Purification and properties of DAHP synthase from Nocardia mediterranei

3-Deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase, the first enzyme of the shikimate pathway was isolated from Nocardia mediterranei. It has a molecular weight of approx. 135,000, and four identical subunits, each with a molecular weight of 35,000. The Km values for phosphoenolpyruvate (PEP) and D-erythrose 4-phosphate (E-4-P) were 0.4 and 0.25 mM, respectively, and kinetic study showed that LTrp inhibited DAHP synthase activity, but was not competitive with respect to PEP or E-4-P. The enzyme activity was inhibited by excess of E-4-P added in the incubation system. D-ribose 5-phosphate (R-5-P), D-glucose 6-phosphate (G-6-P) or D-sedoheptulose 7-phosphate (Su-7-P) etc. inhibited DAHP synthase in cell-free extract, but on partially purified enzyme no inhibitory effect was detected. The indirect inhibition of R-5-P and other sugar phosphates was considered to be due to the formation of E-4-P catalyzed by the related enzymes present in cell-free extract.