Glutamine Phosphoribosylpyrophosphate Amidotransferase: Purification, substructure, amino acid composition, and absorption spectra

Abstract

Abstract A purification procedure has been developed for the isolation of phosphoribosylpyrophosphate amidotransferase (EC 2.4.2.14, ribosylamine 5-phosphate:pyrophosphate phosphoribosyltransferase) from pigeon liver. It has been shown that the enzyme molecule of molecular weight 200,000 dissociates readily into 100,000 mol wt subunits and that these, in the presence of thiols, dissociate into 50,000 mol wt subunits. These basic 50,000 mol wt subunits, or monomers, are identical electrophoretically. Each 100,000 mol wt subunit, or dimer, contains 6 nonheme iron atoms some of which are involved in the catalytic reaction, whereas others serve primarily a structural function. The visible absorption spectrum of the enzyme reveals a maximum at 415 mµ, which disappears with the removal of iron from the protein. The tetramer, dimer, and monomer forms, as isolated, are enzymatically active and subject to purine ribonucleotide inhibition. The molecular weight of the functionally active unit, however, is unknown.