Sialic acids comprise a varied group of nine-carbon amino sugars that are widely distributed among mammals and higher metazoans. Some human commensals and bacterial pathogens can scavenge sialic acids from their environment and degrade them for use as a carbon and nitrogen source. The enzymeN-acetylmannosamine kinase (NanK; EC 2.7.1.60) belongs to the transcriptional repressors, uncharacterized open reading frames and sugar kinases (ROK) superfamily. NanK catalyzes the second step of the sialic acid catabolic pathway, transferring a phosphate group from adenosine 5′-triphosphate to the C6 position ofN-acetylmannosamine to generateN-acetylmannosamine 6-phosphate. The structure of NanK fromFusobacterium nucleatumwas determined to 2.23 Å resolution by X-ray crystallography. Unlike other NanK enzymes and ROK family members,F. nucleatumNanK does not have a conserved zinc-binding site. In spite of the absence of the zinc-binding site, all of the major structural features of enzymatic activity are conserved.