Crystal Structure of the IIBSor Domain of the Sorbose Permease from Klebsiella pneumoniae Solved to 1.75 Å Resolution

The phosphoenolpyruvate transferase system (PTS) is the major pathway by which bacteria import hexose sugars across the plasma membrane. The PTS transfers a phosphoryl group sequentially via several components from the glycolytic intermediate phosphoenolpyruvate (PEP) to the translocated sugar. It is comprised of the two general proteins enzyme I and HPr, and a sugar-specific enzyme II complex. Sugar translocation is through the membrane domain of the enzyme II complex. The enzyme II complex can belong to one of six families based upon sequence similarity, with the sorbose transporter from Klebsiella pneumoniae a member of the mannose family. The structure of the IIBSor domain was solved to 1.75 Å resolution by molecular replacement. It has a central core of seven parallel β-strands surrounded by a total of six α-helices. Three helices cover the front face, one the back face with the remaining two capping the central β-sheet at the top and bottom. The catalytic His15 residue is situated on the surface-exposed loop between strand 1 and helix 1. In addition to the features previously observed in the homologous IIBLev domain from Bacillus subtilis we see new features in the IIBSor structure. First, the catalytic His15 side-chain is fixed in a specific conformation by forming a short hydrogen bond with Asp10, which in turn makes a salt-bridge with Arg8. Second, as observed in other phosphoproteins, an arginine residue (Arg12) is well poised to stabilize a phosphoryl group on His15. Third, we see an Asp/His pair reminiscent of that observed in the IIAMan domain from Escherichia coli. Finally, docking of IIAMan to IIBSor shows that Arg12 in its current conformation is well positioned to assist the subsequent transfer of the phosphoryl group onto the sugar in line with previous mutagenesis studies.