Lipid Metabolism/ β- Oxidation of Fatty Acids

Lipid Metabolism/ β- Oxidation of Fatty Acids

Fatty acids are the monocarboxylic acids with hydrocarbon chain ranging from 4-36 in number. Fatty acids are considered the major energy reservoir of the living system. After the consumption of the fatty meal, they need to get digested in the intestine with the help of different digestive enzymes (bile for an example).Then after the digested fats undergo series of the reactions to yield energy. The energy is released by the oxidation of fatty acids. Fatty acids are oxidized by different ways (α, β, ω- oxidation). Among these ways of the fatty acids oxidation, β- oxidation is the major way of fatty acid metabolism.

β- Oxidation can be defined as the oxidation of fatty acids on the β- Carbon atom with the sequential release of two carbon fragments in the form of acetyl-CoA. It occurs in the mitochondrial matrix of the eukaryotic cells and in the cytosol of prokaryotic cells. The β- oxidation of fatty acids involve three stages:

1) Activation of fatty acids occurring in the cytosol.

2) Transport of fatty acids into the mitochondria (in eukaryotes)

3) β- oxidation proper in mitochondrial matrix/ β- oxidation cycle.

1) Activation of Fatty Acids Occurring in Cytosol:

The fatty acids to get oxidized, need to get activated before they are brought about the oxidation. This process is carried out in the cytosol (outside the mitochondrial matrix). The required enzymes that catalyze the oxidation of fatty acids are found in the matrix of the mitochondria. The fatty acids without their activation cannot be brought inside the mitochondrial matrix. Once the fatty acids are activated outside, they are degraded by the process of β- oxidation.

For the activation of a fatty acid molecule, there is the formation of fatty-acyl-CoA molecule. This reaction is catalyzed by the enzyme, Thiokinase or Acyl-CoA synthase. This enzyme is a combined group of three isoenzymes that act against the fatty acids with their specific chain length. Hence there are three types of Thiokinase each of which is specific to short, medium and long chain of fatty acids. In the course of this reaction, there is the formation of the thioester bond between a molecule of coenzyme A and Fatty acid each. This is an endergonic reaction which requires energy in the form of ATP. The ATP molecule is cleaved into AMP and pyrophosphate. The bond between the two terminal phosphate groups from ATP gets cleaved one by one and AMP remains uncleaved. This investment of energy is equivalent to the investment of two ATPs, as there is the cleavage of bonds of phosphate groups of ATP two times (hydrolysis of two phospho- anhydride bonds).

The reaction of activation of Fatty acids occur in two steps:

1) The reaction of fatty acids and ATP to form Acyl AMP and pyrophosphate.

2) The reaction of an acyl-AMP molecule and a coenzyme A molecule to form a fatty acyl-CoA molecule and free a molecule of AMP.

The reaction seems to be as given in above figure.

Thus formed fatty-acyl-CoA molecule is said to be an activated fatty acid molecule, which now is ready to get shuttled inside the mitochondrial matrix.

2) Transport of Fatty Acids into Mitochondria:

This step is regarded as the rate-limiting step and is observed only in eukaryotic cells. The inner mitochondrial membrane is highly non-permeable to fatty acids. So, fatty acyl-CoA thus formed cannot directly enter the matrix of mitochondria. So, this step of reaction takes help from a specific carrier enzyme named carnitine (β- hydroxy-trimethyl aminobutyrate). Fatty acids get attached to the hydroxyl group of carnitine outside the mitochondrial matrix. But it is not clear yet whether this attachment of fatty acyl-CoA and carnitine is carried out in the cytosol or the inter-mitochondrial membrane space. Now via the process of facilitated diffusion, thus formed fatty acyl carnitine is transported across the mitochondrial membrane to the mitochondrial matrix.

This process is carried out in four steps:

1) At first, the fatty acyl group is transferred to carnitine producing fatty acyl carnitine. This reaction is catalyzed by the enzyme, carnitine acyltransferase I.

2) The fatty acyl carnitine is transferred into the mitochondrial matrix by a carrier protein that is found in the inter-mitochondrial membrane by the facilitated diffusion process.

3) An enzyme named carnitine acyl transferase II (CAT II), which is found in the inter-mitochondrial membrane catalyzes the acyl group from fatty acyl carnitine to CoA and free carnitine.

(Note: It should be noted that the Coenzyme A that participates in the formation of fatty-acyl-CoA in cytosol region is not same to the Coenzyme A inside the mitochondrial matrix to form fatty-acyl-CoA. Remember, there are separate cytosolic and mitochondrial pools of CoA.)

4) The free carnitine is regenerated for another transfer of fatty acids.

3) β- oxidation proper in mitochondrial matrix/ β- oxidation cycle:

β- Oxidation can be defined as the oxidation of fatty acids on the β- Carbon atom with the sequential release of two carbon fragments in the form of acetyl-CoA. The actual oxidation of fatty acid is held in three stages:

i) Oxidation of fatty-acyl-CoA and removal of 2 carbon fragments in the form of acetyl-CoA:

Fatty acids get oxidized by the removal of two carbon fragments in the form of acetyl-CoA. The process continues until all the carbon chain of the provided fatty acid is totally converted to acetyl CoA. This process may slightly be differed depending upon the nature of fatty acid chain i.e. the process of oxidation of saturated fatty may be slightly different to that of unsaturated fatty acids and the process of oxidation of even-chained fatty acids may be slightly different to that of odd-chained fatty acids. The detail of β- oxidation of fatty acids with even numbered chain and the saturated fatty acids been discussed in next topic.

ii) Oxidation of acetyl-CoA to CO₂ in TCA cycle:

Acetyl-CoA, the product of β- oxidation of fatty acids are then brought to TCA cycle for actual energy yielding process where they are oxidized to CO₂. The detail reaction of TCA cycle has already been discussed in the topic "Reactions of TCA cycle". By the oxidation of each molecule of acetyl CoA, there is the yield of 1 GTP, 3 NADH and 1 FADH₂ (ultimately 12 ATPs altogether from one complete TCA cycle)

iii) Oxidation of the NADH and FADH₂ generated during the oxidative removal of acetyl-CoA and from TCA cycle:

NADH, FADH₂, and nucleosides are produced as the energy-rich side products during the oxidative removal of acetyl-CoA and from TCA cycle. These molecules after taking part in electron transport chain/ Oxidative phosphorylation yield ATPs.

1 molecule of NADH is equivalent to 3 ATPs.

1 molecule of FADh₂ is equivalent to 2 ATPs.

1 molecule of GTP is equivalent to 1 ATP.

Calculation of total numbers of ATP generated has been discussed in next topic.