Digesting Fatty Acids (continued)
The process by which we digest fatty acids is called β-oxidation
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- This is a cycle of four reactions.
- At the end of each complete cycle the fatty acid is two carbons shorter.
- The cycles continue until the fatty acid is reduced to 2 or 3 carbons.
- The fatty acid remains bound to Coenzyme A throughout the process.
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For palmitic acid, for example, the full process can be summarized as follows:
Palmitic acid yields 7 molecules of FADH2, 7 of NADH, and 8 acetyl CoA's. The electrons from FADH2 and NADH are sent into the electron transport pathway of the mitochondria where they yield energy and the acetyl groups are transferred from CoA into the TCA cycle where they too yield energy.
Some fatty acids have an odd number of carbons. Here the last cycle of β -oxidation will give one molecule of acetyl-CoA and one of propionyl-CoA. A pathway requiring Vitamin B12 converts propionyl-CoA into succinyl-CoA, which enters the TCA cycle.
Some fatty acids are unsaturated. Additional enzymes are required to re-arrange the double bonds so that the fatty acid can be digested by β-oxidation. The bottom line is that for every double bond in a fatty acid, one of the cycles will lack the acyl-CoA dehydrogenase step. Hence, one less FADH2 and thus less energy is produced. Another way of putting this is that one gram of butter is more fattening than one gram of olive oil.
Very Long-Chain Fatty Acids are largely oxidized by peroxisomes. These organelles have enzymes analogous to those in mitochondria, except that the acyl dehydrogenase reaction is very different and leads to production of H2O2.
