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J. Biol. Chem., Vol. 277, Issue 17, 14764-14770, April 26, 2002

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Kinetic Characterization of Human Glutamine-fructose-6-phosphate Amidotransferase I
POTENT FEEDBACK INHIBITION BY GLUCOSAMINE 6-PHOSPHATE^*

Kay O. Broschat^§, Christine Gorka, Jimmy D. Page^¶, Cynthia L. Martin-Berger^¶, Michael S. Davies, Horng-chih Huang^**, Eric A. Gulve, William J. Salsgiver, and Thomas P. Kasten

From the  Cardiovascular & Metabolic Disesases, Pharmacia Corporation, St. Louis, Missouri 63167, ^¶ Discovery Biology, Pharmacia Corporation, Skokie, Illinois 60077, and  Biochemistry & Molecular Biology and ^** Discovery Chemistry, Pharmacia Corporation, Chesterfield, Missouri 63198

Glutamine-fructose-6-phosphate amidotransferase (GFAT) catalyzes the first committed step in the pathway for biosynthesis of hexosamines in mammals. A member of the N-terminal nucleophile class of amidotransferases, GFAT transfers the amino group from the L-glutamine amide to D-fructose 6-phosphate, producing glutamic acid and glucosamine 6-phosphate. The kinetic constants reported previously for mammalian GFAT implicate a relatively low affinity for the acceptor substrate, fructose 6-phosphate (Fru-6-P, K_m 0.2-1 mM). Utilizing a new sensitive assay that measures the production of glucosamine 6-phosphate (GlcN-6-P), purified recombinant human GFAT1 (hGFAT1) exhibited a K_m for Fru-6-P of 7 µM, and was highly sensitive to product inhibition by GlcN-6-P. In a second assay method that measures the stimulation of glutaminase activity, a K_d of 2 µM was measured for Fru-6-P binding to hGFAT1. Further, we report that the product, GlcN-6-P, is a potent competitive inhibitor for the Fru-6-P site, with a K_i measured of 6 µM. Unlike other members of the amidotransferase family, where glutamate production is loosely coupled to amide transfer, we have demonstrated that hGFAT1 production of glutamate and GlcN-6-P are strictly coupled in the absence of inhibitors. Similar to other amidotransferases, competitive inhibitors that bind at the synthase site may inhibit the synthase activity without inhibiting the glutaminase activity at the hydrolase domain. GlcN-6-P, for example, inhibited the transfer reaction while fully activating the glutaminase activity at the hydrolase domain. Inhibition of hGFAT1 by the end product of the pathway, UDP-GlcNAc, was competitive with a K_i of 4 µM. These data suggest that hGFAT1 is fully active at physiological levels of Fru-6-P and may be regulated by its product GlcN-6-P in addition to the pathway end product, UDP-GlcNAc.

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