Fatty Acid Synthesis

00:01 Now we have covered pretty much there fatty acid oxidation including the longer chains and the unsaturated fatty acids as well as the saturated ones.

00:09 Now let's turn our attention to how we make fatty acids.

00:12 Fatty acid synthesis doesn't occur in the mitochondrion it occurs in the cytoplasm. It is sequestered from the fatty acid oxidation and this simplifies considerably the regulation needed to control the two pathways.

00:28 Fatty acid synthesis is similar chemically to the reverse of oxidation.

00:33 For example looking at the bottom we see that in synthesis we are doing the joining of two things instead of the splitting of two things.

00:41 The next step moving upwards involves the reduction whereas in fatty acid oxidation it involved oxidation.

00:49 The next step above that involves the loss of water whereas in fatty acid oxidation that was a gain of water.

00:55 And finally the last step involves a reduction whereas in fatty acid oxidation it was an oxidation. So in many ways it's very much the reverse of the oxidation process.

01:06 Now there are some differences and one of the differences is on the end of the fatty acid.

01:10 Fatty acid synthesis occurs with the fatty acid group being carried by a molecule called acyl-carrier protein or ACP as people most commonly call it.

01:22 That's different from the CoA that was on the fatty acid during the oxidation process.

01:27 Notably also the electron source in fatty acid synthesis is NADPH.

01:33 The electron carriers in fatty acid oxidation were, of course, FAD and NAD+.

01:39 Now let's step through the process to learn a little bit about how this happens.

01:44 The very first step is little unusual in fatty acid synthesis.

01:48 It's the only regulated reaction in the entire synthesis of fats and this reaction occurs, as I noted, in the cytoplasm.

01:56 ACP will get involved as a carrier.

02:00 But the very firt step involves the synthesis of a three carbon intermediate. That three carbon intermediate is known as malonyl-CoA that you can see right here.

02:13 Now malonyl-CoA is made from acetyl-CoA by the addition of a carboxyl group by the enzyme acetyl-CoA carboxylase.

02:24 And you can see that this reaction is also a very energy requiring reaction; because, ATP is being converted to AMP.

02:31 That's the equivalent of burning two ATP molecules.

02:36 The enzyme acetyl-CoA carboxylase is the only enzyme that is regulated in fatty acid synthesis, as I noted.

02:42 You will also see that this enzyme is using biotin as a co-enzyme.

02:47 Biotin is a co-enzyme that is involved in grabbing carbons to add them to things as we have seen with the previous reactions in some of these presentations.

02:58 Acetyl-CoA carboxylase is regulated by two different mechanisms, a covalent modification and the allosteric binding of small molecules.

03:09 Now we can see this depicted on the screen here.

03:11 The carboxylase that is the acetyl-CoA carboxylase can be phosphorylated by a protein known as AMP activated protein kinase.

03:20 That enzyme converts the activate carboxylase into an inactive form by putting a phosphate onto it.

03:25 That also has the effect of changing the organization of the acetyl-CoA carboxylase.

03:32 The active carboxylase is actually a polymer that is a bunch of linear chains of individual units that are attached to each other and that polymer is the most active form of the enzyme.

03:44 Adding phosphates to individual units as happens with the AMP activated protein kinase converts that polymer into individual monomers.

03:51 And the individual monomers are significantly less active than the polymer.

03:56 You can see that phosphates are taken off by an enzyme known as protein phosphatase 2.

04:03 Now one of the things that can be happen to the inactive carboxylase, however, is it can be activated a little bit at least by binding of an allosteric factor known as citrate.

04:14 Citrate is an intermediate in the citric acid cycle and citrate accumulates when cells have a lot of energy.

04:22 So if cells have a lot of energy what they do is they store that energy as something and one of the things they stored as is fatty acid. So this citrate is indicating that their energy level is high that energy level being high favors the synthesis of fatty acids.

04:39 Now this enzyme, as I said, is located in the cytoplasm and it is also feedback inhibited by the end product of the pathway.

