Kinetic Characterization and Computational Modeling of the Escherichia coli Heptosyltransferase II: Exploring the Role of Protein Dynamics in Catalysis for a GT-B Glycosyltransferase

Glycosyltransferases (GTs) are enzymes that are uniquely adapted to promote the formation of a glycosidic bond between a sugar molecule and a wide variety of substrates. Heptosyltransferase II (HepII) is a GT involved in the lipopolysaccharide (LPS) biosynthetic pathway that transfers the seven-carbon sugar (L-glycero-D-manno-heptose; Hep) onto a lipid anchored glycopolymer (heptosylated Kdo-Lipid A, Hep-Kdo-Lipid A or HLA). LPS plays a key role in Gram-negative bacterial sepsis as a stimulator of the human immune response and has been used as an adjuvant in vaccines. As such, ongoing efforts towards inhibition of LPS biosynthetic enzymes to aid development of novel antimicrobial therapeutics has driven significant effort towards the characterization of these enzymes. Three heptosyltransferases are involved in the inner-core biosynthesis, with E. coli HepII being the last to be quantitatively characterized in vivo, as described herein. HepII shares modest sequence similarity with heptosyltransferase I (HepI) while maintaining a high degree of structural homology. Here we report the first kinetic and biophysical characterization of HepII and demonstrate the properties of HepII that are shared by HepI to include sugar donor promiscuity, and sugar acceptor induced secondary structural changes which results in significant thermal stabilization. HepII also has an increased catalytic efficiency and a significantly tighter binding affinity for both of its substrates, with an insensitivity to the number of acyl chains on the sugar acceptor. Additionally, a structural model of the HepII ternary complex, refined by molecular dynamics simulations, was developed to probe potentially important substrate-protein contacts and revealed the potential of Tryptophan (Trp) residues responsible for reporting on ligand binding. As was previously described for HepI, Tryptophan fluorescence in HepII allowed observation of substrate induced changes in Trp fluorescence intensity which enabled determination of substrate dissociation constants. Combined, these efforts meaningfully enhance our understanding of the Heptosyltransferase family of enzymes and will aid in future efforts to design novel, potent and specific inhibitors for this family of enzymes.