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 (40 kDa) by LC-MS and N-terminal sequencing analyses. The cleavage results in the formation of two inactive fragments of 33 and 7 kDa. 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.