The phenylalanine-sensitive 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAH 7-P) synthase (phe),
a key enzyme involved in the biosynthesis of the aromatic amino acid phenylalanine, expressed by the
Escherichia coli gene aroG, which catalyzes the condensation of d-erythrose 4-phosphate with phosphoenolpyruvate (PEP) to give DAH 7-P, was cloned into the expression vector pT7-7 for overexpression in E.
coli. Purified enzyme from this expression system was used to demonstrate that DAH 7-P synthase (phe) also
catalyses the aldol-type condensation of PEP with the 5-carbon analogues d-arabinose 5-phosphate, d-ribose
5-phosphate, and 2-deoxy-d-ribose 5-phosphate to yield 3-deoxy-d-manno-octulosonate 8-phosphate, 3-deoxy-d-altro-octulosonate 8-phosphate, and 3,5-dideoxy-d-gluco(manno)-octulosonate 8-phosphate, respectively, as
determined by 1H NMR and other standard analytical methods. The kinetic parameters, K
m and V
max, for
these reactions were determined. The 3- and 6-carbon phosphorylated monosaccharides, d,l-glyceraldehyde
3-phosphate and d-glucose 6-phosphate, as well as the nonphosphorylated 5-carbon analogues d-arabinose
5-phosphate, d-ribose 5-phosphate, and 2-deoxy-d-ribose 5-phosphate were not substrates.
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