Characterization of the Enzyme Complex Involving the Folate-Requiring Enzymes of de Novo Purine Biosynthesis

Evidence is presented for a functional association of GAR TFase and the trifunctional protein within the protein complex consisting of GAR TFase, AICAR TFase, Ser HMase, and trifunctional protein. Resolution of the trifunctional protein from the remaining enzymes in the complex causes a loss of GAR TFase activity which is regained upon recombination. The minimum stoichiometry for GAR TFase reactivation is 3 : 1 GAR TFase-trifunctional protein. Determination by ultracentrifugation of the sedimentation coefficient as a function of protein concentration disclosed that the complex is in mobile equilibrium with the individual proteins. However, mixed GAR TFase-trifunctional protein species can be detected by trapping with cleavable bifunctional crosslinking reagents. Additional support for their interaction is found in the kinetic coupling of the trifunctional protein and GAR TFase activities that leads to a fourfold more efficient formation of formylglycinamide ribotide commencing with formate rather than with 5, 10-methenyl-H₄folate triglutamate.