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Questions and Answers
What is the regulation mechanism of the genes encoding Tyr- and Trp-sensitive isoenzymes?
What is the regulation mechanism of the genes encoding Tyr- and Trp-sensitive isoenzymes?
What is the proposed mechanism for the synthesis of aromatic amino acids in plants?
What is the proposed mechanism for the synthesis of aromatic amino acids in plants?
Where are proteins synthesized in plants?
Where are proteins synthesized in plants?
What is the role of chloroplasts in the synthesis of aromatic amino acids?
What is the role of chloroplasts in the synthesis of aromatic amino acids?
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What is the evidence for the localization of the shikimate pathway in the plastids?
What is the evidence for the localization of the shikimate pathway in the plastids?
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What is the fate of the shikimate pathway enzymes in plastids?
What is the fate of the shikimate pathway enzymes in plastids?
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What is the percentage of aromatic amino acid biosynthesis attributed to the chloroplast-localized biosynthetic activity?
What is the percentage of aromatic amino acid biosynthesis attributed to the chloroplast-localized biosynthetic activity?
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What is the characteristic of the regulation of plant DAHP synthases?
What is the characteristic of the regulation of plant DAHP synthases?
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What is the compartment where the aromatic amino acids are synthesized in plants?
What is the compartment where the aromatic amino acids are synthesized in plants?
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What is the significance of the shikimate pathway in plants?
What is the significance of the shikimate pathway in plants?
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Study Notes
Shikimate Pathway
- 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).
Enzymes of Shikimate Pathway
- 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.
Regulation of Shikimate Pathway
- 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.
Subcellular Localization of Shikimate Pathway
- 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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Description
This quiz covers the shikimate pathway, a 7-step metabolic process involving 6 enzymes. It's a key pathway for the synthesis of aromatic amino acids and phenylpropanoids.
