Isolation of a Multifunctional Protein with Aminoimidazole Ribonucleotide Synthetase, Glycinamide Ribonucleotide Synthetase, and Glycinamide Ribonucleotide Transformylase Activities: Characterization of Aminoimidazole Ribonucleotide Synthetase

5-Aminoimidazole ribonucleotide (AIR) synthetase, glycinamide ribonucleotide (GAR) synthetase, and GAR transformylase activities from chicken liver exist on a single polypeptide of M_r110000 [Daubner, C. S., Schrimsher, J. L., Schendel, F. J., Young, M., Henikoff, S., Patterson, D., Stubbe, J., & Benkovic, S. J. (1985) Biochemistry 24, 7059–7062]. Details of copurification of these three activities through four chromatographic steps are reported. The ratios of these activities remain constant throughout the purification. AIR synthetase has an absolute requirement for K⁺ for activity and under these conditions has apparent molecular weights of 330000, determined by Sephadex G-200 chromatography, and 133 000, determined by sucrose density gradient ultracentrifugation. Incubation of ¹⁸O-labeled formylglycinamidine ribonucleotide (FGAM) with AIR synthetase results in stoichiometric production of AIR, ADP, and [¹⁸O]Pi. NMR spectra of β-FGAM and β-AIR are reported.