Aromatic amino acid biosynthesis in Alcaligenes eutrophus H16. I. Properties and regulation of 3-deoxy-d-arabino heptulosonate 7-phosphate synthase.

Properties and regulation of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAHP-synthase), EC4.1.2.15, from Alcaligenes eutrophus H16 were investigated. DAHP synthase was unstable during manipulations such as dialysis, dilution, ammonium sulfate fractionation, chromatography on DEAE-cellulose or Sephadex G-200. For kinetic measurements Sephadex G-25 treated crude extracts were used. The enzyme was not affected by thiol reagents, EDTA or divalent metal ions. The activation energy, deltaH, amounted to 16100 cal/mole. Between pH 7.2 and pH 8.2 there was little change of enzyme activity. The Km-values for the two substrates were found to be 0.043 mM phosphoenolpyruvate and 0.055 mM erythrose-4-phosphate. DAHP-synthase was inhibited by 0.5 mM phenylalanine for 60% and by 0.5 mM tyrosine for 20%. In the presence of both amino acids cumulative inhibition occurred amounting to about 70%. No other amino acid exerted inhibitory effects. A repression of DAHP-synthase by the aromatic amino acids was not observed. Some other strains of hydrogen bacteria were included in this study. The DAHP synthase from strain 12/60/X and Corynebacterium autotrophicum 7C was unregulated. The enzyme from strain 33/X was subject to retro-tyrosine inhibition and from strain 3/2, H1 and H20 were subject to cumulative inhibition.