ABSTRACT

Exosialidases are glycoside hydrolases thatremove a single terminal sialic acid residue fromoligosaccharides. They are widely distributed inbiology, having been found in prokaryotes,eukaryotes, and certain viruses. Most characterizedprokaryotic sialidases are from organisms that arepathogenic or commensal with mammals.However, in this study, we used functionalmetagenomic screening to seek microbial sialidasesencoded by environmental DNA isolated from anextreme ecological niche, a thermal spring. Usingrecombinant expression of potential exosialidasecandidates and a fluorogenic sialidase substrate, wediscovered an exosialidase having no homology toknown sialidases. Phylogenetic analysis indicatedthat this protein is a member of a small family ofbacterial proteins of previously unknown function.Proton NMR revealed that this enzyme functionsvia an inverting catalytic mechanism, a biochemicalproperty that is distinct from those of knownexosialidases. This unique inverting exosialidasedefines a new CAZy glycoside hydrolase family wehave designated GH156.