Catalyzes the four-electron oxidation of UDP-N-acetyl-D-mannosamine (UDP-ManNAc), reducing NAD(+) and releasing UDP-N-acetylmannosaminuronic acid (UDP-ManNAcA). H2O + 2 NAD(+) + UDP-N-acetyl-alpha-D-mannosamine = 3 H(+) + 2 NADH + UDP-N-acetyl-alpha-D-mannosaminouronate Activated by N-acetylglucosamine-1-P or K(+) at low UDP-ManNAc concentrations. Optimum pH is 10.0. Bacterial outer membrane biogenesis; enterobacterial common antigen biosynthesis. Homodimer. Mutants do not synthesize Und-PP-GlcNAc-ManNAcA (lipid II) and enterobacterial common antigen (ECA). Belongs to the UDP-glucose/GDP-mannose dehydrogenase family. WecC subfamily.