The tyrosine-sensitive allosteric first enzyme of the aromatic amino acid biosynthetic pathway, 3-deoxy-D-arabinoheptulosonate 7-phosphate synthetase (7-Phospho-2-keto-3-deoxy-D-arabino-heptonate D-erythrose 4-phosphate-lyase (pyruvate phosphorylating), EC 4.1.2.15) has been purified from a mutant strain of Escherichia coli. 2. The enzyme activity was inhibited to 50% at 2-10(-5) M tyrosine and to 90% at 2-10(-4) M tyrosine concentration. At tyrosine concentrations lower than 2-10(-5) M a cooperative interaction between tyrosine binding sites was observed. 3. Co2+ increased the enzyme activity about 2-2.5-fold. The presence of Co2+ ions stabilized the enzyme. EDTA inhibited the enzyme activity, and this inhibition was reversed by Co2+. Tyrosine-sensitive DAHP synthetase seems to be a metal containing enzyme. 4. Kinetic experiments were carried out to study the catalytic action. Contrary to earlier suggestions it is concluded, that the reaction mechanism appears to be more complex--with either the ping-pong or sequential type predominating, depending on conditions.
