UDP-galactose 4-epimerase from Escherichia coli: existence of a catalytic monomer.

UDP-galactose 4-epimerase from Escherichia coli is a homodimer of molecular mass 39 kDa/subunit and requires NAD as a co-factor. X-ray crystallographic studies indicate two pyridine nucleotide co-factor-binding sites of the dimeric molecule situated in a symmetry-oriented manner. Size-exclusion HPLC of an equilibrium intermediate at 3 M urea suggests a monomeric holoenzyme structure that is catalytically active. Ultracentrifugal studies of the native enzyme in a 5-20% sucrose gradient at low protein concentration also indicate existence of a catalytic monomer. The monomer resembles the dimeric protein in stability and most of its physico-chemical properties.