Beta-Oxidation

So, did you, like me, think that you could forget everything that you learned in first year about β-oxidation? Well, think again!

Describe the reactions in, regulation of and cellular location of beta-oxidation.

Beta-oxidation, which is the breakdown of fatty acids to form acetyl-CoA, has three stages. In the first stage, fatty acids in the cytosol are activated. In the next stage, they are transported into the mitochondria. In the third and final stage, the actual β-oxidation process occurs, breaking down the acyl chains to form acetyl CoA, which can then enter the citric acid cycle.

First stage: Activation

In the activation state, ATP reacts with the fatty acid to form an acyl-adenylate intermediate and pyrophosphate. The acyl-adenylate intermediate reacts with acetyl-CoA to form a tetrahedral intermediate which breaks apart into fatty acyl-CoA and AMP. This reaction is catalysed by acyl-CoA synthetase, and can be summarised as follows:

Fatty acid + CoASH (i.e. acetyl-CoA) + ATP -> Fatty acyl-CoA + AMP + pyrophosphate

Second stage: Transport

Since the mitochondrial membrane is tough to cross, there is a convoluted transport system in place that involves carnitine transporters (CPTI and CPTII). CPTI, located on the outer mitochondrial membrane, reacts fatty acyl-CoA in the cytosol with carnitine in the space between membranes to form CoASH in the cytosol and acylcarnitine between the membranes. Acylcarnitine then passes through a translocase to CPTII, which is located in the inner mitochondrial membrane. Here, it combines with CoASH from the mitochondrial matrix. This produces carnitine, which is released back into the space between membranes (essentially recycling it) and acyl-CoA in the mitochondrial matrix.

Third stage: β-oxidation

The process of β-oxidation itself consists of four steps, which can be summarised as oxidation, hydration, oxidation and cleavage:

1. Oxidation of the fatty acyl-CoA to trans-Δ²-enoyl-CoA via acyl-CoA dehydrogenase. Produces FADH₂.
2. Hydration of trans-Δ²-enoyl-CoA to 3-L-hydroxyacyl-CoA via enoyl-CoA hydratase.
3. Oxidation of 3-L-hydroxyacyl-CoA to β-ketoacyl-CoA via 3-L-hydroxylacyl-CoA dehydrogenase. Produces NADH.
4. Cleavage of β-ketoacyl-CoA to a shorter fatty acyl-CoA and acetyl-CoA via β-ketoacyl-CoA thiolase.

Each round of β-oxidation produces one acetyl-CoA molecule (which can go on to produce 12 ATP), 1 FADH₂ (= 2 x ATP) and 1 NADH (= 3 x ATP). (The exception is the last round, as that produces two acetyl-CoA molecules, along with 1 FADH₂ and 1 NADH.) As fatty acid chains are quite long, this cycle will proceed several times, producing quite a lot of ATP per fatty acid chain. Remember that you need to subtract 2 x ATP to get the overall yield, as 2 ATP molecules were consumed in the activation step.

β-oxidation is not the only process available for the breakdown of fatty acids. There are other minor pathways that can help with the breakdown of fatty acids with an odd number of carbon atoms, unsaturated fatty acids, fatty acids with methyl groups and so on.

Explain why ketone bodies are important and how they are formed.

Ketone bodies are a lipid-based yet water-soluble energy source. They are more commonly used by the skeletal and cardiac muscles, but can also be used by the CNS in cases of starvation. The main ketone bodies are β-hydroxybutyrate and acetoacetate. Most ketone bodies are formed in the liver.

There are several steps in the formation of ketone bodies:

1. Formation of acetoacetyl-CoA from acetyl-CoA via thiolase
2. Formation of HMG-CoA via HMG-CoA synthase (requires another acetyl-CoA as a cofactor)
3. Formation of acetoacetate via HMG-CoA lyase
4. Formation of β-hydroxybutyrate (requires NADH).

Ketone bodies can also be converted back into acetyl-CoA:

1. β-hydroxybutyrate converted into acetoacetate via β-hydroxybutyrate dehydrogenase (restores NADH)
2. Acetoacetate converted into acetoacetyl-CoA via β-ketoacyl-CoA transferase
3. Acetoacetyl-CoA converted into acetyl-CoA via thiolase

Have an understanding of the nomenclature of the common fatty acids.

See earlier post: Chemistry of Fatty Acids and Lipids

Appreciate the metabolic changes resulting from starvation.

See earlier post: Weight Loss and Exercise. Remember, glycogen is preferentially used first in a fasting stage. However, glycogen gets used up after around 24 hours, whereas fatty acids can be stored for weeks.