Regulates and initiates biosynthesis of N-acetylneuraminic acid (NeuAc), a precursor of sialic acids. Required for normal sialylation in hematopoietic cells. Sialylation is implicated in cell adhesion, signal transduction, tumorigenicity and metastatic behavior of malignant cells. Plays an essential role in early development (By similarity). H2O + UDP-N-acetyl-alpha-D-glucosamine = H(+) + N-acetyl-D-mannosamine + UDP an N-acyl-D-mannosamine + ATP = ADP + an N-acyl-D-mannosamine 6-phosphate + H(+) Allosterically regulated (By similarity); feedback inhibited by cytidine monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac), the end product of neuraminic acid biosynthesis. Activity is dependent on oligomerization. The monomer is inactive, whereas the dimer catalyzes only the phosphorylation of N-acetylmannosamine, and the hexamer is fully active for both enzyme activities. Up-regulated after PKC-dependent phosphorylation. Amino-sugar metabolism; N-acetylneuraminate biosynthesis. Homodimer and homohexamer. Widely expressed. Highest expression is observed in liver. Phosphorylated by PKC. In the N-terminal section; belongs to the UDP-N-acetylglucosamine 2-epimerase family. In the C-terminal section; belongs to the ROK (NagC/XylR) family.