Shikimic acid

Biosynthesis

Phosphoenolpyruvate and erythrose-4-phosphate condense to form 3-deoxy-D-arabinoheptulosonate-7-phosphate (DAHP), in a reaction catalyzed by the enzyme DAHP synthase. DAHP is then transformed to 3-dehydroquinate (DHQ), in a reaction catalyzed by DHQ synthase. Although this reaction requires nicotinamide adenine dinucleotide (NAD) as a cofactor, the enzymic mechanism regenerates it, resulting in the net use of no NAD.

:[[Image:Shikimate pathway 1.svg|left|676px|thumb|Biosynthesis of 3-dehydroquinate from phosphoenolpyruvate and erythrose-4-phosphate]]

DHQ is dehydrated to 3-dehydroshikimic acid by the enzyme 3-dehydroquinate dehydratase, which is reduced to shikimic acid by the enzyme shikimate dehydrogenase, which uses nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor.

:[[Image:Shikimate pathway 2.svg|left|462px|thumb|Biosynthesis of shikimic acid from 3-dehydroquinate]]

Shikimate pathway

Main article: Shikimate pathway

Biosynthesis of the aromatic amino acids

The shikimate pathway, named after shikimic acid as important intermediate, is a seven-step metabolic route used by bacteria, fungi, algae, parasites, and plants for the biosynthesis of aromatic amino acids (phenylalanine, tyrosine, and tryptophan). This pathway is not found in animals; therefore, phenylalanine and tryptophan are essential nutrients and must be obtained from the animal's diet. Tyrosine is not essential, as it can be synthesized from phenylalanine, except for individuals unable to hydroxylate phenylalanine to tyrosine.

Use in biosynthesis

Phenylalanine and tyrosine are the precursors used in the phenylpropanoids biosynthesis. The phenylpropanoids are then used to produce the flavonoids, coumarins, tannins and lignin. The first enzyme involved is phenylalanine ammonia-lyase (PAL) that converts L-phenylalanine to trans-cinnamic acid and ammonia.

Gallic acid biosynthesis

Gallic acid is formed from 3-dehydroshikimate by the action of the enzyme shikimate dehydrogenase to produce 3,5-didehydroshikimate. This latter compound spontaneously rearranges to gallic acid.

Other compounds

Shikimic acid is a precursor for:

• indole, indole derivatives and aromatic amino acid tryptophan and tryptophan derivatives such as the psychedelic compound dimethyltryptamine
• many alkaloids and other aromatic metabolites

Mycosporine-like amino acids

Mycosporine-like amino acids are small secondary metabolites produced by organisms that live in environments with high volumes of sunlight, usually marine environments.

Uses

In the pharmaceutical industry, shikimic acid from the Chinese star anise (Illicium verum) is used as a base material for production of oseltamivir (Tamiflu). Although shikimic acid is present in most autotrophic organisms, it is a biosynthetic intermediate and in general found in very low concentrations. The low isolation yield of shikimic acid from the Chinese star anise is blamed for the 2005 shortage of oseltamivir. Shikimic acid can also be extracted from the seeds of the sweetgum (Liquidambar styraciflua) fruit, which is abundant in North America, in yields of around 1.5%. For example, 4 kg of sweetgum seeds is needed for fourteen packages of Tamiflu. By comparison, star anise has been reported to yield 3% to 7% shikimic acid. Biosynthetic pathways in E. coli have recently been enhanced to allow the organism to accumulate enough material to be used commercially.{{cite journal

Protecting groups are more commonly used in small-scale laboratory work and initial development than in industrial production processes because their use adds additional steps and material costs to the process. However, the availability of a cheap chiral building block can overcome these additional costs, for example, shikimic acid for oseltamivir.

Aminoshikimic acid is also an alternative to shikimic acid as a starting material for the synthesis of oseltamivir.

Target for drugs

Shikimate can be used to synthesise (6S)-6-fluoroshikimic acid, an antibiotic which inhibits the aromatic biosynthetic pathway. More specifically, glyphosate inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). "Roundup Ready" genetically modified crops overcome that inhibition.

Occurrence

It occurs in tree fern fronds, a specialty called fiddlehead (furled fronds of a young tree fern in the order Cyatheales, harvested for use as a vegetable). These fronds are edible, but can be roasted to remove shikimic acid.

Shikimic acid is also the glycoside part of some hydrolysable tannins. The acid is highly soluble in water and insoluble in nonpolar solvents, and this is why shikimic acid is active only against Gram-positive bacteria, due to outer cell membrane impermeability of Gram-negatives.

References

Books

References

1. Eykman, J. F.. (1881). "The botanical relations of ''Illicium religiosum'' Sieb., ''Illicium anisatum'' Lour.". American Journal of Pharmacy.
2. (April 2008). "''Liquidambar styraciflua'': a renewable source of shikimic acid". Tetrahedron Letters.
3. "Gallic acid pathway".
4. (7 November 2010). "Maine pine needles yield valuable Tamiflu material".
5. (4 August 2011). "Facile Syntheses of (6S)-6-Fluoroshikimic Acid and (6R)-6-Hydroxyshikimic Acid". Chemical Research in Chinese Universities.
6. (February 1994). "(6S)-6-fluoroshikimic acid, an antibacterial agent acting on the aromatic biosynthetic pathway". Antimicrobial Agents and Chemotherapy.
7. (29 August 2006). "Molecular basis for the herbicide resistance of Roundup Ready crops". Proceedings of the National Academy of Sciences.
8. (July 1974). "Carcinogenicity of bracken and shikimic acid". Nature.
9. (30 September 2009). "Evaluation of the Biological Activity of Extracts from Star-Anise (Illicium verum)". Preventive Nutrition and Food Science.