The next topic that we're going to focus on will be fatty acid metabolism. In this lecture, I'd like to discuss what fatty acids are and what the cells of our body actually use fatty acids for. I'd also like to introduce the processes of fatty acid breakdown and fatty acid synthesis. What are fatty acids?
Fatty acids are these biological molecules that consist of a long hydrocarbon chain and a terminal carboxylate group. The terminal carboxylate group gives the molecule hydrophilic polar properties, while the long hydrocarbon chain gives the fatty acids hydrophobic non-polar properties. So what do our cells use fatty acids for? Well, fatty acids have four important functions inside our cells.
Number one is fatty acids are fuel molecules, and as we'll shortly see, our cells can break down fatty acids to actually generate high-energy ATP molecules. Number two is fatty acids are actually used to build molecules that exist within cell membrane. So things like glycolipids and phospholipids are built from fatty acids. Number three is we can modify proteins by attaching fatty acids onto them. And what this does is, is it increases and diversifies the functionality of the proteins.
Number four is, molecules such as hormones and other intracellular messenger molecules are built from fatty acids. So we have many hormones which are built from fatty acids. Now let's discuss the breakdown and the synthesis of fatty acids and as we'll about and as we'll see in just a moment these two processes are essentially mirror images of one another. They're the reverse of one another and let's begin by discussing the breakdown of fatty acids.
So the breakdown of fatty acids basically consists of four steps. We have an oxidation step, we have a hydration step, we have another oxidation step, and we have a cleavage. And so these four processes together make up one cycle of fatty acid breakdown. And this process is in fact an oxidative process. So what we're essentially doing is we're extracting electrons and we ultimately...
One to basically cleave a sigma bond and by cleaving that sigma bond We're essentially shortening that fatty acid chain by two carbons. So each cycle removes two carbon component molecules So we begin with an activated fatty acid and activated simply means we've done something to the fatty acid We attached a specific group onto that fatty acid to make it more reactive and in this particular case The group we have attached is the R prime group and this is usually the coenzyme molecule, and that makes it more active. So we take the activated fatty acid and we allow it to undergo an oxidation step, and here we're essentially extracting a lack electrons.
More specifically, we extract an H atom from this carbon, an H atom from this carbon, and the electrons left over on these two carbons, we have one electron left over here, one electron left over here, that is used to actually generate a pi bond. So going from this molecule to this molecule, we ultimately form a double bond. Now, once we form the double bond, the next step is a dehydration step.
And what that ultimately tries to achieve is to attain a attach a hydroxyl group onto this carbon here. So going from here to here, we break that pi bond and we also attach that hydroxyl group as shown here. So this gives us an alcohol group.
In the next step, which is once again an oxidation step just like this step, we essentially want to extract electrons. In the process, we want to transform this hydroxyl group into a carbonyl group. And once we form that carbonyl group, we basically form the ketone. And now we can undergo a cleavage process in which coenzyme A is basically used to cleave this bond. And we form these two product molecules.
Now, one of these two product molecules is the two carbon component that we basically removed. And this is this activated acetyl unit. So acetyl simply means we have one, two carbons. And activated means we still have this coenzyme A molecule, which is given by R prime.
Now, this is the activated acyl unit. And if this molecule... has a fully saturated hydrocarbon chain and it contains an even number of carbon atoms, then this process can basically take place again and again and again until we break down that acetyl unit into these acetyl units.
If the fatty acid that we're breaking down is fully saturated and it contains an even number of carbon atoms, then this process can cycle over and over and over until we completely break down that fatty acid into these acetyl units. Now, once we form the acetyl units, they can then enter the citric acid cycle, and that can ultimately be used to generate high-energy ATP molecules. So once again, to summarize, the breakdown of fatty acids is an oxidative process that releases activated acetyl coenzyme A units, thereby shortening or decreasing the size of that hydrocarbon chain by two carbons.
And once we form the activated coenzyme A units, they can enter the citric acid cycle where they're ultimately used to actually generate the high energy ATP molecules. Thank you. Now what about fatty acid synthesis? What if we actually want to synthesize fatty acids? Why would we want to synthesize fatty acids?
Well, if we have plenty of ATP molecules we don't want to form anymore, our cells will essentially take these acetyl units and will synthesize fatty acids from them. Or if we want to build some type of hormone or modify a protein or build up our cell membrane, we can also build up these fatty acids and use them in those processes. Now as we'll see in just a moment, the synthesis is actually the opposite of the breakdown. They're mirror images. And to see exactly what we need, let's take a look at this process here.
So here we have an oxidation reaction. Here we have a reduction reaction. So the first step in this process, the first step in the breakdown process is an oxidation step, the last step in the synthesis process is a reduction step.
Now the second process, the second step in the breakdown process is a hydration and the second to last step is the dehydration. Then we have an oxidation, we have a reduction. and then we have a cleavage here and we have a condensation. So we see that this process is the reverse of this process, this is the reverse of this process, this is the reverse of this process, and this is the reverse of this process.
On top of that, all the steps are actually reversed in this. synthesis process and that's what we mean by these processes being mirror images of one another so in this particular case we want to actually synthesize these fatty acids we begin with the activated acyl units and the activated malonyl units or malonyl units Now we take the activated malonyl units and we basically undergo a condensation reaction with this activated acyl unit and what that does is it helps us generate that sigma bond and once we generate that sigma bond we essentially generate that same molecule that we had in this particular case. The next process is a reduction step because that's the opposite of this oxidation step.
So we're going this way here. And that means we essentially want to add an extra number of electrons along with H atoms. And so we generate, we transform this ketone group into this alcohol group. Once we form that, we want to remove that hydroxyl. And the way that we remove that hydroxyl is via dehydration step.
So we form this pi bond. And in the final step, we once again want to reduce this. We want to add the H atoms along with our electrons, and we generate the activated fatty acid.
And so we see that in this particular case, one cycle of the breakdown of fatty acids basically shortens that carbon chain by two carbons. But in this particular case, it increases that carbon chain by two carbons. So once again, the synthesis process is a reductive process that builds up the hydrocarbon chain of fatty acids. Beginning with the activated acyl units and the malonyl units, the reverse processes of fatty acid degradation are followed in the opposite order. And we'll talk about the details of these individual steps in the lectures to come.