Catalyzes the fifth step in the biosynthesis of pseudaminic acid, a sialic-acid-like sugar that is used to modify flagellin. Catalyzes the condensation of phosphoenolpyruvate with 2,4-diacetamido-2,4,6-trideoxy-beta-l-altropyranose, forming pseudaminic acid. 2,4-diacetamido-2,4,6-trideoxy-beta-L-altrose + H2O + phosphoenolpyruvate = phosphate + pseudaminate kcat is 0.65 sec(-1). Optimum pH is 7.0. Belongs to the pseudaminic acid synthase family.