title: Characterization and activation of naturally occurring abortive complexes of UDP-galactose 4-epimerase from Escherichia coli.
abstract: UDP-galactose 4-epimerase catalyzes the interconversion of UDP-galactose and UDP-glucose. The enzyme from Escherichia coli is a dimeric protein with an overall molecular weight of 79,000 that contains NAD+ very tightly but noncovalently bound in the enzymatic active site. NAD+ is the coenzyme for epimerization and is transiently reduced to NADH in the course of catalysis. All samples of highly purified UDP-galactose 4-epimerase contain significant amounts of NADH, and that purified after overexpression in E. coli cells contains a substantial amount of NADH. To the degree that NADH replaces enzyme bound NAD+ in the coenzyme binding site, the epimerase activity is decreased. The extinction coefficient at 345 nm for NADH in its binding site is estimated to be 3.3 mM-1 cm-1. 31P NMR spectroscopic and enzymatic analyses reveal that UDP-glucose, UDP-galactose, UDP, and UMP are gradually released from the purified enzyme upon addition of UMP or P1-5'-uridine-P2-methyl diphosphate (MeUDP). It is concluded that NADH associated with the purified enzyme is a component of inactive, abortive complexes (E-NADH-uridine nucleotide) that contain tightly bound uridine nucleotides in place of the epimerization intermediate UDP-4-keto-alpha-D-hexoglucopyranose. These complexes are produced in vivo in the course of bacterial growth. The enzymatic activity of purified epimerase is increased by reaction with 1,2-naphthoquinone-4-sulfonate, which oxidizes the NADH to NAD+. Compositionally defined abortive complexes (E-NADH-uridine nucleotide) containing UMP, UDP, or UDP-hexoses (Glc/Gal) have been prepared in vitro and subjected to activation by 1,2-naphthoquinone-4-sulfonate. All are activated at rates comparable to that for the purified enzyme, although those containing UMP and UDP-hexose are more readily activated than those containing UDP. The activity of the reactivated enzyme approaches that of the most highly active epimerase that has been reported from E. coli.
authors.affiliation: Institute for Enzyme Research, Graduate School.
authors.forename: J L
authors.initials: JL
authors.lastname: Vanhooke
authors.forename: P A
authors.initials: PA
authors.lastname: Frey
mesh_terms: D002238:Carbohydrate Epimerases / Q000378:metabolism*; D004789:Enzyme Activation; D004926:Escherichia coli / Q000201:enzymology; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D009243:NAD / Q000378:metabolism; D010084:Oxidation-Reduction; D013056:Spectrophotometry, Ultraviolet; D013329:Structure-Activity Relationship; D014534:UDPglucose 4-Epimerase; D014500:Uracil Nucleotides / Q000378:metabolism
chemical_list: D014500:Uracil Nucleotides; D009243:NAD; D002238:Carbohydrate Epimerases; D014534:UDPglucose 4-Epimerase; C020551:galactose epimerase