Plays a role in the biosynthesis of B-band O antigen for serotype O5. Catalyzes the epimerization of UDP-2,3-diacetamido-2,3-dideoxy-alpha-D-glucuronic acid (UDP-alpha-D-GlcNAc3NAcA) to UDP-2,3-diacetamido-2,3-dideoxy-alpha-D-mannuronic acid (UDP-alpha-D-ManNAc3NAcA). Exhibits high specificity towards the substrate as UDP-alpha-D-GlcNAc, UDP-alpha-D-GlcNAcA (UDP-2-acetamido-2-deoxy-alpha-D-glucuronic acid) and UDP-alpha-D-GlcNAc3NAc (UDP-2,3-diacetamido-2,3-dideoxy-alpha-D-glucose) cannot act as substrates. UDP-2,3-diacetamido-2,3-dideoxy-alpha-D-glucuronate = UDP-2,3-diacetamido-2,3-dideoxy-alpha-D-mannuronate Optimum pH is 6.0. Optimum temperature is 30-37 degrees Celsius. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Belongs to the UDP-N-acetylglucosamine 2-epimerase family.