The protein or protein complex carrying chorismate mutase and prephenate dehydratase activities has been partially purified from Salmonella typhimurium. End-product inhibition of both activities by phenylalanine is obtained. Inhibition by phenylalanine is partially competitive with chorismate and with prephenate. Analysis of inhibition by the Hill equation yields values of n′ of 2.0 and 1.7 for chorismate mutase and prephenate dehydratase activities, respectively, and suggests cooperative interaction of phenylalanine sites. Sucrose gradient centrifugation experiments show that phenylalanine and dithiothreitol cause large changes in the S20.wvalue of the enzyme. Treatment with 1 mM phenylalanine causes a change in S20.wfrom 5.3 to 6.3 S; treatment with 0.1 mM dithiothreitol causes a change from 5.3 to 6.9 S. Analysis of substrate saturation data, obtained in the absence of phenylalanine, by the Hill equation yields a value of n of 1.1 for both chorismate mutase and prephenate dehydratase activities and suggests little or no cooperative interaction of substrate sites under these conditions. Cooperativity is suggested by maximal values of n of 2.0 and 1.9 for chorismate mutase and prephenate dehydratase activities obtained in the presence of 0.24 mM phenylalanine. In the over-all conversion of chorismate into phenylpyruvate, prephenate dissociates from the enzyme and accumulates in the reaction mixture. After a lag prephenate is converted into phenylpyruvate. © 1969, American Chemical Society. All rights reserved.
