Oxidation of Unsaturated and Other Fatty Acids

00:01 Now the oxidation of unsaturated fatty acids posses a little bit of a problem for a cell; because, if you recall the unsaturated fatty acids have cis-double bonds whereas the beta oxidation, I have just described to you, creates intermediates that have trans double bonds.

00:16 So those have to be dealt with appropriately.

00:18 I show on the screen the way in which this is handled and it's handled interesting by only two enzymes that it requires to do it.

00:25 So I show the fatty acids that has 2 double bonds on the top and the oxidation of that fatty acid proceeds by normal beta oxidation until the oxidation process gets close to that cis-double bond.

00:38 When that happens we create an intermediate that looks here like this one that has a double at positions 3 and 4.

00:45 Now you remember in the beta oxidation that the trans double bond was at positions 2 and 3 whereas we have a cis at positions 3 and 4 in the structure. Now the cis 3,4 double bond in this is handled very easily by an enzyme called Enoyl-CoA isomerase.

01:02 The cis 3,4 double bond is converted to a trans 2,3 double bond.

01:05 Now that trans 2,3 double bond is an intermediate normally in beta oxidation, we can see that happening right here.

01:14 That bond is then metabolized and broken down and two carbons are broken off just like it has happened before in beta oxidation.

01:22 After that two carbon pieces being released we start the process again of oxidizing what remains and we get to an intermediate that has a trans 2,3 bond next to a cis 4,5. Now this is getting a little tacky but the important thing is that the cell can't handle those.

01:42 So rather than deal, we tried to deal with those two use another enzyme that comes into play that's start to simplify that situation.

01:48 And that enzyme is shown here, it's 2,4 Dienoyl-CoA reductase.

01:54 And it uses electrons, because, it has to do a reduction at this point in order to convert two double bonds into one double bond and that's what this enzyme is doing.

02:04 So it's converting the two double bonds that you see here into one double bond and the one double bond is between carbons 3 and 4.

02:14 Well, the one double bond between carbons 3 and 4 is converted into a double bond between carbons 2 and 3 by the same enzyme we saw before, Enoyl-CoA isomerase.

02:24 At this point we have an intermediate that has a trans bond in positions 2 and 3 and beta oxidation can continue as normal.

02:32 Now long chain fatty acids are metabolized slightly differently than shorter chain fatty acids and by long chain, I typically mean fatty acids that are 20-22 carbons or greater.

02:45 The oxidations of these starts not in the mitochondrion but in another organelle called the peroxisome.

02:52 And fatty acids with odd numbers of carbons which the cell occasionally encounters are oxidized in a little bit different process; because, most fatty acids have even numbers of carbons and even numbers of carbons work really well.

03:05 Because if you are chopping off two carbon pieces every time when you get to the last 4 carbon pieces you split it in half and you have two units of 2 carbons each acetyl-CoA.

03:16 Now if you have an odd number of carbons in your fatty acid chain then that means when you get down to the end, and instead of having a 4 carbon piece you have a 3 carbon piece and that 3 carbon piece is called propionyl-CoA.

03:28 Propionyl-CoA has to be altered in some way for the cell to be able to handle it and it's handled in this way by a certain unusual scheme.

03:38 First there is an enzyme called propionyl-CoA carboxylase which combines with bicarbonate to add a carboxyl group to the propionyl-CoA creating methylmalonyl-CoA that you can see right here.

03:52 Now you will think move forward from there but the cell doesn't move from there. It actually converts the forms and it converts the forms from the D to the L configuration before going forwards. As it goes forwards then it takes that carboxyl group that it added to the end and rearranges it so that it creates a new molecule called auccinyl-CoA.

04:15 That enzyme that catalyzes this reaction is called methylmalonyl-CoA mutase, mouth full of a name.

04:22 But this enzyme requires cobalt and this enzyme requires vitamin B12.

04:28 So, it's one of the reasons that we have to have B12 in our diet; because, this reaction very much depends on it.

04:34 Succinyl-CoA is an intermediate in the citric acid cycle and can be metabolized there.

04:42 Now each round of fatty acid oxidation creates one molecule of FADH2 and one NADH, as we have seen.

04:47 Along with one acetyl-CoA and a fatty acid is been shorten by two carbons.

04:52 Each acetyl-CoA that's released in the matrix of the mitochondrion and this is important; because, this is where the citric acid cycle actually uses those. So it doesn't have to go anywhere else.