Shikimate Pathway: Aromatic Amino Acids and Phenylpropanoids

Study Notes

The shikimate pathway consists of 7 metabolic steps, involving 6 enzymes.
The pathway starts with the condensation of phosphoenolpyruvate (PEP) and erythrose-4-phosphate (Ery4P) to yield 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP).

DAHP Synthase: The first enzyme of the shikimate pathway, catalyzing the condensation of PEP and Ery4P to yield DAHP. E. coli has three feedback inhibitor-sensitive DAHP synthase isoenzymes: a Tyr-sensitive, a Phe-sensitive, and a Trp-sensitive enzyme.
DHQ Synthase: The second enzyme, catalyzing the elimination of phosphate from DAHP to generate 3-dehydroquinate (DHQ). The enzyme from E. coli requires divalent cations for activity and is activated by inorganic phosphate.
DHQ Dehydrogenase: The third enzyme, catalyzing the dehydration of DHQ to form 3-dehydroshikimate (DHS). There are two types of DHQ dehydrogenase: type I (heat-labile) and type II (heat-stable).
Shikimate Kinase: The fourth enzyme, catalyzing the phosphorylation of shikimate to form shikimate-3-phosphate. E. coli has two isoenzymes of shikimate kinase, both can function in vitro for aromatic amino acid biosynthesis.
5-Enolpyruvyl-Shikimate-3-Phosphate Synthase (EPSPS): The fifth enzyme, catalyzing the reversible formation of 5-enolpyruvyl-shikimate-3-phosphate (EPSP) from shikimate-3-phosphate and PEP. EPSPS is the target of the broad-spectrum herbicide, glyphosate.
Chorismate Synthase: The sixth enzyme, catalyzing the trans-1,4 elimination of phosphate from EPSP to yield chorismate. Chorismate synthase requires a reduced flavin nucleotide (FMNH2) as a cofactor.

The shikimate pathway is regulated in response to the demand for phenolic metabolites during plant development and environmental stress.
In bacteria, DAHP synthase and shikimate kinase are regulated by feedback inhibition and gene expression.
In plants, DAHP synthases are not subject to feedback inhibition by the aromatic amino acids.

The shikimate pathway is localized in the plastids, supported by molecular analysis and in vitro experiments.
The isolated chloroplasts from spinach were able to synthesize aromatic amino acids from CO2 or shikimate, indicating the existence of an aromatic biosynthetic pathway in chloroplasts.

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