Structural characterization of Escherichia coli sialic acid synthase.

Sialic acid synthase encoded by the neuB gene of Escherichia coli catalyzes the condensation of N-acetylmannosamine and phosphoenolpyruvate to form N-acetylneuraminic acid. This report demonstrates the first structural information on sialic acid synthase by CD, MALDI-TOF, and chemical cross-linking studies. Also, a specific cleavage by endogenous protease(s) has been identified at Lys280 of the enzyme (40kDa) by LC-MS and N-terminal sequencing analyses. The cleavage results in the formation of two inactive fragments of 33 and 7kDa. The structural analysis indicates that the fragmentation is associated with a significant change of the enzyme from a tetrameric to trimeric form, and alterations in both secondary and native quaternary structures.