Escherichia coli heptosyltransferase I: investigation of protein dynamics of a GT-B structural enzyme.

Heptosyltransferase I (HepI), the enzyme responsible for the transfer of l-glycero-d-manno-heptose to a 3-deoxy-α-d-manno-oct-2-ulopyranosonic acid (Kdo) of the growing core region of lipopolysaccharide, is a member of the GT-B structural class of enzymes. Crystal structures have revealed open and closed conformations of apo and ligand-bound GT-B enzymes, implying that large-scale protein conformational dynamics play a role in their reaction mechanism. Here we report transient kinetic analysis of conformational changes in HepI reported by intrinsic tryptophan fluorescence and present the first real-time evidence of a GT-B enzyme undergoing a substrate binding-induced transition from an open to closed state prior to catalysis.