Identification and characterization of a deaminoneuraminic acid (Kdn)-specific aldolase from Sphingobacterium species

Sialic acid (Sia) is a group of acidic sugars with a 9-carbon backbone, and classified into 3 species based on the substituent group at C5 position: N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), and deaminoneuraminic acid (Kdn). In Escherichia coli, the sialate aldolase or N-acetylneuraminate aldolase (NanA) is known to catabolize these Sia species into pyruvate and the corresponding 6-carbon mannose derivatives. However, in bacteria, very little is known about the catabolism of Kdn, compared with Neu5Ac. In this study, we found a novel Kdn-specific aldolase (Kdn-aldolase), which can exclusively degrade Kdn, but not Neu5Ac or Neu5Gc, from Sphingobacterium sp., which was previously isolated from a Kdn-assimilating bacterium. Kdn-aldolase had the optimal pH and temperature at 7.0-8.0 and 50 °C, respectively. It also had the synthetic activity of Kdn from pyruvate and mannose. Site-specific mutagenesis revealed that N50 residue was important for the Kdn-specific reaction. Existence of the Kdn-aldolase suggests that Kdn-specific metabolism may play a specialized role in some bacteria.