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3-Deoxy-D-arabino-Heptulosonate 7-Phosphate Synthase from Carrot Root (Daucus carota) Is a Hysteretic Enzyme ¹

Department of Biochemistry, Purdue University, West Lafayette, Indiana 47907

Roots of carrots (Daucus carota) contain three activities of 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase, the enzyme that catalyzes the first step of the shikimate pathway. The three activities, enzymes I, II, and III, are separated by chromatography on phosphocellulose. Enzyme III, purified to electrophoretic homogeneity, has a native molecular weight of 103,000 and consists of two identical subunits of 53,000 daltons each. Double reciprocal plots of reaction velocity versus substrate concentration yield K_m values of 0.03 and 0.07 millimolar for P-enolpyruvate and erythrose-4-P, respectively. Both products, DAHP and orthophosphate, inhibit the enzyme. Enzyme III is a hysteretic enzyme that is activated by physiological concentrations of L-tryptophan and Mn²⁺, both of which also partially eliminate the hysteretic lag. Feedback activation of carrot DAHP synthase by tryptophan is interpreted to be an early regulatory signal for polyphenol biosynthesis. The three carrot DAHP synthase isoenzymes share antigenic determinants.

¹ Supported by United States Department of Agriculture Grant GAM-8400608 to K. M. H. and a Purdue Agricultural Experiment Station Fellowship to J. A. S. This is Journal Paper 9511 from the Purdue University Agricultural Experiment Station.

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