Acyl-CoA is a group of coenzymes involved in the metabolism of fatty acid

Fatty acid

In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s. It is a temporary compound formed when coenzyme A

Coenzyme A

Coenzyme A is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All sequenced genomes encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it as a substrate...

 (CoA) attaches to the end of a long-chain fatty acid inside living cells. The compound undergoes beta oxidation

Beta oxidation

Beta oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle....

, forming one or more molecules of acetyl-CoA

Acetyl-CoA

Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...

. This, in turn, enters the citric acid cycle

Citric acid cycle

The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

, eventually forming several molecules of ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

.

Fatty acid activation

To be oxidatively degraded, a fatty acid must first be activated in a two-step reaction catalyzed by acyl-CoA synthetase. First, the fatty acid displaces the diphosphate group of ATP, then coenzyme A (HSCoA) displaces the AMP

Adenosine monophosphate

Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...

 group to form an acyl-CoA. The acyladenylate product of the first step has a large free energy of hydrolysis and conserves the free energy of the cleaved phosphoanhydride bond in ATP. The second step, transfer of the acyl group to CoA (the same molecule that carries acetyl groups as acetyl-CoA), conserves free energy in the formation of a thioester bond. Consequently, the overall reaction has a free energy change near zero:

Fatty acid + CoA + ATP <=> Acyl-CoA + AMP + PP_i

Subsequent hydrolysis of the product PP_i (by the enzyme inorganic pyrophosphatase

Inorganic pyrophosphatase

Pyrophosphatase is an enzyme that catalyzes the conversion of one molecule of pyrophosphate to two phosphate ions. This is a highly exergonic reaction, and therefore can be coupled to unfavorable biochemical transformations in order to drive these transformations to completion...

) is highly exergonic, and this reaction makes the formation of acyl-CoA spontaneous and irreversible.

Fatty acids are activated in the cytosol, but oxidation occurs in the mitochondria. Because there is no transport protein for CoA adducts, acyl groups must enter the mitochondria via a shuttle system involving the small molecule carnitine

Carnitine

Carnitine is a quaternary ammonium compound biosynthesized from the amino acids lysine and methionine. In living cells, it is required for the transport of fatty acids from the cytosol into the mitochondria during the breakdown of lipids for the generation of metabolic energy. It is widely...

.

See also

• Acetyl-CoA
  Acetyl-CoA
  Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
  
  
• Beta oxidation
  Beta oxidation
  Beta oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle....
  
  
• Coenzyme A
  Coenzyme A
  Coenzyme A is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All sequenced genomes encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it as a substrate...
  
  
• Acyl CoA dehydrogenase
  Acyl CoA dehydrogenase
  Acyl-CoA dehydrogenases are a class of enzymes that function to catalyze the initial step in each cycle of fatty acid β -oxidation in the mitochondria of cells. Their action results in the introduction of a trans double-bond between C2 and C3 of the acyl-CoA thioester substrate...
  
  
• Fatty acid metabolism
  Fatty acid metabolism
  Fatty acids are an important source of energy and adenosine triphosphate for many cellular organisms. Excess fatty acids, glucose, and other nutrients can be stored efficiently as fat. Triglycerides yield more than twice as much energy for the same mass as do carbohydrates or proteins. All cell...