Catalyzes the first step in the biosynthesis of pseudaminic acid, a sialic-acid-like sugar that is used to modify flagellin. Has both C6 dehydratase and C5 epimerase activities that result in the production of both UDP-2-acetamido-2,6-dideoxy-beta-L-arabino-4-hexulose and UDP-2-acetamido-2,6-dideoxy-alpha-D-xylo-4-hexulose. UDP-N-acetyl-alpha-D-glucosamine = H2O + UDP-2-acetamido-2,6-dideoxy-beta-L-arabino-hex-4-ulose Homohexamer. Belongs to the polysaccharide synthase family.