| Abstract: | Prephenate dehydratase from Bacillus subtilis was found to exist in three states of aggregation. A high molecular weight (210,000) species was fully active and the catalytic activity was unaffected by the effectors methionine or phenylalanine. Low concentrations of phenylalanine caused dissociation to a Mr = 55,000 dimer. Heating to 32 degrees C also caused dissociation, but cooling and adding substrate or methionine favored association. When no effectors were present the enzyme eluted from Sephadex columns as a monomer. Both methionine and phenylalanine shifted the equilibrium from the inactive monomer to the active dimeric enzyme. In the presence of a saturating methionine concentration, the dimer possessed the same high activity as did the 210,000-dalton form. Phenylalanine inhibited the dimer, but not the higher molecular weight form. A model involving only three types of sites (catalytic, association-activation, and inhibition) is consistent with the data. It is proposed that phenylalanine is the preferred metabolite for binding both effector sites on the dimer; it binds the association-activation site with higher affinity than the inhibition site, but binding at the latter site has a greater effect on the catalytic rate. Methionine, like phenylalanine, has a hydrophobic side chain but is accommodated only at the association-activation site. |
