to-tail” model of this beautiful little enzyme:
As you can see by this wonderfully labelled image, there are six enzymatic domains that each perform different kinds of chemistry on our substrate, malonyl-CoA. Of course, before we do any of that chemistry, we first have to bind malonyl-CoA and acetyl-CoA to the enzyme.
This is a covalent interaction, and it happens when acetyl-CoA and malonyl-CoA both exchange thioester linkages with ACP. First, acetyl-CoA binds to MAT (Malonyl/Acetyl Transacetylase), and its thioester bond is traded with a thioester bond to ACP. Acetyl-ACP is then transferred to KR. Malonyl-CoA then binds to MAT, and its thioester is, too, exchanged with one on ACP. This is sometimes called “priming,” and it’s reversible (which makes sense, since all that’s happening is an exchange of thioester bonds).
Now that our FAS has malonyl-CoA and acetyl-CoA (now malonyl-ACP and acetyl-ACP) in its death-grip, it’s time to do some real chemistry. The first thing that happens is the condensation of malonyl-ACP and acetyl-ACP by the beta-Ketoacyl-Synthase (KS) enzyme domain.
Here’s how it goes down: first, the enzyme mediates the decarboxylation of malonyl-ACP. When this happens, a carbanion is generated. This reacts with the carbonyl of the acetyl-ACP (which is actually tethered to the KS domain, as I said above). This generates acetoacetyl-ACP and carbon dioxide. The cleavage of the thioester bond on the acetyl-ACP when it binds to the KS domain provides energy that makes this spontaneous; the removal of carbon dioxide from our body helps, too.
Once this happens, acetoacetyl-ACP is reduced to 3-hydroxybutyrl-ACP by the KR domain. This uses NADPH as an electron source.
After the formation of 3-hydroxybutyryl-ACP, DH is used to dehydrate this substrate to crotonyl-ACP. As you’d expect, this produces water.
Finally, ER is used to reduce crotonyl-ACP to butyrl-ACP. This uses another NADPH.
Once this stage is reached, the butyrl-ACP is transferred to the KS enzymatic domain, and this whole things starts back again at the beginning. Carbons are added to the growing fatty acid, two at a time, until the final length is reached, at which point the fatty acid is liberated from the enzyme.
In the case of our target fatty acid, we need to build up sixteen carbons using cycles that each add two carbons to a two-carbon starting material. If we do a little mental math, we’ll find that we need to do seven cycles (14 + 2) in order to get our fatty acid to this length. Granted that we consume an ATP and two NADPH with each cycle, a seven-cycle synthesis is consistent with the net equation that I put up top.
Now, before we wrap things up, let me put in a brief word about fatty acids that extend beyond the capability of this enzyme: FAS can’t make fatty acids that are longer than about sixteen or eighteen carbons or fatty acids that are unsaturated. In both cases, premade fatty acids are modified in the smooth endoplasmic reticulum (smooth ER). Elongation occurs by elongases (which use the same substrates as FAS), and desaturation occurs by desaturases (which use molecular oxygen and NADH).
If you’re thinking back to previous knowledge of unsaturated fatty acids, you’re probably thinking about all of those essential unsaturated fatty acids (PUFAs—Polyunsaturated Fatty Acids) that we’re told to be on the lookout for in our diets. If we can desaturate our saturated fatty acids, why do we need those? Well, dear reader, we, as mammals, only have Δ5, Δ6 and Δ9 desaturases—we can’t interconvert omega-3s and omega-6s. We have to depend on our food to make those for us.
Phew! All right! That took a sufficiently long time! (You have no idea. I started this at like, 2:00 pm, and since, I’ve gone on a grocery trip, baked a ham, taught my youngest brother about the wave-particle duality, and waltzed around our kitchen. My mom says that she wants me to study, hahaha…) Now that we’re done, though, we can move on to something a little more interesting—our friends, the polar lipids!
(Edit: I lied. It’s been almost a full day. Oh well. There are probably inconsistencies and errors in this, but it’s Thanksgiving Break. I get some slack, right?)
Questions? Comments? Songs in foreign languages that have good ukulele parts? Services that can remove awkwardness from twelve-year-old boys? Put ’em below!