Fatty Acid Activation & Transport

Fatty Acid Activation

Fatty acid activation occurs in the cytosol, but fatty acids are oxidized in the mitochondria.

Enzymes of Fatty acid activation

1. Acyl CoA ligases (thiokinases).
2. Inorganic pyrophosphatase.

Steps involved in fatty acid activation

1. Formation of Acyl-CoA
2. Exergonic hydrolysis of PP_i

Formation of Acyl-CoA

1. There are different Acyl-CoA Synthases for fatty acids of different chain lengths.
2. It is associated with endoplasmic reticulum membranes and the outer mitochondrial membrane and catalyzes the activation of long-chain fatty acids, esterifying them to a coenzyme.
3. Adenosine triphosphate (ATP) drives the formation of a thioester linkage between the carboxyl group of a fatty acid and the sulfhydryl group of CoA.

Exergonic hydrolysis of PPi

1. Paul Berg showed that the activation of a fatty acid is accomplished in two steps.
2. First, the fatty acid reacts with ATP to form an acyladenylate.
3. The carboxyl group of a fatty acid is bonded to the phosphoryl group of AMP. The other two phosphoryl groups of the ATP substrate are released as pyrophosphate.
4. The sulfhydryl group of CoA then attacks the acyl adenylate, which is tightly bound to the enzyme, to form acyl-CoA and AMP.
5. The acyl-coenzyme A product includes one “high-energy” thioester linkage

Fatty Acid Transport Into Mitochondria

Carnitine takes long-chain fatty acids into mitochondria through a special transport system as a carnitine shuttle.

Carnitine

1. Carnitine, derived from an amino acid, is found in nearly all cells of the body. Its name is derived from the Latin word carnus, meaning “flesh”.
2. Carnitine plays a critical role in energy production
3. It transports long-chain fatty acids into the mitochondria so they can be oxidized (“burned”) to produce energy.
4. Carnitine is concentrated in tissues that utilize fatty acids as a dietary fuel.

1. The carnitine shuttle transports long-chain fatty acids across the barrier of the inner mitochondrial membrane to access the enzymes of beta-oxidation.
2. The carnitine shuttle consists of three enzymes:
   • carnitine palmitoyltransferase 1 (CPT1A & CPT1B)
   • Carnitineacylcarnitinetranslocase (SLC25A20)  
   • Carnitine palmitoyltransferase 2 (CPT2)
   • Carnitine

Enzymes Involved in Transport of Fatty acids

• Enzymes specific for very long-chain fatty acids are associated with the inner mitochondrial membrane, facing the matrix
  • Carnitine Palmitoyl Transferase I: Catalyzes the transfer of a fatty acid from ester linkage with the thiol of coenzyme A to the hydroxyl on carnitine.
  • Carnitine Acyltransferase: Mediates trans-membrane exchange of fatty acyl carnitine for carnitine.
  • Carnitine Palmitoyl Transferase II: Catalyzes the transfer of the fatty acid from carnitine to coenzyme A

Steps of Fatty Acid Transport

A specialized transport mechanism is required to carry long-chain acyl-CoA molecules across the inner mitochondrial membrane.

Formation of Acyl Carnitine

1. Activated long-chain fatty acids are transported across the membrane by conjugating them to carnitine, a zwitterionic alcohol.
2. The acyl group is transferred from the sulfur atom of CoA to the hydroxyl group of carnitine, forming acylcarnitine.
3. This reaction is catalyzed by carnitine acyltransferase I.
4. Fatty acyl-carnitine is carried across the inner mitochondrial membrane by a specific transporter.

Formation of fatty acyl-CoA with free carnitine

1. The fatty acyl group is enzymatically transferred from carnitine to intramitochondrial coenzyme A by carnitine acyltransferase II.
2. Regenerates fatty acyl-CoA and releases it, along with free carnitine, into the matrix.
3. Carnitine re-enters the space between the inner and outer mitochondrial membranes via a transporter.

Cytosolic and mitochondrial pools of coenzyme A

1. Cytosolic and mitochondrial pools of coenzyme A, which have different functions.
2. The mitochondrial pool of coenzyme A is largely used in the oxidative degradation of pyruvate, fatty acids, and some amino acids.
3. The cytosolic pool of coenzyme A is used in the biosynthesis of fatty acids.

Carnitine Regulation

1. This carnitine shuttle is a rate-limiting step in the oxidation of fatty acids in the mitochondria, and thus fatty acid oxidation can be regulated at this step.
2. Malonyl CoA, an intermediate of fatty acid synthesis present in the cytosol, is an inhibitor of carnitine acyltransferase I.

Importance

1. Carnitine plays a crucial role in the transport of fatty acids across the mitochondrial membrane and thus gains access to the enzymes for the breakdown of long-chain fatty acids.
2. Thus, deficiency of carnitine can lead to decreased ability of the tissue to utilize long-chain fatty acids as energy. 

Carnitine Transporter Deficiency

1. It is one type of fatty acid oxidation disorder.
2. People with CTD have problems using fat as a source of energy for the body. 
3. CTD occurs when an enzyme, called the “carnitine transporter” (CT), is either missing or not working properly.
4. This enzyme’s job is to carry a substance called carnitine into our cells.
5. Carnitine helps the body make energy from the fat in food.
6. Characterized by progressive muscular weakness and aching muscle cramps.
7. The symptoms were aggravated by fasting, exercise, and a high-fat diet.

Conclusion

The inner mitochondrial membrane is impermeable to fatty acids, and a specialized transport system operates to activate fatty acids from cytosol to mitochondria & peroxisomes.