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Current time:0:00Total duration:9:20

Fatty Acid Oxidation - Part I

Video transcript

what I've drawn out here are three compartments found inside of a cell and what I want to go ahead and do now is kind of label what I have to run out here because these three compartments are going to be very important when discussing the three kind of major phases involved in oxidizing and extracting ATP from fatty acids inside of the cells so starting up here this compartment I'm going to label as the cell's cytoplasm and these two lines down here are both representing the two membranes around the mitochondria so if you remember the mitochondria has two membranes it has the outer mitochondrial membrane facing the cytoplasm and then below it it has the inner mitochondrial membrane and I'm abbreviating this as oh mmm and I mmm and so of course the space between these is the inter membrane space and if you remember this is where the proton gradient was built up for the electron transport chain and then finally down here this is going to be the inside of the mitochondria which is something that we also commonly refer to as the mitochondrial matrix now you might recall that fatty acid synthesis takes place inside of the cytoplasm where the enzymes required for that process are located now on the other hand turns out that the enzymes required for the oxidation of fatty acids to obtain all of that ATP from that molecule are located inside of the mitochondria but remember that cells take up fatty acids into their cytoplasm from the bloodstream so we need some way to be able to transport these fatty acids from the cytoplasm into the mitochondrial matrix and this might look familiar to you it might kind of ring a bell from fatty acid synthesis because remember in fatty acid synthesis we were also having to transport something across the cytoplasm of course you were going the opposite direction in fatty acid synthesis where we wanted to transport acetyl co a into the cytoplasm but I actually want to come back at the end of the video to touch on kind of the seemingly roundabout way to have to transport things across this mitochondrial membrane because it turns out it has a very interesting implication for how our body is able to regulate fatty acid synthesis and oxidation and so I'll come back again and touch on that but just going back to this transport process it turns out that in order to be able to transport these fatty acids across this mitochondrial membrane there's a specific pathway that requires us to actually activate so to say a fatty acid molecule with another molecule in order for this transport machinery to work and so altogether there are kind of three major phases involved in being able to ultimately extract ATP from a fatty acid so let's go ahead and cover these three phases in turn alright so starting off with the activation step let me go ahead and draw out the chemical structure of a fatty acid starting off with its carboxylate head group and of course it also has a long chain of carbons and hydrogen's that comprise its fatty acid tail which we usually refer to it as but instead of drawing this tail out every single time or drawing or writing out all the carbons and hydrogen's I'm just going to abbreviate that using the letter R to keep things in our diagram simple now as I mentioned before the the goal here is to activate this molecule quote-unquote with another molecule so that were able to transport it into the mitochondria so how does the body do that well the body does that by reacting as fatty acid with one of the most versatile metabolites in our body which is coenzyme a because the structure this molecule is is quite large we end up just abbreviating it as co a and usually at most textbooks we'll highlight this one functional group this sulfur hydrogen group is style group because it's involved in forming a bond with this carbon here so let's kind of see what that looks like what the product of this reaction is so we end up preserving this acyl group and the acyl group just refers to this kind of part of the molecule here we end up forming a bond with the co and a through the sulphur atom as such and the name of this molecule is acyl Co a and I remember when I was first learning about this I confused this often with acetyl co a which remember just has a methyl group instead of this long fatty acid chain so just keep those two things in mind making sure not to confuse this acyl Co a with acetyl co a now as a quick disclaimer I want apologize in advance for any geometry calculations that don't seem to be right perhaps like this oxygen atom here where is this oxygen atom going and that's that's due in part because I'm abbreviating some things in these molecules and so the oxygen atoms might be hidden somewhere but but mostly it's due to the fact that I'm not going over the entire mechanistic pathway by which this reaction occurs it ends up there there are a couple steps involved in this reaction and so I encourage you if there's something that doesn't make sense in terms of the stoichiometry to just do a quick google search and the entire mechanism will be illuminated but i'm just trying to give you the big picture and keep our diagram a little bit neater here by just kind of giving you a big picture here and so going back to this particular reaction it turns out that like any reaction where we have to activate something with a a higher energy functional group perhaps so to say we need some input of chemical energy and indeed this reaction is coupled to the hydrolysis of ATP and in fact ATP we normally think about it as ATP going to ADP and a free phosphate group but in this case we actually go all the way to a mono phosphate group and produce what we call a pyro phosphate group which is just two phosphate groups stuck together and what really makes this reaction thermodynamically favorable just kind of as a a fun factor is that it turns out that when this reacts with with the water it splits up into two individual phosphate groups and this hydrolysis reaction right here ends up being having a very negative Delta G value and so that's kind of what drives this overall reaction forward so just to summarize here we've successfully transformed our acid into an acyl Co a group which is what we refer to as an activated fatty acid for the transport process that we'll talk about next but before we do that as you want to mention at the enzyme that catalyzes the reaction is actually located on this outer mitochondrial membrane here and it's called acyl synthetase so I kind of just think that we're synthesizing essentially another acyl group an acyl Kawai acyl synthetase that's kind of how I try and remember it by all right so what happens to our acyl cooling molecule next well it turns out that there is another molecule inside of the cell by the name of carnitine and if this usually has a kind of bulky chemical structure to draw out so I won't you're at it completely but what I will drought is the fact that it has a hydroxyl group here and I'm drawing this hydroxyl group this oxygen bound to hydrogen because if I put on my organic chemistry hat for a moment I remind myself that this oxygen can serve as a nucleophile and form a bond with the carbon on this acyl Co a molecule like such and the coenzyme a group that we added can serve as a leaving group and the sulfur essentially can take back its electrons and so ultimately what we've done is we've to this carnitine molecule which I'll abbreviate here as just C now for simplicity we've attached via this oxygen right here our fatty acid and so we have a structure that looks something like this now just as a quick aside one way that I kind of remember the name of this molecule and its function in fatty acid oxidation is that I kind of think about it as being carnivorous carnitine being carnivorous for this fatty acids so it kind of essentially but takes a bite into this to see this acyl Co way through its oxygen group and and is able to attach it to itself like this so just a quick aside in case that helps you remember anything but going back to this particular reaction this ends this reaction also has an enzyme that carries this out and it's called carnitine acyl transferase and it's located on again the outer mitochondrial membrane and so I'll go ahead and write that out so carnitine acyl transferase so pretty a logical name right it's transferring the acyl group onto the carnitine molecule and it's actually denoted as carnitine acyl transferase one because you will meet another one of these enzymes on the inside of the mitochondrial matrix in just a bit so keep that at the back of your mind