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Fatty Acid Oxidation
FATTY ACID OXIDATION
Not all cellular energy comes from glucose. Fats are important in providing necessary energy for the cell. Triglycerides first get broken down into fatty acids plus glycerol. Fatty acids ultimately get broken down into two carbon units in the making of what’s called acyl-CoA. In order to be oxidized, the fatty acids must be attached to coenzyme A and moved into the mitochondria. The coenzyme A must detach at the mitochondrial membrane so the fatty acid can attach to carnitine. This carnitine complex gets transported across the inner mitochondrial membrane, where it can then be reattached to coenzyme A inside the matrix of the mitochondrion. Carnitine just keeps getting recycled.
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So, how does fatty acid oxidation work? The fatty acid is attached to coenzyme A. It looks like a long chain attached to this coenzyme A molecule. There is dehydrogenation between carbons 2 and 3, with number one being attached to coenzyme A. There will be a double bond between the two carbon atoms. Then, a hydroxyl group gets added to the third carbon atom and there is oxidation to make a ketone. There is a cleavage at this point to release an acetyl CoA molecule plus the rest of the fatty acid. The fatty acid left over can get oxidized again.
These reactions that cause beta oxidation are very similar to the last half of the Krebs cycle. One molecule of FADH2 is made as is one molecule of NADH. The process goes on in a cycle with two carbon atoms knocked off at a time until only two carbon atoms are left. These last two carbon atoms make acetyl CoA and the process ends.
As you can imagine, this process works well for fatty acids that have an even number of carbon atoms but is not as helpful for those few fatty acids that have an odd number of carbon atoms. In these cases, the end result is a three-carbon molecule called propionyl-CoA. It can’t be oxidized any further. Instead, it gets turned into methyl malonyl-CoA and gets rearranged to make succinyl-CoA. This can go into the citric acid cycle.
Alpha oxidation may also need to happen if there are branches in the fatty acid chain. This is a minor metabolic pathway that only becomes significant if it does not work. In
