Inactivation of Escherichia coli phosphoribosylpyrophosphate synthetase by the 2',3'-dialdehyde derivative of ATP. Identification of active site lysines

The enzyme 5-phosphoribosyl-α-1-pyrophosphate (PRPP) synthetase from Escherichia coli was irreversibly inactivated on exposure to the affinity analog 2',3'-dialdehyde ATP (oATP). The reaction displayed complex saturation kinetics with respect to oATP with an apparent K(D) of approximately 0.8 mM. Reaction with radioactive oATP demonstrated that complete inactivation of the enzyme corresponded to reaction at two or more sites with limiting stoichiometries of approximately 0.7 and 1.3 mol of oATP incorporated/mol of PRPP synthetase subunit. oATP served as a substrate in the presence of ribose-5-phosphate, and the enzyme could be protected against inactivation by ADP or ATP. Isolation of radioactive peptides from the enzyme modified with radioactive oATP, followed by automated Edman sequencing allowed identification of Lys ¹⁸¹ Lys ¹⁹³, and Lys ²³⁰ as probable sites of reaction with the analog. Cysteine 229 may also be labeled by oATP. Of these four residues, Lys ¹⁹³ is completely conserved within the family of PRPP synthetases, and Lys ¹⁸¹ is found at a position in the sequence where the cognate amino acid (Asp ¹⁸¹) in human isozyme I PRPP synthetase has been previously implicated in the regulation of enzymatic activity. These results imply a functional role for at least two of the identified amino acid residues.