Abstract
Abstract The bifunctional enzyme chorismate mutase-prephenate dehydratase catalyzes the first two reactions specific for phenylalanine biosynthesis in Salmonella typhimurium. Both activities are subject to feedback inhibition by phenylalanine. In the presence of phenylalanine the sedimentation coefficient of the enzyme is increased from approximately 5.6 to 8.0 S and the molecular weight is increased from approximately 109,000 to 220,000. This apparent dimerization is dependent upon the concentrations of phenylalanine and enzyme. Chorismate mutase-prephenate dehydratase that has been rendered insensitive to feedback inhibition either chemically or by mutational alteration does not dimerize. Therefore, dimerization appears somehow related to end product control. However, dimerization is not obligatory for feedback inhibition of either activity. At low protein concentration some enzyme does not dimerize in the presence of phenylalanine. Such enzyme can still be inhibited by phenylalanine. It is therefore suggested that phenylalanine binds to the monomer (smallest active species) to produce an inhibited monomer which dimerizes at appropriate protein concentrations.
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