Coenzyme A

Coenzyme A (CoA, CoASH, or HSCoA) 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 (or a thioester, such as acetyl-CoA) as a substrate.^[1] It is adapted from cysteamine, pantothenate, and adenosine triphosphate.

Biosynthesis

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

1. Pantothenate (vitamin B₅) is phosphorylated to 4'-phosphopantothenate by the enzyme pantothenate kinase (PanK; CoaA; CoaX)
2. A cysteine is added to 4'-phosphopantothenate by the enzyme phosphopantothenoylcysteine synthetase (PPC-DC; CoaB) to form 4'-phospho-N-pantothenoylcysteine (PPC)
3. PPC is decarboxylated to 4'-phosphopantetheine by phosphopantothenoylcysteine decarboxylase (CoaC)
4. 4'-phosphopantetheine is adenylylated to form dephospho-CoA by the enzyme phosphopantetheine adenylyl transferase (CoaD)
5. Finally, dephospho-CoA is phosphorylated using ATP to coenzyme A by the enzyme dephosphocoenzyme A kinase (CoaE).

Function

Since coenzyme A is, in chemical terms, 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'.

Coenzyme A is also the source of the phosphopantetheine group that is added as a prosthetic group to proteins such as acyl carrier protein and formyltetrahydrofolate dehydrogenase.^[2][3]

List of 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 (used in heme biosynthesis)
  • Hydroxymethylglutaryl-CoA (used in isoprenoid biosynthesis)
  • Pimelyl-CoA (used in biotin biosynthesis)
  • fatty acyl-CoA (activated form of all fatty acids; only the CoA esters are substrates for important reactions such as mono-, di-, and triacylglycerol synthesis, carnitine palmitoyl transferase, and cholesterol esterification)
• Benzoyl CoA
• Phenylacetyl CoA
• Butyryl CoA

Additional images

References

Bibliography

• Karl Miller (1998). Beta Oxidation of Fatty Acids. Retrieved May 18, 2005.
• Charles Ophard (2003). Acetyl-CoA Crossroads. Retrieved May 18, 2005.
• Lehninger: Principles of Biochemistry, 4th edition, David L. Nelson, Michael M. Cox
• http://www.elmhurst.edu/~chm/vchembook/621fattyacidrx.html