Novel glycoside hydrolase family enzymes fromEscherichia coliare associated with osmo-regulated periplasmic glucan synthesis

Most Gram-negative bacteria synthesize osmo-regulated periplasmic glucans (OPG) in the periplasm or extracellular space. Many pathogens lose their pathogenicity by knocking out opgG , an OPG-related gene indispensable for OPG synthesis. However, the biochemical functions of OpgG and OpgD, a paralog of OpgG, have not been elucidated. In this report, structural and functional analyses of OpgG and OpgD from Escherichia coli revealed that these proteins are β-1,2-glucanases with remarkably different activity, establishing a new glycoside hydrolase family. Furthermore, a reaction mechanism with an unprecedentedly long proton transfer pathway is proposed for OpgD. The conformation of the region that forms the reaction pathway differs noticeably between OpgG and OpgD, which explains the observed low activity of OpgG. The findings enhance our understanding of OPG biosynthesis and provide insights into functional diversity for this novel enzyme family.