Chorismate Mutase/Prephenate Dehydratase from Escherichia coli K12. Effects of Chemical Modification on the Enzymic Activities and Allosteric Inhibition

1 The modification of one cysteine residue per subunit of chorismate mutase/prephenate dehydratase by N-ethylmaleimide results in complete inactivation of the dehydratase activity with only 5 % loss of the mutase activity. 2 The reaction of N-ethylmaleimide with the second and third cysteine residues in the subunit leads to a desensitization of the remaining mutase activity to inhibition by phenylalanine. 3 Reaction of four cysteine residues per enzyme subunit with tetrathionate also causes a differential inactivation of only dehydratase activity. 4 Modification by tetranitromethane of two tyrosine residues per enzyme subunit results in complete inactivation of the dehydratase activity while only 30 % of the mutase activity is lost. 5 In contrast to the above observations, modification of a single lysine residue per enzyme subunit with 2,4,6-trinitrobenzene sulphonate leads to a complete inactivation of the mutase activity with only 20 % loss of the dehydratase activity. 6 Modification of tryptophan residues with dimethyl (2-hydroxy-5-nitrobenzyl)-sulphonium bromide leads to partial loss of both the mutase and dehydratase activities and to densensitization of both activities to inhibition by phenylalanine. 7 On the basis of these results it is suggested that the mutase and dehydratase activities are catalysed at separate sites on the enzyme, or that the sites are at most only slightly overlapping, and that the phenylalanine-binding site is topographically distinct from the active sites of the enzyme.