What is Beta Oxidation of Fatty Acids

The beta oxidation

The attempts of Knoop

In 1904, the German chemist F. Knoop made an interesting experiment which produced very decisive insights into the breakdown of fatty acids in the cells of mammals. A number of dogs were given fat to eat - not ordinary fat, but fat in which the fatty acids at the omega end were specially marked. Radioactive markings or markings with fluorescent dyes were not invented at the time, but chemical markings could already be used. Knoop used fatty acids, at the omega end of which an H atom was replaced by a benzene ring.

The experiments of F. Knoop

Part of the fatty acids consisted of one straight Number of carbon atoms (top row), another part of the fatty acids had one odd Number of carbon atoms.

Knoop was then able to detect the breakdown products of the fatty acids in the urine of the dogs. In the example above in Figure 1, the number of fatty acid C atoms has been reduced from 6 to 2, in the example below from 5 to 1. That means that it has always been accurate four C atoms degraded. In the case of similarly prepared fatty acids with 7 and 8 carbon atoms, exactly six carbon atoms were always broken down, and so on.

Knoop's findings from these experiments can be formulated in one sentence: When fatty acids are broken down, the carbon atoms are always removed in groups of two.

In the picture you can see that the COOH group is retained at the alpha end of the fatty acid. That's not exactly correct. The COOH group of the fatty acid is removed together with the second carbon atom. The third carbon atom, the so-called beta carbon atom, is then oxidized again, so that a new COOH group is formed.

The beta-oxidation process[1][4]

The beta-oxidation "lightening" the fatty acid by two carbon atoms and thereby gaining a molecule of acetic acid, so to speak. As is so often the case in biochemistry, this process also takes place in several steps.

Oxidation by FAD

Oxidation of a fatty acid by FAD

Briefly about the terminology of the carbon atoms: If you start with the carbon atom of the former COOH group, the directly adjacent second carbon atom is called the alpha carbon atom. The third carbon atom is then the beta carbon atom.

In the first step of beta oxidation, the alpha and beta carbon atoms are oxidized by removing hydrogen (dehydrogenation). The coenzyme FAD acts as a hydrogen acceptor, which becomes FADH2 is reduced. This oxidation creates a trans- double bond.

For chemistry beginners:
In the case of C = C double bonds, one can choose between trans- and cis-Configuration differ. In the transConfiguration, there is one alkyl group on one side of the double bond and the other alkyl group on the opposite side. In the cisConfiguration, both alkyl groups are on the same side. If you would like to know details about this, go to the lexicon page cis-trans-Isomerism.

This step is necessary because the enzyme that is responsible for the next step only contains fatty acids trans-Configuration as substrate recognizes, which is of course due to the structure of the active center of this enzyme (lock and key principle).

Hydration

Addition of water to the new double bond

In the second step, a water molecule is added to the newly created double bond. The beta-carbon atom thus receives an OH group. Anyone who has a little knowledge of biochemistry already knows what comes next: The OH group is oxidized, so that a new carbonyl group is created.

Oxidation by NAD+

Oxidation of the OH group

The reaction product now has two C = O groups: one in the former COOH group and one on the beta-carbon atom. If you look closely now, you can see an acetic acid molecule in the reaction product. For the people who don't see that yet, I've changed the above picture a bit:

Oxidation of the OH group

The acetyl group is basically an acetic acid residue. However, one should not confuse this residue with the negatively charged acetate ion: acetyl residue = CH3-C = O-; Acetate ion = CH3-COO-.

Thiolysis

The fatty acid that is now twice oxidized (once by FAD, once by NAD+) now separates the acetyl group with the attached coenzyme A molecule:

Cleavage of acetyl-CoA

Another CoA molecule is added and attaches to the remaining fatty acid, which is now shortened by two carbon atoms.

Overall view

The beta oxidation, summary

Here we see all four steps of beta-oxidation again in one summary. In the case of a fatty acid such as stearic acid with 18 carbon atoms, this beta-oxidation has to take place nine times before all of the fatty acid is broken down.

Problems

In the case of odd-numbered saturated fatty acids, acetyl-CoA is also formed from two carbon atoms. There is only a problem when the last three carbon atoms have to be broken down. This problem is solved by the fact that in the end it is not acetyl-CoA that is produced, but propionyl-CoA, i.e. a compound with one more carbon atom.

There are also problems with unsaturated fatty acids because the beta-oxidation enzymes cannot cope with the cis double bonds. These cis double bonds must first be converted into trans double bonds by further steps and further enzymes so that hydration can take place.

Energy yield

As can easily be seen in Figure 7, one FADH is produced per "round" of beta oxidation2- and a NADH / H+-Unit. So these are two reduction equivalents that can be fed directly into the respiratory chain of the mitochondria.

Not to be forgotten is acetyl-CoA, which is actually the main product of beta-oxidation. The acetyl-CoA can be fed directly into the citric acid cycle, where it is broken down into reduction equivalents, which then flow back into the respiratory chain.

Example palmitic acid[1]

In his book, Löffler set up the energy balance for the breakdown of palmitic acid and comes to the following result:

Complete oxidation of the palmitic acid (16 carbon atoms) produces 8 acetyl-CoA molecules. To do this, the beta oxidation has to be run through 7 times, with 7 FADH2 and 7 NADP / H+ arise.

When FADH in the respiratory chain2 is oxidized, 2 molecules of ATP are created. In the oxidation of NADP / H+ there are even 3 ATP molecules. That means 5 molecules of ATP per run of the beta-oxidation, i.e. 35 molecules of ATP for a seven-time run.

If acetyl-CoA is fed into the citric acid cycle, reduction equivalents are also produced. For each acetyl-CoA, 12 ATP are then produced in the respiratory chain. With 8 molecules of acetyl-CoA, that's 96 ATP molecules.

Together, this results in the impressive number of 131 ATP molecules that can be obtained from one palmitic acid molecule. However, you have to subtract 2 ATP molecules that were required for the formation of the activated fatty acid. In the overall balance, one palmitic acid molecule can deliver 129 ATP molecules.

Comparison of long-chain and short-chain fatty acids

It becomes interesting when you compare the energy yield of long-chain and short-chain fatty acids. A short-chain fatty acid with - let's say - 6 carbon atoms undergoes beta oxidation twice. This creates 3 acetyl-CoA molecules and 2 FADH2 and 2 NADP / H+, which are used in the respiratory chain to 10 ATP. The 3 acetyl-CoA molecules produce 36 ATP, together that's 46 ATP. From these 46 ATP, however, 2 ATP must be subtracted for the activation of the fatty acid. Of the 46 ATP gained, only 44 remain, that's 95.7%. On the other hand, when palmitic acid is broken down, 129 of the 131 ATP obtained remain, that is 98.5%.

The longer a fatty acid is, the cheaper it is for generating energy. This is because a fatty acid only needs to be activated once, regardless of how many carbon atoms it has[5].

Swell:

  1. Löffler, Functional Biochemistry, Berlin 1994
  2. State education server Baden-Württemberg, "Breakdown of glycerine"
  3. Römpp Chemie-Lexikon, 9th edition 1992
  4. Wikipedia, article "beta oxidation"
  5. Schlieper, Basic Questions in Nutrition, Hamburg 2017