Catalyzes the phosphorylation of N-acetylmannosamine (ManNAc) to ManNAc-6-P. Has also low level glucokinase activity in vitro. ATP + N-acetyl-D-mannosamine = ADP + H(+) + N-acetyl-D-mannosamine 6-phosphate Catalytic efficiency with N-acyl-D-mannosamine as substrate is 136-fold higher than that with D-glucose. Amino-sugar metabolism; N-acetylneuraminate degradation; D-fructose 6-phosphate from N-acetylneuraminate: step 2/5. Homodimer. Negatively regulated by the transcriptional repressor NanR. Induced by N-acetylneuraminate, via inactivation of NanR. Belongs to the ROK (NagC/XylR) family. NanK subfamily. Extended N-terminus.