L-Glutamine:D-fructose-6-phosphate aminotransferase

L-Glutamine D-fructose 6-phosphate aminotransferase (EC 2.6.1.16) is widely distributed in mammalian tissue,430,431 and in E. coli,4MNeurospora cras a,4S0 and B. subtilis.432,433 The enzyme catalyzes the formation of 2-amino-2-deoxy-D-glucose 6-phosphate ... 

Another, more classical, control mechanism exists for the feedback regulation of key enzymes for the synthesis of Gm-6-P04 and for N-acetylmannosamine. Each of these is the first enzyme in the metabolic commitment to hexosamine and sialic acid synthesis. Komfeld et al, (1964) showed that UDP-GlcNAc is an efficient feedback inhibitor for L-glutamine-D-fructose-6-phosphate aminotransferase and that CMP-NAN also inhibits UDP-GlcNAc-2-epimerase, which is responsible for the synthesis of A/ -acetylmannosamine. This is an example of the by now familar endproduct inhibition of the first enzyme of a metabolic pathway (see Figure 6). They were also able to demonstrate that in vivo administration of puromycin to rats, which inhibits de novo protein synthesis and also depresses sialic acid and hexosamine utilization, does not lead to an accumulation of UDP-GlcNAc. Furthermore, the turnover of the UDP-hexosamine pool was shown to be slowed down. These data suggest that impairment of the utilization of UDP-hexosamine leads to decreased synthesis of UDP-hexosamines or their precursors (i.e., classical feedback inhibition). 

Winterbum and Phelps (1970) have suggested that the L-glutamine-D-fructose-6-phosphate aminotransferase does not display classical feedback kinetic behavior in the presence of UDP-GlcNAc. Furthermore, they suggest that there is a 10-fold excess of enzyme over that necessary for a steady state for hexosamine metabolism. 

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