Galactose-1-phosphate uridylyltransferase. Purification of the enzyme and stereochemical course of each step of the double-displacement mechanism

Abstract

A convenient new procedure for purifying galactose-1-phosphate uridylyltransferase from Escherichia coli is described. It departs from earlier methods by introducing the use of a Cibacron Blue-agarose (Bio-Rad Affi-Gel Blue) at an early stage. Purification is completed by ion-exchange chromatography using DEAE-Sephadex A-50. The procedure is substantially shorter than earlier methods and reproducibly yields enzyme of high specific activity suitable for use in structural work such as characterization of the intermediate uridylyl-enzyme. The first step of the galactose-1-P uridylyltransferase reaction is the transfer of the uridylyl group from UDP-glucose to N3 of a histidine residue in the enzyme to form the covalent uridylyl-enzyme and glucose-1-P. The uridylyl-enzyme intermediate then reacts in a second step with galactose-1-P to form UDP-galactose. The enzyme accepts (RP)-UDP alpha S-glucose as a good substrate, converting it to (RP)-UDP alpha S-galactose, i.e., with overall retention of configuration. In this paper we show that reaction of the enzyme with (RP)-[2-14C]UDP alpha S-glucose produces a [2-14C]uridylyl alpha S-enzyme that can be converted by base-catalyzed cyclization to (RP)-[2-14C]cUMPS. Inasmuch as cyclization must have proceeded with inversion of configuration at phosphorus, the corresponding configuration in the intermediate must have been the inverse of that in the substrate. Therefore, formation of uridylyl alpha S-enzyme from (RP)-UDP alpha S-glucose proceeds with inversion of configuration, and overall retention arises from inversion in each of the two steps. The results support the authenticity of the isolated uridylyl-enzyme as the true reaction intermediate.(ABSTRACT TRUNCATED AT 250 WORDS)