Separation and Properties of Isozymes of 3-Deoxy-D-arabino-heptulosonate-7-phosphate Synthetase from Saccharomyces cerevisiae

Abstract

The phenylalanine-sensitive and the tyrosine-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthetases in yeast were separated by affinity chromatography and several properties of the separated isozymes were studied. The nature of the chromatographic process was also investigated. Both DAHP synthetases were inactivated by EDTA. The inactivation was reversed by Co²⁺ or Mn²⁺ and to a lesser extent by Zn²⁺. Phosphoenolpyruvate protected both enzymes from inactivation by EDTA or by heat treatment. Both enzymes showed a broad pH optimal range between 6.5 and 8.0 and a molecular weight of 65 000. The concentration of effector for 50% inhibition of each isozyme was approximately 5 × 10⁻⁵ M phenylalanine and 2 × 10⁻⁴ M tyrosine. A number of tyrosine and phenylalanine analogues also inhibited the enzyme reaction. Kinetic data were obtained for each of the separated isozymes both in the presence and in the absence of inhibitors. Considerable departure from Michaelis–Menten kinetics was observed in several instances. Our kinetic results are significantly different from those previously reported for the ammonium sulfate fractionated isozymes. These differences may be due to structural changes in the enzymes caused by the ammonium sulfate procedure.