مسار شيكيمي

(تم التحويل من Shikimate pathway)

المسار الشيكيمي (shikimate pathway)، مسار حمض الشيكيميك، هو مسار أيضي من سبع خطوات تستخدمه البكتريا، العتائق، الفطريات، الأشنيات، الأوليات، والنباتات لعملية التخليق الحيوي للفولات والأحماض الأمينية العطرية (التريپتوفان، الفنايل‌ألانين، والتايروسين). لا يوجد هذا المسار في الثدييات.

The five enzymes involved in the shikimate pathway are 3-dehydroquinate dehydratase, shikimate dehydrogenase, shikimate kinase, EPSP synthase, and chorismate synthase. In bacteria and eurkaryotes, the pathway starts with two substrates, phosphoenol pyruvate and erythrose-4-phosphate, are processed by DAHP synthase and 3-dehydroquinate synthase to form 3-dehydroquinate. In archaea, 2-amino-3,7-dideoxy-D-threo-hept-6-ulosonate synthase condenses L-Aspartic-4-semialdehyde with a sugar to form 2-amino-3,7-dideoxy-D-threo-hept-6-ulosonate, which is then turned by 3-dehydroquinate synthase II into 3-dehydroquinate. Both pathways end with chorismate (chrorismic acid), a substrate for the three aromatic amino acids. The fifth enzyme involved is the shikimate kinase, an enzyme that catalyzes the ATP-dependent phosphorylation of shikimate to form shikimate 3-phosphate (shown in the figure below).[1][2] Shikimate 3-phosphate is then coupled with phosphoenol pyruvate to give 5-enolpyruvylshikimate-3-phosphate via the enzyme 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase. Glyphosate, the herbicidal ingredient in Roundup, is a competitive inhibitor of EPSP synthase, acting as a transition state analog that binds more tightly to the EPSPS-S3P complex than PEP and inhibits the shikimate pathway.

Chorismate pathway 1.png

Then 5-enolpyruvylshikimate-3-phosphate is transformed into chorismate by a chorismate synthase.

Chorismate pathway 2.png

Prephenic acid is then synthesized by a Claisen rearrangement of chorismate by chorismate mutase.[3][4]

Prephenate biosynthesis.png

Prephenate is oxidatively decarboxylated with retention of the hydroxyl group to give p-hydroxyphenylpyruvate, which is transaminated using glutamate as the nitrogen source to give tyrosine and α-ketoglutarate.

Tyrosine biosynthesis.svg

المصادر

  1. ^ Morar, Mariya; White, Robert H.; Ealick, Steven E. (1 September 2007). "Structure of 2-Amino-3,7-dideoxy- d - threo -hept-6-ulosonic Acid Synthase, a Catalyst in the Archaeal Pathway for the Biosynthesis of Aromatic Amino Acids ,". Biochemistry. 46 (37): 10562–10571. doi:10.1021/bi700934v.
  2. ^ Herrmann, K. M.; Weaver, L. M. (1999). "The Shikimate Pathway". Annual Review of Plant Physiology and Plant Molecular Biology. 50: 473–503. doi:10.1146/annurev.arplant.50.1.473. PMID 15012217.
  3. ^ Helmut Goerisch (1978). "On the mechanism of the chorismate mutase reaction". Biochemistry. 17 (18): 3700–3705. doi:10.1021/bi00611a004. PMID 100134.
  4. ^ Peter Kast; Yadu B. Tewari; Olaf Wiest; Donald Hilvert; Kendall N. Houk; Robert N. Goldberg (1997). "Thermodynamics of the Conversion of Chorismate to Prephenate: Experimental Results and Theoretical Predictions". J. Phys. Chem. B. 101 (50): 10976–10982. doi:10.1021/jp972501l.

المراجع

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