[Functionally important residues of glutamic acid in E. coli pyrophosphatase. I. Chemical modification and...

Inorganic pyrophosphatase of E. coli is rapidly and irreversibly inactivated by 5-ethyl-5-phenylisoxazolium-3'-sulfonate (Woodward's reagent K). The appearance in the absorption spectrum of a maximum at 340 nm testifies to the formation of an enzyme enol ester with the inhibitor. The non-hydrolyzable substrate analog CaPP1 partly protects the enzyme from inactivation. A peptide has been isolated from a tryptic hydrolysate of inactivated enzyme which contains an amino acid residue whose modification is critical for the enzyme activity. This peptide corresponds to residues 95-104 of pyrophosphatase and contains four dicarboxylic acid residues. A peptide containing a modified glutamic acid residue was isolated from modified pyrophosphatase hydrolyzed by protease v8. This peptide represents a fragment of a tryptic modified peptide and has a Glu-Ala-Gly-Glu (residues 98-1C1) structure. It is concluded that inactivation of E. coli pyrophosphatase by Woodward's reagent K is a result of selective modification of Glu98, apparently by the most reactive dicarboxylic amino acid within the enzyme active center.