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coenzyme A

Wednesday 30 July 2008

Functions

Coenzyme A may act as an acyl group carrier to form acetyl-CoA and other related compounds; this is a way to transport carbon atoms within the cell.

The transfer of carbon atoms by coenzyme A is important in cellular respiration, as well as the biosynthesis of many important compounds such as fatty acids, cholesterol, and acetylcholine.

Biosynthesis

Coenzyme A is synthesized in a five-step process from pantothenate:

- 1. Pantothenate is phosphorylated to 4’-phosphopantothenate by the enzyme pantothenate kinase
- 2. A cysteine is added to 4’-phosphopantothenate by the enzyme phosphopantothenoylcysteine synthetase to form 4’-phospho-N-pantothenoylcysteine (PPC)
- 3. PPC is decarboxylated to 4’-phosphopantetheine by phosphopantothenoylcysteine decarboxylase
- 4. 4’-phosphopantetheine is adenylylated to form dephospho-CoA by the enzyme phosphopantetheine adenylyl transerase
- 5. Finally, dephospho-CoA is phosphorylated using ATP to coenzyme A by the enzyme dephosphocoenzyme A kinase.

Acyl group carrier

Since coenzyme A is chemically a thiol, it can react with carboxylic acids to form thioesters, thus functioning as an acyl group carrier.

It assists in transferring fatty acids from the cytoplasm to mitochondria. A molecule of coenzyme A carrying an acetyl group is also referred to as acetyl-CoA. When it is not attached to an acyl group it is usually referred to as ’CoASH’ or ’HSCoA’.

- citric acid cycle

Acetyl-CoA is used in the condensation of oxaloacetate to citrate at the initiation of the citric acid cycle (TCA cycle or Krebs cycle).

- fatty acid biosynthesis

From the citric acid cycle, acetyl-CoA can also initiate the fatty acid synthesis pathway.

Coenzyme A activated acyl groups

- Acetyl-CoA
- Propionyl-CoA
- Acetoacetyl-CoA
- Coumaroyl-CoA (used in flavonoid and stilbenoid biosynthesis)

- Acyl derived from dicarboxylic acids

  • Malonyl-CoA
  • Succinyl-CoA
  • Hydroxymethylglutaryl-CoA (used in isoprenoid biosynthesis)
  • Pimelyl-CoA (used in biotin biosynthesis)

- Butyryl CoA

Pathology

- pantothenic acid deficiency (vitamin B5)

  • Pantothenic acid is used in the synthesis of coenzyme A (CoA). Since pantothenic acid participates in a wide array of key biological roles, it is considered essential to all forms of life. As such, deficiencies in pantothenic acid may have numerous wide-ranging effects.

See also

- citric acid cycle
- fatty acid beta oxidation
- cholesterol biosynthesis

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