Somekinetic properties ofthetryptophan-sensitive 3-deoxy-D-arabino- heptulosonate 7-phosphate synthase fromNeurospora crassa

3-deoxy-D-arabino- heptulosonate 7-phosphate synthase fromNeurospora crassa wereexamined. The results suggest thattheenzymeobeysaRapid-Equilibrium Ordered mechanism, in whichphosphoenolpyruvate isthefirst substrate tobindand3-deoxy-D-arabino- heptulosonate 7-phosphate isthesecond product tobereleased, rather thanaPingPong mechanism ashasbeenreported previously. Theinhibition bytryptophan wasfound to beparabolic competitive withrespect toD-erythrose 4-phosphate andparabolic non-competitive withrespect tophosphoenolpyruvate. Theenzymewasinactivated by EDTA,andcould beprotected against this inactivation byphosphoenolpyruvate or 3-deoxy-D-arabino-heptulosonate 7-phosphate butnotbyD-erythrose 4-phosphate, tryptophan orP.Thissuggests that theenzyme maybeametalloenzyme. 3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase(7-phospho-2-keto-3-deoxy-D-arabino- heptonate D-erythrose 4-phosphate-lyase (pyruvate- phosphorylating), EC4.1.2.151 catalyses theforma-