04:49 So this is catalyzing the first step in the pathway that leads to fatty acids and as long chain fatty acids start to accumulate they will come back as allosteric factors and inhibit the enzyme.

05:01 So the enzyme can be inhibited by phosphorylation and binding of long chain fatty acids and it can be stimulated by dephosphorylation and the binding of citrate.

05:12 Polymerization, as I noted, favors activation and depolymerization favors inactivation.

05:19 Now the fatty acid synthesis proceeds with an intermediate carrier called ACP, as I noted.

05:26 That carrier is added in this reaction as you can see here and you also see that there is two starting molecules.

05:32 The two starting molecules are the malonyl-CoA that we started with that is converted into malonyl-ACP. And an acetyl-CoA that is converted into acetyl-ACP. Now those two are the starting points for the synthesis of the fatty acid.

05:52 The reaction proceeds, as you can see here, by the joining of these two components.

05:57 And the joining of these two components creates a molecule that has four carbons even though we started with two components that had 5 carbons.

06:06 Well that means, of course, in the process that one of those carbons had to get lost.

06:11 The enzyme catalyzing, as you can see on the screen there, and one of the ACPs gets lost as well.

06:16 So that carbon is added and then the carbon is lost in the process so that's seems inefficient process. But that's the way cell have evolved to do this.

06:27 By the way all the reactions that occur in fatty acid synthesis also occur between carbons 2 and 3 or the α and β as we have described before.

06:37 We see that the ketone on carbon number 3 of this acyl-ACP group is reduced in the next reaction by 3-ketoacyl-ACP reductase.

06:50 That enzyme uses electrons from NADPH creating an ADP and produces the intermediate shown on the right.

06:57 Now the intermediate in the right is in the opposite configuration to the same intermediate we saw in oxidation.

07:05 In oxidation, the hydroxyl on position 3 was in the L form and you can see here it's in the D form.

07:11 In this reaction we can see that there is a dehydration that happens, a removal of water to create a double bond.

07:17 This reaction which is catalyzed by 3-ketoacyl-ACP dehydrase creates a trans double bond an intermediate very much like what we saw in fatty acid oxidation.

07:27 In the last step of this round of the process of making fatty acids the double bond is reduced to a single bond by addition of electrons that come from NADPH catalyzed by the enzyme enoyl-ACP reductase.

07:42 Now at this point we have created a molecule that is grown by two carbons from the first round of the cycle and the continuing synthesis of the fatty acid only requires repeating the cycle over and over, until we get to a molecule that has 16 carbons.

07:57 At 16 carbons we have made a fatty acid called palmitic acid and it's released from this complex that occurs.

08:05 The release of palmitic acid from the palmitoyl-ACP is procuded by the catalysis of the enzyme, thioesterase which simply hydrolysis the bond between ACP and the palmitic acid to create the intermediate that you see on the right.

08:21 Now I have shown you a series of reactions that goes through the cycle of adding two carbons to a growing acyl-CoA. But what I didn't tell you was that all of those enzymatic properties are contained within one giant enzyme.

08:36 And that giant enzyme is known as fatty acid synthase.

08:39 Now fatty acid synthase is a big structure like what you have seen on the screen in blue.

08:45 Now I am gonna step you through the reactions to show you how they are performed on individual parts of this bigger enzyme.

08:52 But each of the individual activity is part of a big complex and this is true for fatty acid synthesis in most cells from bacteria all the way up to humans.

09:02 So if we look at the reactions that we have just gone through, I have highlighted in green on the screen the place in this fatty acid synthase complex where each activity is located. You can see the green here in the first reaction, the joining of 2 carbons occurring in the second reaction and the location of the activity is moving.

09:22 Some have compared the movement of the catalysis around the enzyme to the moving the hands of the clock.

09:30 Although as you will see the clock actually has to run backwards in some cases as it has does here.

09:36 Last we get to the step where the palmitoyl ACP is cleaved into palmitic acid and released.

09:41 And is because of the limitations of this enzymes in terms of how big of a fatty acid that can handle. That fatty acids up to 16 carbons are only made here.