Opens both the alpha- and beta-forms of N-acetylneuraminate (sialic acid; Neu5Ac) to provide aceneuramate, the preferred substrate for NanA. 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. N-acetyl-alpha-neuraminate = aceneuramate N-acetyl-beta-neuraminate = aceneuramate The Zn(2+) ion may play a catalytic role. Inhibited by 1,10-phenanthroline. Negatively regulated by the transcriptional repressor NanR. Induced by N-acetylneuraminate, via inactivation of NanR. A His-tag on the N-terminus blocks activity, while a C-terminal His-tag does not. No visible effect on growth on Neu5Ac at pH 7.0 and 37 degrees Celsius. Shows slight systematic growth delay at pH 6.0 and 20 degrees Celsius. No visible phenotype on glucose. Belongs to the NanQ anomerase family.