Catalyzes the interconversion of N-acetylglucosamine to N-acetylmannosamine (PubMed:10502668, PubMed:12499362, PubMed:9990133). Involved in the N-glycolylneuraminic acid (Neu5Gc) degradation pathway: although human is not able to catalyze formation of Neu5Gc due to the inactive CMAHP enzyme, Neu5Gc is present in food and must be degraded (PubMed:9990133).
