Interconversion of different molecular weight forms of the orotate phosphoribosyltransferase.orotidine-5'-phosphate decarboxylase enzyme complex from mouse Ehrlich ascites cells.

Complex U consists of orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidineG’-phosphate decarboxylase (EC 4.1.1.23), the last two enzymes of the de nova pathway for UMP biosynthesis. Earlier studies had shown that orotate and magnesium 5-phosphoribosyl-1-pyrophosphate, substrates for the transferase, and orotidine 5’-monophosphate (OMP), the product of the transferase and also the substrate for the decarboxylase, could alter the conformation of Complex U as measured by changes in sedimentation on sucrose gradients (Traut, T. W., and Jones, M. E. (1977) J. Biol. Chem. 252, 8374-8381). More extensive studies using both density gradient sedimentation and gel filtration chromatography reveal that Complex U has four different aggregation states: a 3.6 S species (Mr = 45,400) which is probably a monomer, a 4.7 S species (M,. = 78,000), a 5.1 S dimer (Mr = 92,800), and a 5.6 S species (iI& = 118,000). The 4.7 S and 5.6 S species are interpreted as hybrids formed with either the monomer or the dimer of Complex U and an as yet unidentified molecule, whose M, is about 29,000. A wide variety of compounds (Pi, 5-phosphoribosyl1-pyrophosphate, chloride, OMP, UMP, azaUMP) promote the formation of the dimer. Of these effecters, OMP is a substrate for the decarboxylase, and the rest are all competitive inhibitors (uersus OMP) of the decarboxylase reaction. The data suggest that formation of the dimer is achieved by the binding of these effector molecules to the decarboxylase catalytic site since each of these inhibitors, at a concentration 10 times its Ki uersus OMP, will completely convert the 3.6 S monomer to the 5.1 S dimer. Only the nucleotides OMP, UMP, and azaUMP can convert Complex U entirely to the 5.6 S species.