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Fatty Acid Metabolism
Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is fatty acid metablism?

Fatty Acid Metabolism is basically how you get energy from eating fatty stuff! Put in more technical terms, it is the process by which fatty acids are metabolised to obtain energy. This is done by breaking them down into smaller pieces, and putting each piece through something called the TCA Cycle. A series of reactions take place, outlined in the following:

The fat, found in the diet generally as something called triacylglycerol, is broken down into glycerol and 3 fatty acids by adipose tissue lipase, an enzyme. It's the fatty acids we want.

Using energy from ATP, coenzyme A is added to a fatty acid, giving it a handle. Because of the energy required to complete this step, it is considered an irreversible reaction - going back the other way simply wouldn't be possible! It is also important that we've used a molecule of ATP - it means that when we total up how many are produced at the end, it's 1 less.

The fatty acyl CoA then swaps its Coenzyme A for something called carnitine so that it can enter the mitochondrion. Mitochondria are parts of a cell in which some reactions in metabolism take place, and fatty acids have to get into one mitochondrion in order to be broken down. It can't get through when it's just bound to CoA, so it binds to carnitine. Then, when inside the mitochondrion, it gets rid of the carnitine and binds to CoA again. Ultimately we're left with the same molecule but in a different place, and everything is now ready for the molecule to enter the Beta Oxidation Cycle.

Inside the mitochondrion, the coenzyme FAD comes along and grabs a couple of the hydrogen atoms. This causes the fatty acyl CoA to form a double bond, and the product is called Δ2-trans-enoyl-CoA.

This time it is the turn of NAD to come along and steal a couple of hydrogens. This leaves the beta carbon with just the oxygen (no longer any hydrogens) attached to it, and is considered to be further oxidation - hence more reason to be calling this the beta oxidation cycle!!

This is the most important part, and perhaps the bit that's most difficult to get your head round! The last enzyme (thiolase) breaks off the end of the beta ketoacyl CoA, producing something called Acetyl CoA. Another Coenzyme A molecule is added to the end of the reminaing atoms, producing another fatty acyl CoA molecule, only this one will obviously be two carbon atoms shorter.

The clever thing here is that the beta oxidation cycle part of the above reactions can occur over and over again - each time it starts with pretty much the same molecule: Fatty Acyl CoA. Each time the cycle runs through, the carbon chain is shortened by 2 carbons, and eventually the end will be reached and it will be over. However, something with, say, 18 carbon atoms in it can go through this cycle 9 times, producing 9 molecules of Acetyl CoA which then go into the TCA Cycle. Since the beta oxidation cycle produces FADH2 and NADH each time it goes round, this cycle itself produces energy - in something called oxidative phosphorylation. However, because it also produces acetyl CoA to go into the TCA cycle, it also produces energy that way. This means that fatty acids produce a heck of a lot of energy - which is part of the reason why we don't need much in our diet!!!


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