Catalyzes the interconversion between UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylmannosamine (UDP-ManNAc). Involved in the biosynthesis of the capsular polysaccharides. In vitro, can also use several chemoenzymatically synthesized UDP-ManNAc derivatives as substrates, with lower efficiency. UDP-N-acetyl-alpha-D-glucosamine = UDP-N-acetyl-alpha-D-mannosamine Activated by UDP-GlcNAc and inhibited by 2-acetamidoglucal and UDP. Activity is strongly decreased in the presence of Co(2+) and abolished in the presence of Mn(2+) or Zn(2+). kcat is 31 sec(-1) with UDP-GlcNAc as substrate. kcat is 52 sec(-1) with UDP-ManNAc as substrate. kcat is 4.2 sec(-1) with UDP-ManNGc as substrate. kcat is 7.8 sec(-1) with UDP-ManNPr as substrate. kcat is 6.7 sec(-1) with rUDP-ManNAc as substrate. Optimum pH is 8-0-9.0. Capsule biogenesis; capsule polysaccharide biosynthesis. Belongs to the UDP-N-acetylglucosamine 2-epimerase family.