The Enzymes of the Galactose Operon in Escherichia coli

Abstract Uridine diphosphogalactose 4-epimerase of Escherichia coli had previously been shown to have a molecular weight of 7.9 x 104 and to contain 1 molecule of tightly bound DPN (Wilson, D. B., and Hogness, D. S., J. Biol. Chem., 239, 2469 (1964)). This enzyme is further characterized here with regard to (a) amino acid composition, (b) number of tryptic peptides, (c) amino-terminal residues, and (d) molecular weight in guanidine hydrochloride. The number of tryptic peptides indicates that an amino acid sequence of 360 residues is represented twice within the native enzyme. The finding of only one pair of amino-terminal residues (aspartic acid) per enzyme molecule and the reduction of the molecular weight by a factor of 2 in guanidine hydrochloride are consistent with the simplest conclusion that the sequence of 360 residues is contained within a single polypeptide. Uridine diphosphogalactose 4-epimerase therefore appears to be a complex consisting of two identical polypeptides and one molecule of DPN held together by noncovalent bonds. Some implications regarding the size of the structural gene for this enzyme and the nature of the binding sites for DPN are discussed.