FORMYL PHOSPHATE - A PROPOSED INTERMEDIATE IN THE REACTION CATALYZED BY ESCHERICHIA-COLI PURT GAR TRANSFORMYLASE

The Escherichia coli purT encoded glycinamide ribonucleotide transform ylase (GAR transformylase) serves as an alternate enzyme in the produc tion of formyl GAR for use in de novo purine biosynthesis. This enzyme differs from the previously known purN encoded enzyme in size, sequen ce, and substrates; ATP and formate are required as opposed to formyl tetrahydrofolate. Kinetic studies of the wild-type PurT enzyme describ ed here demonstrate that formyl phosphate behaves as a chemically and kinetically competent intermediate. The requirement for ATP and GAR in these reactions is consistent with previous steady-state kinetic resu lts, which demonstrated that all substrates must be bound before catal ysis. Kinetic characterization of a mutant, which releases formyl phos phate into solution, and positional isotope exchange studies also supp ort the assignment of formyl phosphate as a plausible intermediate.