Opens both the alpha- and beta-forms of N-acetylneuraminate (sialic acid; Neu5Ac) to provide aceneuramate, the preferred substrate for NanA (Probable). Has preferential activity on the beta-anomer rather than the alpha-anomer. Accelerates a reaction that is spontaneous at slightly alkaline pH, facilitates the reaction at acidic pH (By similarity). N-acetyl-alpha-neuraminate = aceneuramate N-acetyl-beta-neuraminate = aceneuramate Has been crystallized with Cu(2+) but reconstitutes with Zn(2+). Belongs to the NanQ anomerase family.