A GLUTAMATE RESIDUE IN THE CATALYTIC CENTER OF THE YEAST CHORISMATE MUTASE RESTRICTS ENZYME-ACTIVITY TO ACIDIC CONDITIONS

Chorismate mutase acts at the first branchpoint of aromatic amino acid biosynthesis and catalyzes the conversion of chorismate to prephenate , Comparison of the x-ray structures of allosteric chorismate mutase f rom the yeast Saccharomyces cerevisiae with Escherichia coli chorismat e mutase/prephenate dehydratase suggested conserved active sites betwe en both enzymes, We have replaced all critical amino acid residues, Ar g-16, Arg-157, Lys-168, Glu-198, Thr-242, and Glu-246, of yeast choris mate mutase by aliphatic amino acid residues, The resulting enzymes ex hibit the necessity of these residues for catalytic function and provi de evidence of their localization at the active site, Unlike some bact erial enzymes, yeast chorismate mutase has highest activity at acidic pH values, Replacement of Glu-246 in the yeast chorismate mutase by gl utamine changes the pH optimum for activity of the enzyme from a narro w to a broad pH range, These data suggest that Glu-246 in the catalyti c center must be protonated for maximum catalysis and restricts optima l activity of the enzyme to low pH.