UDP-D-glucuronate 4-epimerase involved in the synthesis of the negatively charged monosaccharide that forms the backbone of pectic cell wall components. UDP-alpha-D-glucuronate = UDP-alpha-D-galacturonate Inhibited by UDP-Xylose. Equilibrium between UDP-glucuronate and UDP-D-galacturonate established at 1:1.3. Optimum pH is 7.6. Active from pH 6 to 8.9. Homodimer. In root stele, leaves, siliques, flowers, pollen and stems. Belongs to the NAD(P)-dependent epimerase/dehydratase family.